Voet D., Voet Ju.G. Biochemistry

Подождите немного. Документ загружается.

NANA group to G

,forming G

, or by adding an N-acetyl-

glucosamine unit to lactosyl ceramide before NANA addi-

tion, forming G

. Hundreds of different gangliosides are

known.

e. Sphingoglycolipid Degradation and

Lipid Storage Diseases

Sphingoglycolipids are lysosomally degraded by a series

of enzymatically mediated hydrolytic reactions (Fig. 25-93).

These reactions are catalyzed at the lipid–water interface by

soluble enzymes, often with the aid of sphingolipid activator

proteins (SAPs; including saposins, G

-activator protein,

and SAP-A through SAP-D). These nonenzymatic ancil-

lary proteins are thought to increase the accessibility of the

carbohydrate moiety of the sphingolipid to the degradation

enzyme. For example, G

-activator binds G

and helps

expose it to the surface of the membrane.The G

-activator–

complex can then bind hexosaminidase A, an  dimer

that hydrolyzes N-acetylgalactosamine from G

at the

lipid–water interface (Fig. 25-94).

The hereditary absence of one of the sphingolipid hy-

drolases or a SAP results in a sphingolipid storage disease

(Table 25-2). One of the most common such conditions is

Tay–Sachs disease, an autosomal recessive deficiency in

hexosaminidase A. The absence of hexosaminidase A

activity results in the neuronal accumulation of G

shell-like inclusions (Fig. 25-95).

Although infants born with Tay–Sachs disease at first

appear normal, by ⬃1 year of age, when sufficient G

has

accumulated to interfere with neuronal function, they be-

come progressively weaker, retarded, and blinded until

they die, usually by the age of 3 years. It is possible, how-

ever, to screen potential carriers of this disease by a simple

serum assay. It is also possible to detect the disease in

utero by assay of amniotic fluid or amniotic cells obtained

by amniocentesis. The assay involves use of an artificial

hexosaminidase substrate, 4-methylumbelliferyl--

D-N-

acetylglucosamine, which yields a fluorescent product on

hydrolysis:

4-Methylumbelliferyl--D-

N-acetylglucosamine

4-Methylumbelliferone

(fluorescent in alkaline medium)

GlcNAc

hexosaminidase

Section 25-8. Phospholipid and Glycolipid Metabolism 1011

Figure 25-92 The biosynthesis of globosides and G

gangliosides.

Glc β (1 1) ceramide

Glucocerebroside

UDP

UDP–Gal

Gal β (1 4) Glc β (1 1) ceramide

Lactosyl ceramide

Gal β (1 4) Glc β (1 1 ) ceramide

Trihexosyl ceramide

Gal α (1 4) Gal β (1 4) Glc β (1 1) ceramide

UDP

UDP–GalNAc

UDP

UDP–GalNAc

GalNAc β (1 4) Gal β (1 4) Glc β (1 1) ceramide

GalNAc β (1 3) Gal α (1 4) Gal β (1 4) Glc β (1 1) ceramide

Globoside

UDP

UDP–Gal

Gal β (1 3) GalNAc β (1 4) Gal β (1 4) Glc β (1 1) ceramide

UDP–Gal

UDP

globoside synthesis

CMP–NANA

CMP

ganglioside synthesis

NANA

α2

NANA

α2

NANA

α2

JWCL281_c25_940-1018.qxd 6/8/10 9:00 AM Page 1011

1012 Chapter 25. Lipid Metabolism

Gal GalNAc Glc CerGal

Ganglioside G

ββββ

NANA

GalNAc Glc CerGal

Ganglioside G

βββ

NANA

Glc CerGal

Ganglioside G

ββ

NANA

Glc CerGal

Lactosyl ceramide

ββ

NANA

Glc Cer

Glucocerebroside

Glc

Ceramide

Fatty acid

Glc CerGal

Trihexosylceramide

ββ

Gal

-galactosidaseβ

gangliosidosis

GalNAc

hexosaminidase A

activator protein

Tay–Sachs disease

ganglioside

neuraminidase

SAP-B

α-galactosidase A

SAP-B

Fabry’s disease

Gal

β-galactosidase

SAP-B

+ SAP-C

CerGal

Galactocerebroside

Gal

galactocerebrosidase

SAP-A, SAP-C

Krabbe’s disease

phosphocholine

glucocerebrosidase

SAP-C

Gaucher’s disease

sphingomyelinase

Niemann–Pick

disease

sphingosine

ceramidase

SAP-D

Farber’s

lipogranulomatosis

Cer-phospho-

choline

(Sphingomyelin)

Sulfatide

CerGal

arylsulfatase A

SAP-B

Metachromatic

leukodystrophy

–

hexosaminidase A and B

Sandhoff’s disease

Glc CerGal

Globoside

ββ

Gal

GalNAc

2–

-activator protein

Ganglioside G

Hexosaminidase A

degradation

Figure 25-93 The breakdown of sphingolipids by lysosomal

enzymes. The genetic diseases caused by the corresponding

enzyme deficiencies are noted in red.

Figure 25-94 Model for G

-activator protein–stimulated

hydrolysis of ganglioside G

by hexosaminidase A. G

-activator

protein binds and lifts G

out of the membrane so that it can be

recognized and cleaved by the  dimer of hexosaminidase A.

[After Kolter,T. and Sandhoff, K., Angew. Chem. Int. Ed. 38, 1532

(1999).]

JWCL281_c25_940-1018.qxd 4/20/10 1:59 PM Page 1012

Since this substrate is also recognized by hexosaminidase

B, which is unaffected in Tay–Sachs disease, the hex-

osaminidase B is first heat inactivated since it is more heat

labile than hexosaminidase A.As a result of mass screening

efforts, the tragic consequences of this genetic enzyme de-

ficiency are being averted. The other sphingolipid storage

diseases, although less common, have similar consequences

(Table 25-2). In recent years, DNA sequencing techniques

that screen potential carriers for the most common muta-

tions have become economically feasible.

Chapter Summary 1013

Figure 25-95 Cytoplasmic membranous body in a neuron

affected by Tay–Sachs disease. [Courtesy of John S. O’Brien,

University of California at San Diego Medical School.]

Table 25-2 Sphingolipid Storage Diseases

Principal Storage

Disease Enzyme Deficiency Substance Major Symptoms

Gangliosidosis G

-galactosidase Ganglioside G

Mental retardation, liver enlargement,

skeletal involvement, death by age 2

Tay–Sachs disease Hexosaminidase A Ganglioside G

Mental retardation, blindness, death

by age 3

Fabry’s disease -Galactosidase A Trihexosylceramide Skin rash, kidney failure, pain in

lower extremities

Sandhoff’s disease Hexosaminidases A Ganglioside G

and Similar to Tay–Sachs disease but

and B globoside progressing more rapidly

Gaucher’s disease Glucocerebrosidase Glucocerebroside Liver and spleen enlargement, erosion

of long bones, mental retardation in

infantile form only

Niemann–Pick Sphingomyelinase Sphingomyelin Liver and spleen enlargement, mental

disease retardation

Farber’s Ceramidase Ceramide Painful and progressively deformed

lipogranulomatosis joints, skin nodules, death within a

few years

Krabbe’s disease Galactocerebrosidase Deacylated Loss of myelin, mental retardation,

galactocerebroside death by age 2

Metachromatic Arylsulfatase A Sulfatide Mental retardation, death in first

leukodistrophy decade

(Sulfatide lipidosis)

1 Lipid Digestion, Absorption, and Transport Triacyl-

glycerols, the storage form of metabolic energy in animals,

provide up to six times the metabolic energy of an equal

weight of hydrated glycogen. Dietary lipids are digested by

pancreatic digestive enzymes such as lipase and phospholipase

that are active at the lipid–water interface of bile salt–

stabilized emulsions. Bile salts are also essential for the intes-

tinal absorption of dietary lipids, as is fatty acid–binding pro-

tein.Dietary triacylglycerols and those synthesized by the liver

are transported in the blood as chylomicrons and VLDL, re-

spectively. Triacylglycerols present in these lipoproteins are

hydrolyzed by lipoprotein lipase outside the cells and enter

CHAPTER SUMMARY

JWCL281_c25_940-1018.qxd 4/20/10 1:59 PM Page 1013

1014 Chapter 25. Lipid Metabolism

them as free fatty acids. Fatty acids resulting from hydrolysis

of adipose tissue triacylglycerols by hormone-sensitive triacyl-

glycerol lipase are transported in the bloodstream as fatty

acid–albumin complexes.

2 Fatty Acid Oxidation Before fatty acids are oxidized,

they are converted to their acyl-CoA derivatives by acyl-CoA

synthase in an ATP-requiring process, transported into mito-

chondria as carnitine esters, and reconverted inside the mito-

chondrial matrix to acyl-CoA.  Oxidation of fatty acyl-CoA

occurs in 2-carbon increments so as to convert even-chain

fatty acyl-CoAs completely to acetyl-CoA. The pathway in-

volves FAD-dependent dehydrogenation of an alkyl group,

hydration of the resulting double bond, NAD



-dependent ox-

idation of this alcohol to a ketone, and C¬C bond cleavage to

form acetyl-CoA and a new fatty acyl-CoA with two fewer

carbon atoms.The process then repeats itself.Complete oxida-

tion of the acetyl-CoA, NADH,and FADH

is achieved by the

citric acid cycle and oxidative phosphorylation. Oxidation of

unsaturated fatty acids and odd-chain fatty acids also occurs

by  oxidation but requires the participation of additional en-

zymes. Odd-chain fatty acid oxidation generates propionyl-

CoA, whose further metabolism requires the participation of

(1) propionyl-CoA carboxylase, which has a biotin prosthetic

group, (2) methylmalonyl-CoA racemase, and (3) methyl-

malonyl-CoA mutase, which contains AdoCbl (coenzyme

). Methylmalonyl-CoA mutase catalyzes a carbon skeleton

rearrangement reaction via a free radical mechanism in which

the free radical is generated by the homolytic cleavage of

AdoCbl’s C¬Co(III) bond.

 Oxidation of fatty acids takes place in the peroxisomes in

addition to the mitochondrion. The peroxisomal pathway dif-

fers from the mitochondrial pathway in that the FADH

pro-

duced in the first step, rather than generating ATP by oxida-

tive phosphorylation, is directly oxidized by O

to produce

. Peroxisomal enzymes are specific for long-chain fatty

acids and function in a chain-shortening process via  oxida-

tion. The resultant intermediate chain-length products are

transferred to the mitochondrion for complete oxidation.

3 Ketone Bodies A significant fraction of the acetyl-

CoA produced by fatty acid oxidation in the liver is converted

to acetoacetate and

D--hydroxybutyrate, which, together

with acetone, are referred to as ketone bodies. The first two

compounds serve as important fuels for the peripheral tissues.

4 Fatty Acid Biosynthesis Fatty acid biosynthesis differs

from fatty acid oxidation in several respects. Whereas fatty

acid oxidation occurs in the mitochondrion utilizing fatty acyl-

CoA esters, fatty acid biosynthesis occurs in the cytosol with

the growing fatty acids esterified to acyl-carrier protein

(ACP).The redox coenzymes differ (FAD and NAD



for oxi-

dation; NADPH for biosynthesis), as does the stereochemistry

of the pathway’s intermediate steps. Oxidation produces

acetyl-CoA, whereas malonyl-CoA is the immediate precur-

sor in biosynthesis. Malonyl-CoA is produced by the ATP-

driven reaction of with acetyl-CoA as catalyzed by the

biotin-containing enzyme acetyl-CoA carboxylase, although

this is not incorporated into the final fatty acid prod-

uct. In bacteria, fatty acids synthesis is carried out by a series

of independent enzymes that are collectively known as FAS-I.

In animals, fatty acid synthesis yielding palmitate occurs on a

single X-shaped homodimeric protein known as FAS-II, each

of whose subunits contain all six activities required to do so on

HCO



HCO



separate domains that are serviced by ACP. In fungi, FAS-II is

an 



heterododecamer that forms a barrel-shaped protein

that contains two equivalent reaction chambers. Similar but

more extensive systems are used in the synthesis of the various

polyketides. Acetyl-CoA is transferred from the mitochon-

drion to the cytosol as citrate via the tricarboxylate transport

system and released by citrate cleavage to yield acetyl-CoA

and oxaloacetate. Oxaloacetate is converted to malate and

then to pyruvate for transport back to the mitochondrion, a

process that also generates some of the NADPH required for

biosynthesis. Palmitate is the primary product of fatty acid

biosynthesis in animals. Longer chain fatty acids and unsatu-

rated fatty acids are synthesized from palmitate by elongation

and desaturation reactions. Certain essential unsaturated fatty

acids cannot be synthesized by animals and hence must be ob-

tained from the diet. Triacylglycerols are synthesized from

fatty acyl-CoA esters and glycerol-3-phosphate.

5 Regulation of Fatty Acid Metabolism Fatty acid me-

tabolism is regulated through the allosteric control of

hormone-sensitive triacylglycerol lipase and acetyl-CoA car-

boxylase, phosphorylation/dephosphorylation, and/or changes

in the rates of protein synthesis and breakdown. This regula-

tion is mediated by the hormones glucagon, epinephrine, and

norepinephrine, which activate degradation, and by insulin,

which activates biosynthesis. These hormones interact to con-

trol the cAMP concentration, which in turn controls phos-

phorylation/dephosphorylation ratios via PKA.AMPK, which

senses the level of ATP, is also an important regulator of fatty

acid metabolism.

6 Cholesterol Metabolism Cholesterol is a vital con-

stituent of cell membranes and is the precursor of the steroid

hormones and bile salts. Its biosynthesis, transport, and utiliza-

tion are rigidly controlled. Cholesterol is synthesized in the

liver from acetate in a pathway that involves formation of

HMG-CoA from three molecules of acetate followed by re-

duction, phosphorylation, decarboxylation, and dehydration

to the isoprene units isopentenyl pyrophosphate and dimethyl-

allyl pyrophosphate. Four of these isoprene units are then

condensed via cationic mechanisms to form squalene, which,

in turn, undergoes a cyclization reaction, via a cationic cas-

cade, to form lanosterol, the sterol precursor to cholesterol.

The pathway’s major control point is at HMG-CoA reduc-

tase. This enzyme is regulated by competitive and allosteric

mechanisms, by phosphorylation/dephosphorylation, and,

most importantly, by long-term control of the rates of enzyme

synthesis and degradation. Long-term control is mediated by

the integral membrane protein SREBP, which when the cho-

lesterol level is low, is escorted by SCAP to the Golgi appara-

tus via COPII-coated vesicles. There it is sequentially cleaved

by the proteases S1P and S2P, thereby releasing its soluble

bHLH/Z domain to travel to the nucleus where it induces the

transcription of SRE-containing genes such as those encoding

HMG-CoA reductase and the LDL receptor. In addition,

when sterol levels are high, the rate at which HMG-CoA re-

ductase is proteolytically degraded is greatly increased.

The liver secretes cholesterol into the bloodstream in ester-

ified form as part of the VLDL. This complex is sequentially

converted to IDL and then to LDL. LDL, which is brought into

the cells by receptor-mediated endocytosis, carries the major

portion of cholesterol to peripheral tissues for utilization. Ex-

cess cholesterol is returned to the liver from peripheral tissues

JWCL281_c25_940-1018.qxd 6/8/10 9:00 AM Page 1014

References 1015

by HDL. The cellular supply of cholesterol is controlled by

three mechanisms: (1) long- and short-term regulation of

HMG-CoA reductase; (2) control of LDL receptor synthesis

by cholesterol concentration; and (3) long- and short-term

regulation of acyl-CoA:cholesterol acyltransferase (ACAT),

which mediates cholesterol esterification. Cholesterol is the

precursor to the steroid hormones, which are classified as

progestins, glucocorticoids, mineralocorticoids, androgens, and

estrogens.The only pathway for the excretion of cholesterol in

mammals is through the formation and elimination of bile

salts.

7 Eicosanoid Metabolism: Prostaglandins, Prostacyclins,

Thromboxanes, Leukotrienes, and Lipoxins

Prostaglandins,

prostacyclins, thromboxanes, leukotrienes, and lipoxins are

eicosanoid products produced largely by the metabolism of

arachidonate. These highly unstable compounds have pro-

found physiological effects at extremely low concentrations.

They are involved in the inflammatory response, the produc-

tion of pain and fever, the regulation of blood pressure, and

many other important physiological processes. Arachidonate

is synthesized from linoleic acid, an essential fatty acid, and

stored as phosphatidylinositol and other phospholipids.

Prostaglandins, prostacyclins, and thromboxanes are synthe-

sized via the cyclic pathway, whereas leukotrienes and lipoxins

are synthesized via the linear pathway. Aspirin and other non-

steroidal anti-inflammatory drugs (NSAIDs) inhibit the cyclic

pathway but not the linear pathway. COX-2 inhibitors are

NSAIDs that bind to COX-2 but not COX-1, thereby elimi-

nating the side effects of other NSAIDs. Peptidoleukotrienes

have been identified as the slow reacting substances of ana-

phylaxis (SRS-A) released from sensitized lung after immuno-

logical challenge. Lipoxins and aspirin-induced epi-lipoxins

have anti-inflammatory properties.

8 Phospholipid and Glycolipid Metabolism Complex

lipids have either a phosphate ester or a carbohydrate as their

polar head group and either 1,2-diacyl-sn-glycerol or ceramide

(N-acylsphingosine) as their hydrophobic tail. Phospholipids

are either glycerophospholipids or sphingophospholipids,

whereas glycolipids are either glyceroglycolipids or sphingo-

glycolipids. The polar head groups of glycerophospholipids,

which are phosphate esters of either ethanolamine, serine,

choline, inositol, or glycerol, are attached to 1,2-diacyl-sn-

glycerol’s C3 OH group by means of CTP-linked transferase

reactions. The specific long-chain fatty acids found at the C1

and C2 positions are incorporated by “remodeling reactions”

after the addition of the polar head group. Plasmalogens and

alkylacylglycerophospholipids, respectively, contain a long-

chain alkyl group in a vinyl ether linkage or in an ether linkage

to glycerol’s C1 OH group. Platelet-activating factor (PAF) is

an important alkylacylglycerophospholipid. The only major

sphingophospholipid is sphingomyelin (N-acylsphingosine

phosphocholine), an important structural lipid of nerve cell

membranes. Most sphingolipids contain polar head groups

composed of carbohydrate units and are therefore referred to

as sphingoglycolipids. The principal classes of sphingoglyco-

lipids are cerebrosides, sulfatides, globosides, and gangliosides.

Their carbohydrate units, which are attached to N-acylsphin-

gosine’s C1 OH group by glycosidic linkages, are formed by

stepwise addition of activated monosaccharide units. Several

lysosomal sphingolipid storage diseases, including Tay–Sachs

disease, result from deficiencies in the enzymes that degrade

sphingoglycolipids.

General

Newsholme, E.A. and Leech, A.R., Biochemistry for the Medical

Sciences, Wiley (1983).[Chapters 6–8 contain a wealth of infor-

mation on the control of fatty acid metabolism and its integra-

tion into the overall scheme of metabolism.]

Valle,D. (Ed.), The Online Metabolic & Molecular Bases of Inher-

ited Disease, http://www.ommbid.com/. [Contain numerous

chapters on defects in lipid metabolism.]

Vance, D.E. and Vance J.E. (Eds.), Biochemistry of Lipids,

Lipoproteins, and Membranes (5th ed.), Elsevier (2008).

Lipid Digestion

Berg, O.G., Gelb, M.H., Tsai, M.-D., and Jain, M.K., Interfacial en-

zymology: the secreted phospholipase A

-paradigm, Chem.

Rev. 101, 2613–2653 (2001).

Bhattacharya, A.A., Grüne, T., and Curry, S., Crystallographic

analysis reveals common modes of binding of medium and

long-chain fatty acids to human serum albumin, J. Mol. Biol.

303, 721–732 (2002).

Borgström, B., Barrowman, J.A., and Lindström, M., Roles of

bile acids in intestinal lipid digestion and absorption, in

Danielsson, H. and Sjövall, J. (Eds.), Sterols and Bile Acids,

pp. 405–425, Elsevier (1985).

Brady, L., Brzozowski, A.M., Derewenda, Z.S., Dodson, E., Dod-

son, G., Tolley, S., Turkenburg, J.P., Christiansen, L., Huge-

Jensen, B., Norskov, L., Thim, L., and Menge, U., A serine pro-

tease triad forms the catalytic centre of a triacylglycerol lipase,

Nature 343, 767–770 (1990).

Hermoso, J., Pignol, D., Penel, S., Roth, M., Chapus, C., and

Fontecilla-Camps, J.C., Neutron crystallographic evidence of

lipase–colipase complex activation by a micelle, EMBO J. 16,

5531–5536 (1997).

Scapin, G., Gordon, J.I., and Sacchettini, J.C., Refinement of the

structure of recombinant rat intestinal fatty acid-binding apopro-

tein at 1.2-Å resolution, J. Biol. Chem. 267, 4253–4269 (1992).

van Tilbeurgh, H., Bezzine, S., Cambillau, C., Verger, R., and

Carriére, F., Colipase: structure and interaction with pancreatic

lipase, Biochim. Biophys.Acta 1441, 173–184 (1999).

Fatty Acid Oxidation

Bannerjee, R., Radical peregrinations catalyzed by coenzyme B

dependent enzymes, Biochem. 40, 6191–6198 (2001).

Bartlett, K. and Eaton, S., Mitochondrial -oxidation, Eur. J.

Biochem. 271, 462–469 (2004). [Discusses the reactions and en-

zymes of mitochondrial  oxidation as well as their regulation.]

Bieber, L.L., Carnitine, Annu. Rev. Biochem. 88, 261–283 (1988).

Brownsey, R.W., Boone, A.N., Elliott, J.E., Kulpa, J.E., and Lee,

W.M., Regulation of acetyl-CoA carboxylase, Biochem. Soc.

Trans. 34, 223–227 (2006).

Chou, C.-Y., Yu, L.P.C., and Tong, L., Crystal structure of biotin

carboxylase in complex with substrates and implications for its

catalytic mechanism, J. Biol. Chem. 284, 11690–11697 (2009).

REFERENCES

JWCL281_c25_940-1018.qxd 6/8/10 9:00 AM Page 1015

Kim,J.-J.P.and Battaile,K.P., Burning fat:The structural basis of fatty

acid -oxidation, Curr. Opin. Struct. Biol. 12, 721–728 (2002).

Kim, J.-J.P., Wang, M., and Pashke, R., Crystal structures of

medium-chain acyl-CoA dehydrogenase from pig liver mito-

chondria with and without substrate, Proc. Natl. Acad. Sci. 90,

7523–7527 (1993).

Kindl, H., Fatty acid degradation in plant peroxisomes: Function

and biosynthesis of the enzymes involved, Biochimie 75,

225–230 (1993).

Mancia, R., Smith, G.A., and Evans, P.R., Crystal structure of sub-

strate complexes of methylmalonyl-CoA mutase, Biochem. 38,

7999–8005 (1999).

Marsh, E.N.G. and Drennan, C.L.,Adenosylcobalamin-dependent

isomerases: new insights into structure and mechanism, Curr.

Opin. Chem. Biol. 5, 499–505 (2001).

Rinaldo, P., Matern, D., and Bennett, M.J., Fatty acid oxidation dis-

orders, Annu. Rev. Physiol. 64, 477–502 (2002).

Shoukry, K. and Schulz, H., Significance of the reductase-

dependent pathway for the -oxidation of unsaturated fatty

acids with odd-numbered double bonds: mitochondrial metab-

olism of 2-trans-5-cis-octadienoyl-CoA, J. Biol. Chem. 273,

6892–6899 (1998).

Sudden infant death and inherited disorders of fat oxidation,

Lancet, 1073–1075, Nov. 8, 1986.

Tong, L., Acetyl-coenzyme A carboxylase: crucial metabolic en-

zyme and attractive target for drug discovery, Cell. Mol. Life

Sci. 62, 1784–1893 (2005).

van den Bosch, H., Schutgens, R.B.H.,Wanders, R.J.A., and Tager,

J.M., Biochemistry of peroxisomes, Annu. Rev. Biochem. 61,

157–197 (1992).

Watkins, P.A., Very-long-chain acyl-CoA synthetases, J. Biol.

Chem. 283, 1773–1777 (2008).

Fatty Acid Biosynthesis

Bartlett, K. and Eaton, S., Mitochondrial -oxidation, Eur. J.

Biochem. 271, 462–469 (2004). [Discusses the reactions and en-

zymes of mitochondrial  oxidation as well as their regulation.]

Brownsey, R.W. and Denton, R.M., Acetyl-coenzyme A carboxy-

lase, in Boyer, P.D. and Krebs, E.G. (Eds.), The Enzymes (3rd

ed.),Vol. 18, pp. 123–146, Academic Press (1987).

Brownsey, R.W., Zhande, R., and Boone, A.N., Isoforms of acetyl-

CoA carboxylase: structures, regulatory properties and meta-

bolic functions, Biochem. Soc.Trans. 25, 1232–1238 (1997).

Haapalainen,A.M., Meriläinen, G., and Wierenga, R.K.,The thio-

lase superfamily: condensing enzymes with diverse reaction

specificities, Trends Biochem. Sci. 31, 64–71 (2006).

Jenni, S., Leibundgut, M., Boehringer, D., Frick, C., Mikolásek, B.,

and Ban, N., Structure of fungal fatty acid synthase and impli-

cations for iterative substrate shuttling, Science 316, 254–261

(2007).

Jump, D.B., The biochemistry of n–3 polyunsaturated fatty acids,

J. Biol. Chem. 277, 8755–8758 (2002).

Khosla, C., Tang, Y., Chen, A.Y., Schnarr, N.A., and Cane, D.E.,

Structure and mechanism of the 6-deoxyerythronolide B syn-

thase, Annu. Rev. Biochem. 76, 195–221 (2007).

Leibundgut, M., Maier,T., Jenni, S., and Ban, N., The multienzyme

architecture of eukaryotic fatty acid synthases, Curr. Opin.

Struct. Biol. 18, 714–725 (2008).

Lomakin, I.B., Xiong, Y., and Steitz, T.A., The crystal structure of

yeast fatty acid synthase, a cellular machine with eight active

sites working together, Cell 129, 319–332 (2007); and Lei-

bundgut, M., Jenni, S., Frick, C., and Ban, N., Structural basis

for substrate delivery by acyl carrier protein in the yeast fatty

acid synthase, Science 316, 288–290 (2007).

Los, D.A. and Murata, N., Structure and expression of fatty acid

desaturases, Biochim. Biophys. Acta 1394, 3–15 (1998).

Maier, T., Leibundgut, M., and Ban, N., The crystal structure of a

mammalian fatty acid synthase, Science 321, 1315–1322 (2008).

Smith, S. and Tsai, S.-C., The type I fatty acid and polyketide syn-

thetases: a tale of two megasynthases, Nat. Prod. Rep. 24,

1041–1072 (2007).

Wakil, S.J., Fatty acid synthase, a proficient multifunctional en-

zyme, Biochemistry 28, 4523–4530 (1989).

Walsh, C.T., Polyketide and nonribosomal peptide antibiotics:

Modularity and versatility, Science 303, 1805–1810 (2004).

White, S.W., Zheng, J., Zhang, Y.-M., and Rock, C.O., The struc-

tural biology of type II fatty acid biosynthesis, Annu. Rev.

Biochem. 74, 791–831 (2005).

Regulation of Fatty Acid Metabolism

Eaton, S., Control of mitochondrial -oxidation flux, Prog. Lipid

Res. 41, 197–239 (2002).

Hardie, D.G. and Carling, D., The AMP-activated protein kinase:

fuel gauge of the mammalian cell? Eur. J. Biochem. 246,

259–273 (1997).

Hardie, D.G., Carling, D., and Carlson, M., The AMP-

activated/SNF1 protein kinase subfamily: metabolic sensors of

the eukaryotic cell? Annu. Rev. Biochem. 67, 821–855 (1998).

Munday M.R. and Hemingway C.J.,The regulation of acetyl-CoA

carboxylase—A potential target for the action of hypolipidemic

agents, Adv. Enzym. Regul. 39, 205–234 (1999).

Witters, L.A., Watts, T.D., Daniels, D.L., and Evans, J.L., Insulin

stimulates the dephosphorylation and activation of acetyl-

CoA carboxylase, Proc. Natl. Acad. Sci. 85, 5473–5477 (1988).

Cholesterol Metabolism

Bloch, K., The biological synthesis of cholesterol, Science 150,

19–28 (1965).

Chang,T.Y., Chang, C.C.Y., and Cheng,D.,Acyl-coenzyme A:choles-

terol acyltransferase, Annu. Rev. Biochem. 66, 613–638 (1997).

Durbecq, V., et al., Crystal structure of isopentenyl diphosphate:

dimethylallyl diphosphate isomerase, EMBO J. 20, 1530–1537

(2001).

Edwards, P.A., Sterols and isoprenoids: signaling molecules de-

rived from the cholesterol biosynthetic pathway, Annu. Rev.

Biochem. 68, 157–185 (1999).

Goldstein, J.L. and Brown, M.S., Regulation of the mevalonate

pathway, Nature 343, 425–430 (1990).

Goldstein, J.L., DeBose-Boyd, R.A., and Brown, M.S., Protein

sensors for membrane sterols, Cell 124, 35–46 (2006).

Goldstein, J.L., Rawson, R.B., and Brown, M.S., Mutant mam-

malian cells as tools to delineate the sterol regulatory element

binding protein pathway for feedback regulation of lipid syn-

thesis, Arch. Biochem. Biophys. 397, 139–148 (2002).

Ikonen, E., Cellular cholesterol trafficking and compartmentaliza-

tion, Nature Rev. Mol. Cell Biol. 9, 125–138 (2008).

Istvan, E.S., Bacterial and mammalian HMG-CoA reductases: re-

lated enzymes with distinct architectures, Curr. Opin. Struct.

Biol. 11, 746–751 (2001).

Istvan, E.S. and Deisenhofer, J., Structural mechanism for statin

inhibition of HMG-CoA reductase, Science 292, 1160–1164

(2001).

Knopp, R.H., Drug therapy: drug treatment of lipid disorders,

New Engl. J. Med. 341, 498–511 (1999).

Meyer, M.M., Segura, M.J.R., Wilson, W.K., and Matsuda, S.P.T.,

Oxidosqualene cyclase residues that promote formation of cy-

cloartenol, lanosterol and parkeol, Angew. Chem. Int. Ed. 39,

4090–4092 (2000).

1016 Chapter 25. Lipid Metabolism

JWCL281_c25_940-1018.qxd 4/20/10 1:59 PM Page 1016

Reinert, D.J., Balliano, G., and Schulz, G.E., Conversion of squa-

lene to the pentacarbocyclic hopene, Chem. Biol. 11, 121–126

(2004). [The X-ray structure of squalene-hopene cyclase in

complex with 2-azasqualene.]

Russell, D.W. and Setchell, K.D.R., Bile acid biosynthesis, Bio-

chemistry 31, 4737–4749 (1992).

Thoma, R., Schulz-Gasch, T., D’Arcy, B., Benz, J., Aebl, J.,

Dehmiow, H., Hennig, M., Stihle, M., and Ruf, A., Insight into

steroid scaffold formation from the structure of human oxi-

dosqualene cyclase, Nature 432, 118–122 (2004).

Wang, K.C. and Ohnuma, S.-I., Isoprenyl diphosphate synthases,

Biochim. Biophys.Acta 1529, 33–48 (2000).

Wendt, K.U., Schulz, G.E., Corey, E.J. and Liu, D.R., Enzyme

mechanisms for polycyclic triterpene formation,Angew. Chem.

Int. Ed. 39, 2812–2833 (2000).

Yokode, M., Hammer, R.E., Ishibashi, S., Brown, M.S., and Gold-

stein, J.L., Diet-induced hypercholesterolemia in mice: Preven-

tion by overexpression of LDL receptors, Science 250,

1273–1275 (1990).

Eicosanoid Metabolism

Abramovitz, M.,Wong,E.,Cox,M.E.,Richardson,C.D., Li, C.,and

Vickers, P.J., 5-Lipoxygenase-activating protein stimulates the

utilization of arachidonic acid by 5-lipoxygenase, Eur. J

Biochem. 215, 105–111 (1993).

Chandrasekharan, N.V., Dai, H., Roos, K.L.T., Evanson, N.K.,

Tomsik, J., Elton, T.S., and Simmons, D.L., COX-3, a

cyclooxygenase-1 variant inhibited by acetaminophen and

other analgesic/antipyretic drugs: Cloning, structure, and

expression, Proc. Natl.Acad. Sci. 99, 13926–13931 (2002).

Ferguson, A.D., et al., Crystal structure of inhibitor-bound human

5-lipoxygenase–activating protein, Science 317, 510–512

(2007).

Ford-Huchinson,A.W., Gresser, M., and Young, R.N., 5-Lipoxyge-

nase, Annu. Rev. Biochem. 63, 383–417 (1994).

Gillmor, S.A.,Villaseñor,A., Fletterick, R.,Sigal, E., and Browner,

M.F., The structure of mammalian 15-lipoxygenase reveals

similarity to the lipases and the determinants of substrate

specificity, Nature Struct. Biol. 4, 1003–1009 (1997).

Kurumbail, R.G., Kiefer, J.R., and Marnett, L.J., Cyclooxygenase

enzymes: catalysis and inhibition, Curr. Opin. Struct. Biol. 11,

752–760 (2001).

Phillipson, B.E., Rothrock, D.W., Conner, W.E., Harris, W.S., and

Illingworth, D.R., Reduction of plasma lipids, lipoproteins and

apoproteins by dietary fish oils in patients with hypertriglyc-

eridemia, New Engl. J. Med. 312, 1210–1216 (1985).

Picot, D., Loll, P.J., and Garavito,R.M.,The X-ray crystal structure

of the membrane protein prostaglandin H

synthase-1, Nature

367, 243–249 (1994).

Rådmark, O., Werz, O., Steinhilber, D., and Samuelsson, B.,

5-Lipoxygenase: regulation of expression and enzyme activity,

Trends Biochem. Sci. 32, 332–341 (2007).

Samuelsson, B. and Funk, C.D., Enzymes involved in the biosynthe-

sis of leukotriene B4, J. Biol. Chem. 264, 19469–19472 (1989).

Schwarz, K., Walther, M., Anton, M., Gerth, C., Feussner, I., and

Kuhn, H., Structural basis for lipoxygenase specificity: conver-

sion of the human leukocyte 5-lipoxygenase to a 15-

lipoxygenating enzyme species by site-directed mutagenesis,

J. Biol. Chem. 276, 773–339 (2001).

Serhan, C.N., Lipoxins and novel aspirin-triggered 15-epi-lipoxins

(ATL): a jungle of cell-cell interactions or a therapeutic oppor-

tunity? Prostaglandins 53, 107–137 (1997).

Smith,W.L., Nutritionally essential fatty acids and biologically in-

dispensible cyclooxygenases, Trends Biochem. Sci. 33, 27–37

(2003).

Smith, W.L., DeWitt, D.L., and Garavito, R.M., Cyclooxygenases:

structural, cellular and molecular biology, Annu. Rev. Biochem.

69, 145–182 (2000).

Turini, M.E. and DuBois, R.N., Cyclooxygenase-2: a therapeutic

target, Annu. Rev. Med. 53, 35–57 (2002).

Warner,T.D. and Mitchell, J.A., Cyclooxygenases: new forms, new

inhibitors, and lessons from the clinic, FASEB J. 18, 790–804

(2004).

Phospholipid and Glycolipid Metabolism

Conzelmann, E. and Sandhoff, K., Glycolipid and glycoprotein

degradation, Adv. Enzymol. 60, 89–216 (1987).

Dowhan, W., Molecular basis for membrane diversity: Why are

there so many lipids? Annu. Rev. Biochem. 66, 199–232 (1997).

Kent, C., Eukaryotic phospholipid synthesis, Annu. Rev. Biochem.

64, 315–342 (1995).

Kolter, T. and Sandhoff, K., Sphingolipids—their metabolic path-

ways and the pathobiochemistry of neurodegenerative dis-

eases, Angew. Chem. Int. Ed. 38, 1532–1568 (1999).

Neufield, E.F., Natural history and inherited disorders of a lysoso-

mal enzyme, -hexosaminidase, J. Biol. Chem. 264, 10927–10930

(1989).

Prescott, S.M., Zimmerman, G.A., and McIntire, T.M., Platelet-

activating factor, Biol. Chem. 265, 17381–17384 (1990).

Tifft, C.J. and Proila, R.L., Stemming the tide: glycosphingloipid

synthesis inhibitors as therapy for storage diseases, Glycobiol-

ogy 10, 1249–1258 (2000).

van Echten, G. and Sandhoff, K., Ganglioside metabolism, J. Biol.

Chem. 268, 5341–5344 (1993).

Problems 1017

1. The venoms of many poisonous snakes, including rat-

tlesnakes, contain a phospholipase A

that causes tissue damage

that is seemingly far out of proportion to the small amount of en-

zyme injected. Explain.

2. Explain why individuals with a hereditary deficiency of car-

nitine palmitoyltransferase II have muscle weakness. Why are

these symptoms more severe during fasting?

3. Why are the livers of Jamaican vomiting sickness victims

usually depleted of glycogen?

4. Compare the metabolic efficiencies, in moles of ATP pro-

duced per gram, of completely oxidized fat (tripalmitoyl glycerol)

versus glucose derived from glycogen.Assume that the fat is anhy-

drous and the glycogen is stored with twice its weight of water.

5. Methylmalonyl-CoA mutase is incubated with deuterated

methylmalonyl-CoA. The coenzyme B

extracted from this mu-

tase is found to contain deuterium at its 5¿-methylene group.

Account for the transfer of label from substrate to coenzyme.

6. What is the energetic price, in ATP units, of converting

acetoacetyl-CoA to acetoacetate and then resynthesizing

acetoacetyl-CoA?

PROBLEMS

JWCL281_c25_940-1018.qxd 4/20/10 1:59 PM Page 1017

7. A fasting animal is fed palmitic acid that has a

C-labeled

carboxyl group. (a) After allowing sufficient time for fatty acid

breakdown and resynthesis, what would be the

C-labeling pat-

tern in the animal’s palmitic acid residues? (b) The animal’s liver

glycogen becomes

C labeled although there is no net increase in

the amount of this substance present. Indicate the sequence of re-

actions whereby the glycogen becomes labeled. Why is there no

net glycogen synthesis?

8. What is the ATP yield from the complete oxidation of a

molecule of (a) -linolenic acid (9,12,15-octadecatrienoic acid,

18:3n–3) and (b) margaric acid (heptadecanoic acid, 17:0)? Which

has the greater amount of available biological energy on a per car-

bon basis?

*9. The role of coenzyme B

in mediating hydrogen transfer

was established using the coenzyme B

-dependent bacterial en-

zyme dioldehydrase, which catalyzes the reaction:

The enzyme converts [1-

]1,2-propanediol to [1,2-

H]propi-

onaldehyde with the incorporation of tritium into both C5¿ posi-

tions of 5¿-deoxyadenosylcobalamin’s 5¿-deoxyadenosyl residue.

Suggest the mechanism of this reaction. What would be the prod-

ucts of the dioldehydrase reaction if the enzyme was supplied with

[5¿-

H]deoxyadenosylcobalamin and unlabeled 1,2-propanediol?

Propionaldehyde

1,2-Propanediol

10. Why is it important that liver cells lack 3-ketoacyl-CoA

transferase (Fig. 25-27)?

11. What is the energetic price, in ATP equivalents, of break-

ing down palmitic acid to acetyl-CoA and then resynthesizing it?

12. Is the fatty acid shown below likely to be synthesized in

animals? Explain.

13. What is the energetic price, in ATP equivalents, of synthe-

sizing cholesterol from acetyl-CoA?

14. What would be the

C-labeling pattern in cholesterol if it

were synthesized from HMG-CoA that was

C labeled (a) at C5,

its carboxyl carbon atom, or (b) C1, its thioester carbon atom?

15. Hypercholesterolemic individuals taking statins are some-

times advised to take supplements of coenzyme Q. Explain.

*16. A child suffering from severe abdominal pain is admitted

to the hospital several hours after eating a meal consisting of ham-

burgers, fried potatoes, and ice cream. Her blood has the appear-

ance of “creamed tomato soup” and on analysis is found to con-

tain massive quantities of chylomicrons. As the attending

physician, what is your diagnosis of the patient’s difficulty (the

cause of the abdominal pain is unclear)? What treatment would

you prescribe to alleviate the symptoms of this inherited disease?

17. Although linoleic acid is an essential fatty acid in animals,

it is not required by animal cells in tissue culture. Explain.

18. The inactivation of the peroxidase function of

prostaglandin H synthase (PGHS) also inactivates its cyclooxyge-

nase function but not vice versa. Explain.

CCH CH

–

(

( ))

1018 Chapter 25. Lipid Metabolism

JWCL281_c25_940-1018.qxd 4/20/10 1:59 PM Page 1018

1019

CHAPTER 26

Amino Acid

Metabolism

1 Amino Acid Deamination

A. Transamination

B. Oxidative Deamination: Glutamate Dehydrogenase

C. Other Deamination Mechanisms

2 The Urea Cycle

A. Carbamoyl Phosphate Synthetase: Acquisition of the First

Urea Nitrogen Atom

B. Ornithine Transcarbamoylase

C. Argininosuccinate Synthetase: Acquisition of the Second

Urea Nitrogen Atom

D. Argininosuccinase

E. Arginase

F. Regulation of the Urea Cycle

3 Metabolic Breakdown of Individual Amino Acids

A. Amino Acids Can Be Glucogenic, Ketogenic, or Both

B. Alanine, Cysteine, Glycine, Serine, and Threonine Are

Degraded to Pyruvate

C. Asparagine and Aspartate Are Degraded to Oxaloacetate

D. Arginine, Glutamate, Glutamine, Histidine, and Proline Are

Degraded to ␣-Ketoglutarate

E. Isoleucine, Methionine, and Valine Are Degraded to

Succinyl-CoA

F. Leucine and Lysine Are Degraded to Acetoacetate and/or

Acetyl-CoA

G. Tryptophan Is Degraded to Alanine and Acetoacetate

H. Phenylalanine and Tyrosine Are Degraded to Fumarate

and Acetoacetate

4 Amino Acids as Biosynthetic Precursors

A. Heme Biosynthesis and Degradation

B. Biosynthesis of Physiologically Active Amines

C. Glutathione

D. Tetrahydrofolate Cofactors: The Metabolism of C

Units

5 Amino Acid Biosynthesis

A. Biosynthesis of the Nonessential Amino Acids

B. Biosynthesis of the Essential Amino Acids

6 Nitrogen Fixation

␣-Amino acids, in addition to their role as protein mono-

meric units, are energy metabolites and precursors of many

biologically important nitrogen-containing compounds,

notably heme, physiologically active amines, glutathione,

nucleotides, and nucleotide coenzymes. Amino acids are

classified into two groups: essential and nonessential.

Mammals synthesize the nonessential amino acids from

metabolic precursors but must obtain the essential amino

acids from their diet. Excess dietary amino acids are nei-

ther stored for future use nor excreted. Rather, they are

converted to common metabolic intermediates such as

pyruvate, oxaloacetate, acetyl-CoA, and ␣-keto-glutarate.

Consequently, amino acids are also precursors of glucose,

fatty acids, and ketone bodies and are therefore metabolic

fuels.

In this chapter, we consider the pathways of amino acid

breakdown, synthesis, and utilization.We begin by examin-

ing the three common stages of amino acid breakdown:

1. Deamination (amino group removal),whereby amino

groups are converted either to ammonia or to the amino

group of aspartate.

2. Incorporation of ammonia and aspartate nitrogen

atoms into urea for excretion.

3. Conversion of amino acid carbon skeletons (the ␣-keto

acids produced by deamination) to common metabolic

intermediates.

Many of these reactions are similar to those we have con-

sidered in other pathways. Others employ enzyme cofac-

tors we have not previously encountered. One of our goals

in studying amino acid metabolism is to understand the

mechanisms of action of these cofactors.

After our discussion of amino acid breakdown, we ex-

amine the pathways by which amino acids are utilized in

the biosynthesis of heme, physiologically active amines,

and glutathione (the synthesis of nucleotides and nu-

cleotide coenzymes is the subject of Chapter 28). Next, we

study amino acid biosynthesis pathways. The chapter ends

with a discussion of nitrogen fixation, a process that con-

verts atmospheric N

to ammonia and is therefore the ulti-

mate biological source of metabolically useful nitrogen.

1 AMINO ACID DEAMINATION

The first reaction in the breakdown of an amino acid is

almost always removal of its ␣-amino group with the object

of excreting excess nitrogen and degrading the remaining

carbon skeleton or converting it to glucose. Urea, the pre-

dominant nitrogen excretion product in terrestrial mam-

mals, is synthesized from ammonia and aspartate. Both of

these latter substances are derived mainly from glutamate,

a product of most deamination reactions. In this section we

JWCL281_c26_1019-1087.qxd 4/20/10 9:26 AM Page 1019

examine the routes by which ␣-amino groups are incorpo-

rated into glutamate and then into aspartate and ammonia.

In Section 26-2, we discuss urea biosynthesis from these

precursors.

Most amino acids are deaminated by transamination,

the transfer of their amino group to an ␣-keto acid to yield

the ␣-keto acid of the original amino acid and a new amino

acid, in reactions catalyzed by aminotransferases (alterna-

tively, transaminases). The predominant amino group ac-

ceptor is ␣-ketoglutarate, producing glutamate as the new

amino acid:

Glutamate’s amino group, in turn, is transferred to ox-

aloacetate in a second transamination reaction, yielding

aspartate:

Transamination, of course, does not result in any net

deamination. Deamination occurs largely through the

oxidative deamination of glutamate by glutamate dehydro-

genase (GDH), yielding ammonia. The reaction requires

NAD

⫹

or NADP

⫹

as an oxidizing agent and regenerates ␣-

ketoglutarate for use in additional transamination reactions:

␣-ketoglutarate ⫹ NH

⫹

⫹ NAD(P)H

Glutamate ⫹ NAD(P)

⫹

⫹ H

O Δ

␣-ketoglutarate ⫹ aspartate

Glutamate ⫹ oxaloacetate Δ

␣-keto acid ⫹ glutamate

Amino acid ⫹␣-ketoglutarate Δ

1020 Chapter 26. Amino Acid Metabolism

The mechanisms of transamination and oxidative deami-

nation are the subjects of this section. We also consider

other means of amino group removal from specific amino

acids.

A. Transamination

a. Aminotransferase Reactions Occur in Two Stages

1. The amino group of an amino acid is transferred to

the enzyme, producing the corresponding keto acid and the

aminated enzyme.

2. The amino group is transferred to the keto acid

acceptor (e.g., ␣-ketoglutarate), forming the amino acid

product (e.g., glutamate) and regenerating the enzyme.

To carry the amino group, aminotransferases require

participation of an aldehyde-containing coenzyme, pyri-

doxal-5¿-phosphate (PLP), a derivative of pyridoxine

(vitamin B

; Fig. 26-1a,b). The amino group is accommo-

dated by conversion of this coenzyme to pyridoxamine-5¿-

phosphate (PMP; Fig. 26-1c). PLP is covalently attached to

the enzyme via a Schiff base (imine) linkage formed by the

enzyme ⫹ glutamate

␣-Ketoglutarate ⫹ enzyme¬NH

␣-keto acid ⫹ enzyme¬NH

Amino acid ⫹ enzyme Δ

5⬘

4⬘

2–

Pyridoxal-5ⴕ-

phosphate (PLP)

⌯ ⌷

(a)

2–

Pyridoxamine-5ⴕ-

phosphate (PMP)

(d)

2–

Enzyme–PLP

Schiff base

⌯

(c)

Pyridoxine

(vitamin B

)

⌷⌯

(b)

–

(CH

)

Enzyme

...

Figure 26-1 Forms of pyridoxal-5ⴕ-phosphate.

(a) Pyridoxine (vitamin B

). (b) Pyridoxal-5¿-phosphate (PLP).

forms between PLP and an enzyme ε-amino group.

JWCL281_c26_1019-1087.qxd 4/20/10 9:26 AM Page 1020