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molecule will not migrate (Figure 2). The amino acid

is said to have reached its isoelectric or isoionic point

(pI). (See pH – Principles and Measurement.)

Isomerism

0003 With the obvious exception of glycine, all a-amino

acids contain at least one asymmetric carbon atom,

i.e., their a-carbon atoms are indeed bonded to four

different substituent groups. Most amino acids there-

fore exist in two different spatial arrangements

or isomers, whereas isoleucine, threonine, hydroxy-

lysine, and hydroxyproline each have a second

asymmetric carbon and thus four stereoisomers.

0004 Standard nomenclature of amino acids is preceded

by either a d or an l, indicating whether a structure

corresponds to either that of d(þ)-glyceraldehyde or

l()-glyceraldehyde. This designation is the ‘absolute

configuration’ of an amino acid. The relationship

between the d and l isomers of glyceraldehyde and

alanine can be seen in Figures 3 and 4, respectively.

0005 Since stereoisomers are mirror images of each other

and exhibit characteristic angles of rotation for polar-

ized light, nomenclature also includes either a (þ)or

() (formerly d or l) to indicate that a pure solution

of the amino acid would rotate a beam of polarized

light clockwise (dextrorotation) or counterclockwise

(levorotation), respectively.

Amino Acids as Protein Constituents

0006 Over 95% of the amino acids in a food will be

encountered in the form of proteins. Proteins are

unbranched chains (polypeptides) of l-a-amino (or

a-imino) acids linked together by peptide (amide)

bonds. Combinations of about 20 amino acids com-

monly make up the structures of polypeptides. Differ-

ent sequences made with the same amino acids will

spell out different proteins, each sequence corres-

ponding to a different primary structure. The remain-

der will be found as peptides and free amino acids.

(See Protein: Chemistry.)

0007 Interestingly, only the l isomers of a-amino acids

have been found in all proteins. Although d-amino

acids are not involved in the metabolism of higher

organisms, they can be structural components of cell

walls in certain bacteria and part of some peptide

antibiotics. l-Amino acids will slowly convert to

racemic mixtures of d- and l-amino acids with time

or as a result of alkali exposure or considerable heat

treatment.

0008A peptide bond results from the condensation of

the a-carboxylic group of one amino acid and the a-

amino group of another. Figure 5 shows a dipeptide

linkage between two alanine residues.

0009The peptide bond can be visualized as a plane, in

the middle of which lies the amide (C-N) linkage

(Figure 6). Two opposite corners of the plane are

occupied by the two a-carbons, and each of the

remaining two is occupied by an oxygen and a hydro-

gen atom. The plane is considered a rather rigid struc-

ture (E

rotation

¼ 21 kcal mol

1

) due to the tendency of

α-Carbon

α-Carboxylic

acid group

α-Amino group

COO

−

fig0002 Figure 2 Alanine – a typical a-amino acid.

HO O

Mirror

HCCC

D-Glyceraldehyde L-Glyceraldehyde

fig0003Figure 3 The stereoisomers of glyceraldehyde.

CCH

−

D-Alanine L-Alanine

Mirror

fig0004Figure 4 The stereoisomers of alanine.

184 AMINO ACIDS/Properties and Occurrence

the electron clouds of the system to remain orbitting,

each on a separate side of the plane. Once a free

amino acid has been incorporated into a polypeptide

or protein, all physical, physicochemical, and chem-

ical properties attributed to the free form are altered.

0010 The peptide bond exists in nature in either the trans

or cis configuration. The cis configuration is less

common and results from the presence of proline

residues (Figure 7).

Imino Acids Found in Proteins

0011 Proteins often contain small amounts of the imino

acid l-proline and, more rarely, l-4-hydroxyproline

(Hypro). Unlike a-amino acids (with primary

amines), the imino acids cannot keep their side chains

as far away as possible from those of the neighboring

residues, as the trans configuration would require.

Therefore, the mere presence of a proline (or a

hydroxyproline) residue in the sequence introduces a

‘kink’ or bend in the polypeptide structure (Figure 8).

0012If the imino acid content of a polypeptide is high

(3–10%), then the peptide is likely to fold randomly.

If the imino acid residues are repeated regularly along

the polypeptide (as is true of collagen, *23% Pro þ

Hypro), then the protein is able to fold into a regular

structure.

The Classification of Amino Acids

0013Amino acids can be usefully classified in several ways,

depending on their properties. Two common classifi-

cations are by their nutritional essentiality to animals

and by the polarity of the R-groups at neutral pH

(Tables 2 and 3, respectively). As most proteins and

amino acids function in aqueous environments at

physiological pH, the second criterion is most helpful

when considering the function of an amino acid in a

protein in vivo or in most food systems. Hydrophobic

R-groups tend to ‘hide’ themselves in the interior

of folded proteins. Table 1 highlights the structures

of the 20 common protein amino acids classified

according to their polarity in neutral solutions.

0014A nutritionally essential amino acid is one that

cannot be manufactured, either at all or in sufficient

quantities for optimal growth by the body, and

−

CCNH

OCH

−

OCH

Alanine Alanine

Alanylalaline

fig0005 Figure 5 The formation of the peptide bond between two

molecules of alanine.

−

fig0006 Figure 6 The two structures contributing to the resonance

hybrid structure of the peptide bond.

cis trans

fig0007 Figure 7 The cis and trans configurations of the peptide bond.

α-Carbon

COOH

α-Carboxylic acid group

α-Amino group

fig0008Figure 8 Structure of proline. Note that the a-carbon and

‘amino’ nitrogen are involved in the side chain, and the amine

is thus secondary.

tbl0002Table 2 The nonessential (dispensible) and essential

(indispensible) amino acids for humans

Nonessential Essential

Alanine Arginine

Asparagine Histidine

Aspartate Isoleucine

Cysteine

Leucine

Glutamate Lysine

Glutamine Methionine

Glycine Phenylalanine

Proline Threonine

Serine Tryptophan

Tyrosine

Valine

Essential to the young. Not limiting in most foods.

b, c

Due to one-way interconversions, nutritional evaluation takes into

account the pairs.

AMINO ACIDS/Properties and Occurrence 185

therefore has to be present in the diet. These amino

acids are sometimes termed collectively as ‘indispen-

sible.’ In contrast, nonessential amino acids (indis-

pensible nitrogen) can thus be interconverted in

order for the body to meet its needs.

0015 Although the nutritive or biological value of a pro-

tein can be measured by its contents of essential

amino acids, nonessential amino acids should be

regarded as important, as maximal protein efficiency

will only be attained if an adequate supply of non-

specific nitrogen is guaranteed in the diet. Besides

serving as protein constituents, surplus essential

amino acids are used as precursors of other vital

metabolites and occasionally as energy. Excess

of nonspecific nitrogen, on the other hand, will

preferentially be used as energy. (See Amino Acids:

Metabolism.)

Physical, Chemical, and Sensory Properties of

Amino Acids Found in Proteins

0016 All amino acids in the pure state are crystalline solids

with physical constants much too high for either

organic amines or acids of similar structures and

molecular weights. They are not volatile, melt at

high temperatures, are insoluble in organic solvents,

and have low pK values. Their molecular weights

range from 75 to 204; average molecular weight is

estimated at 110. Therefore, if a protein has a mo-

lecular weight of 33 000, one could estimate it con-

tains approximately 300 residues.

Amino Acids Commonly Found in Proteins

0017Although the following listing refers to those amino

acids actually found in proteins, it should be remem-

bered that they also exist in the free form in both

vegetable and animal tissues.

0018Glycine (mol. wt 75.1) The R-group is a single

hydrogen atom, making it the smallest amino acid.

Because it is the smallest residue, glycine enables a

protein to fold into a compact structure. Collagen can

fold into a tight triple helix because every third amino

acid is glycine. (The uniform sequence of collagen is

abbreviated to [-Gly-Pro-X-]

, where X is often

hydroxyproline.) The hydrogen atoms attached to

the a-carbon atom have too small an influence on

the ionization and high degree of polarity of the a-

amino/a-carboxylic system. Free glycine is respon-

sible for the sweet taste of shrimp. Its name derives

from the same Greek word as glucose (glycos).

0019Alanine (mol. wt 89.1) This is the next simplest

amino acid. The side chain of alanine is a methyl

group (Figure 2) and performs well in both hydro-

phobic and hydrophilic environments. Like glycine,

this amino acid is found in high concentrations in

tightly folded proteins such as keratin and collagen.

0020Serine (mol. wt 105.1) A hydroxyl group at the

end of the side chain makes serine more polar than

alanine, yet uncharged at pH 7. Like the other polar,

tbl0003 Table 3 Genetic code and some physicochemical properties of the common protein amino acids while in the free form

Amino acid Genetic code Solubility (gl

1

)

(a-COO



)PK

(a-NH

)PK

Alanine GN(N) 167.2 2.34 9.69 6.00

Arginine AGA/AGG/CG(N) 855.6 2.17 9.04 12.48 10.76

Asparagine AAU/AAC 28.5 2.02 8.80 5.41

Aspartic acid GAU/GAC 5.0 1.88 9.60 3.65 2.98

Cysteine UGU/UGC 1.96 10.28 8.18 5.07

Glutamic acid GAA/GAG 8.5 2.19 9.67 4.25 3.22

Glutamine CAA/CAG 7.2

2.17 9.13 5.65

Glycine GG(N) 249.9 2.34 9.60 5.97

Histidine CAU/CAC 1.82 9.17 6.00 7.59

Isoleucine AUU/AUC/AUA 34.5 2.36 9.68 6.02

Leucine UUA/UUG/CU(N) 21.7 2.30 9.60 5.98

Lysine AAA/AAG 739.0 2.18 8.95 10.53 9.74

Methionine AUG 56.2 2.28 9.21 5.74

Phenylalanine UUU/UUC 27.6 1.83 9.13 5.48

Proline CC(N) 1620.0 1.94 10.60 6.30

Serine AGU/AGC 422.0 2.20 9.15 5.68

Threonine AC(N) 13.2 2.21 9.15 5.68

Tryptophan AGG 13.6 2.38 9.39 5.89

Tyrosine AUA/UAC 0.4 2.20 9.11 5.66

Valine GU(N) 58.1 2.32 9.62 5.96

Solubility in water at 25



Side chain ionizing groups.

Solubility at 37



186 AMINO ACIDS/Properties and Occurrence

uncharged amino acids (glycine, threonine, cysteine,

tyrosine, asparagine, and glutamine), serine forms

hydrogen bonds with water; with sugars it forms

glycosidic bonds. Serine is essential for the active

sites of enzymes, as in the serine-proteases trypsin

and chymotrypsin. (See Enzymes: Functions and

Characteristics.)

0021 Cystine (mol. wt 240.2) and cysteine (mol. wt

121.1) These amino acids have R-groups that con-

tain sulfur either oxidized as a disulfide or reduced as

a thiol. Cystine is two molecules of cysteine linked

through a disulfide bond (Figure 9). When incorpor-

ated into proteins the disulfide bond between cysteine

residues effectively ‘pins’ the protein structure to-

gether. The fact that most natural proteins contain

disulfide bonds may be a natural mechanism to in-

crease the protein’s structure stability under unfavor-

able conditions and magnify its biochemical function.

Intracellular fluids are mildly reducing, favoring

cysteine, while the extracellular environment is

sufficiently oxidizing to favor cystine. The thiol of

cysteine is essential for the catalytic activity of some

enzymes, such as papain, and is a prime target for

enzyme inactivation by heavy metals and alkylating

reagents. Free cystine has low solubility.

0022 Aspartic acid (mol. wt 133.1) This amino acid is

normally abbreviated to ‘Asp’ but if, when it is deter-

mined, no distinction is made between aspartic acid

and its amide (Asn), it is abbreviated to ‘Asx.’ Aspar-

tate has an acidic, negatively charged R-group at

pH 7. It is electrically neutral at pH 2.98.

0023 Asparagine (mol. wt 132.1) Asparagine is the amide

of aspartate. This was the first amino acid to be

isolated (1806), from asparagus, the plant from

which it derives its name.

0024 Valine (mol. wt 117.1) Valine has a bulky, very

hydrophobic side chain. It is an essential amino acid

for humans (average adults require 0.7 g day

1

) and

can be metabolized in the muscle. Valine, leucine, and

isoleucine are the branched-chain amino acids.

0025 Leucine (mol. wt 131.2) The side chain of leucine

is aliphatic, bulky, and very hydrophobic. It is

essential for humans (average adults require 0.98 g

day

1

) and can be metabolized in the muscle.

Leucine, isoleucine, and valine are the branched-

chain amino acids.

0026Isoleucine (mol. wt 131.2) This amino acid is also

highly hydrophobic. It has two asymmetric carbon

atoms and therefore can exist as any of four isomers.

Only one isomer exists in proteins. It is nutritionally

essential for humans (average adults require 0.7 g

day

1

) and can be metabolized by the muscle. Iso-

leucine, valine, and leucine are the branched-chain

amino acids.

0027Methionine (mol. wt 149.2) Like cysteine, methio-

nine is a sulfur-containing amino acid. The sulfur in

methionine can be oxidized, but is unable to form

disulfide bonds. The R-group renders methionine

quite hydrophobic. In prokaryotic cells a methionine

residue is essential for the initiation of protein synthe-

sis. It is nutritionally essential for humans (average

adults require Met þCys 0.91 g day

1

) and is limiting

in most dietary proteins. It becomes toxic when

ingested in excess of 4% of the protein.

0028Threonine (mol. wt 119.1) This amino acid has a

hydroxyl group as part of the side chain, making it

polar, yet uncharged at physiological pH. It has two

asymmetric carbon atoms and therefore can exist in

any of four isomers. Only one form exists in proteins.

This was the last of the common amino acids to be

identified (1938). It is an essential amino acid for

humans (an average person requires 0.49 g day

1

0029Glutamine (mol. wt 146.1) The abbreviation for

this amino acid is ‘Gln’ (or ‘Q’), but ‘Glx’ (or ‘Z’)is

used when no distinction can be made between glu-

tamic acid and the corresponding amide glutamine.

Glutamine has a polar but uncharged R-group. It is

found in large amounts in the gliadin and glutenin

fractions of wheat.

0030Glutamic acid (mol. wt 147.1) This amino acid has

a g-carboxylate group in the side chain. It is therefore

found on the surface of proteins. It was discovered in

1908 in edible seaweed, when it was recognized as a

flavor potentiator or enhancer. The salt monosodium

glutamate (MSG) has a sweet and salty taste at very

low concentrations. MSG increases sensitivity to sour

and bitter tastes. Other amino acids, such as ibotenic

acid and tricholomic acid, have been reported to have

similar effects.

0031Lysine (mol. wt 146.2) A positively charged side

chain with a second (e-)amino group that can form

COO

−

COO

−

CCHHSSCH

fig0009 Figure 9 Disulfide bond in a cystine molecule linking two

molecules of cysteine.

AMINO ACIDS/Properties and Occurrence 187

salt-bridges or react with other residues to form

covalent cross-links is found in lysine. These cross-

links are important in skin, where they are involved in

aging effects. Lysine is known to react with sugars,

giving rise to different flavor and color compounds

(Maillard reaction). Lysine is an essential amino acid

for humans. Cereal proteins are typically low in

lysine, while legume proteins are high. The average

person requires 0.84 g day

1

.(See Browning: Non-

enzymatic.)

0032 Arginine (mol. wt 174.2) This amino acid has a

positively charged guanidinium group on the end of

the side chain, being a much stronger base than the

e-ammonium group of lysine. Consequently, arginine

is 100% protonated with a net charge of þ1at

physiological pH. Ammonia intoxication results in

children (and cats) if this amino acid is absent from

a fasting diet. It is present in high amounts in imma-

ture seeds like groundnuts.

0033 Histidine (mol. wt 155.2) A weakly protonated imi-

dazole group is present in the side chain of this amino

acid. It is often found in the active sites of enzymes,

where it is essential for the catalytic activity. This

residue is a primary target for enzyme inactivation

by heavy metals and alkylating reagents. Histidine is

the only amino acid with significant buffering power

near the pH of intracellular fluids and blood. Hemo-

globin has a uniquely high content of histidine in

order to counteract the effects of excess carbon

dioxide. Recommended intake for adults varies from

0.56 to 0.84 g day

1

0034 Phenylalanine (mol. wt 165.2) This amino acid has

an aromatic, nonpolar hydrophobic R-group that

makes it ideal for structuring the core of proteins.

When present on the surface, it can have a role in

substrate recognition or hydrophobic binding of

other molecules. Phenylketonuria is a disabling dis-

ease that is estimated to occur in 1% of mentally

handicapped patients. It is caused by a failure of the

liver to metabolize phenylalanine and sufferers must

avoid eating foods containing phenylalanine. As nat-

ural proteins devoid of phenylalanine are almost non-

existent, feeding phenylketonurics is a technological

challenge. Average adults require (Phe þ Tyr) 0.98 g

day

1

0035 Tryptophan (mol. wt 204.2) An indole, nonpolar

hydrophobic R-group makes this amino acid essential

for humans (average adults require 0.25 g day

1

Plant proteins are probably poor in tryptophan due

to diversion of the indole group to auxin biosynthesis.

Pellagra is now an uncommon disease but may occur

in persons subsisting on vegetable proteins that are

poor in tryptophan. Symptoms include skin rashes in

response to sunlight, diarrhea, severe nervous depres-

sion, and partial paralysis. The administration of

tryptophan or nicotinic acid reverses these symptoms.

(See Niacin: Physiology.)

0036Tyrosine (mol. wt 181.2) This amino acid has an

aromatic, polar but uncharged side chain (hydroxy-

lated phenylalanine). Tyrosine is an important inter-

mediate in the synthesis of epinephrine, thyroxin,

melanin, and biogenic amines. Foods such as choc-

olate and cheese which is somewhat rich in tyrosine

have been suggested to be responsible for some

migraine-type headaches. (See Migraine and Diet.)

Posttranslational Modification of

Proteins: Unusual Amino Acids

0037There exists a number of unusual amino acids, de-

rived from some of the 20 standard protein amino

acids. Although a few of these can still be found in the

free form (Table 3), the remainder are essential to

specific proteins and arise from enzymatic modifica-

tions occurring after they have been incorporated into

a peptide, as listed below.

Hydroxylation

0038Hydroxyprolines Proline may be modified to 3-

hydroxyproline or 4-hydroxyproline after it has

been incorporated into a precursor peptide for

collagen. Hydroxylated residues can be further

glycosylated.

00395-Hydroxylysine 5-Hydroxylysine is another un-

usual amino acid found in collagen. Hydroxylysine

results from the modification of lysine after incorpor-

ation into a peptide. Scurvy symptoms occur when

hydroxylation does not take place. (See Scurvy.)

Phosphorylation

0040The reactivity of tyrosine can be augmented in a

peptidic environment. This is illustrated by the fact

that tyrosine residues of certain cell proteins are

particularly targeted by phosphokinase-phophatase-

mediated reactions as an effective way to regulate

division, growth, and general cell metabolism in

both animals and plants. Epidermal-, platelet- and

some insulin like-growth factors (EGF, PDGF, IGF)

are examples of proteins that are activated by tyrosine

phosphorylation. Mice and human oncogenes can

produce tyrosine kinases with uncontrolled phos-

phorylation activities that lead to leukemia, breast,

and prostate cancer.

188 AMINO ACIDS/Properties and Occurrence

Carboxylation

0041 g-Carboxyglutamic acid This amino acid is associ-

ated with calcium binding and is found in proteins

such as prothrombin, the protein involved in blood

clotting, and other proteins in bone tissue.

Methylation

0042 e-N-Methyllysine This amino acid is abundant in

the contractile muscle protein myosin and in cyto-

chrome c.(See Exercise: Muscle.)

Glycosylation

0043 Many proteins have carbohydrate side chains. The

sugars (often galactose and glucose) are attached to

asparagine, serine, or theonine residues or other

hydroxylated amino/imino acid residues. Glycopro-

teins can form gel structures that influence the water

activity and the texture of foods.

Covalent Cross-Linking

0044 Desmosine The structure of desmosine is shown in

Figure 10. It is a derivative of lysine and forms a

covalent cross-link between polypeptides. It is mainly

found in elastin and is relevant to the aging process, as

seen in skin. Other cross-links (aldolic) that occur in

collagen are associated with meat texture.

Sulfur–Selenium Exchange

0045 Selenocysteine and selenomethionine These are

analogs of both cysteine and methionine that result

from sulfur–selenium exchange, occurring mostly in

plants grown in selenium-rich soils. These amino acids

are found in the active site of selenium-dependent

enzymes, such as glutathione peroxidase, which are

essential for the removal of toxic hydrogen peroxide

from cells. (See Selenium: Physiology.)

Occurrence in Foods and Nutritional

Implications

0046All animal foods, such as meat, fish, milk, and eggs,

are protein-rich, whereas most plant foods are not.

Leguminous seeds, like beans and nuts, where protein

and lysine are high, are an exception. These proteins,

however, are deficient in the sulfur-containing amino

acids, particularly methionine. The proteins found in

cereal grains (seeds of maize grain, rice, sorghum), in

turn, are deficient in lysine and tryptophan, but not so

in methionine. Leafy vegetables are somewhat richer

than root vegetables, whereas fruits are very low in

protein. The proteins of seeds of pseudocereals like

quinoa (Chenopodium quinoa Willd.) and amar-

anthus grain (Amaranthus sp.) have amino acid pro-

files that combine the characteristics of both legumes

and cereals. (See Protein: Food Sources.)

0047Humans can synthesize only 10 of the 20 amino

acids required for protein synthesis. They must obtain

the rest from their diet if they are to remain healthy

(Table 2). (See Protein: Synthesis and Turnover.)

0048Interestingly, the metabolisms of children and

adults are slightly different. The growing child has a

greater requirement for arginine and histidine than

the adult, whereas adults synthesize sufficient argin-

ine for their daily requirement.

0049Consistent with food composition data, proteins

rather than free amino acids are the main source of

dietary nitrogen. Some special diets, however, require

the use of predigested proteins or free amino acids.

About 75% of amino acid metabolism in normal

healthy adults is devoted to protein synthesis. This

protein is essential for normal growth, repair, and

defense, including the production of active enzymes,

plasma protein, muscle creatine and, in females,

reproduction and synthesis of milk proteins. The

remaining 25% of nitrogen metabolism produces

intermediates of the tricarboxylic acid (TCA) cycle,

hormones, and neurotransmitters.

0050Amino acids that are not associated with proteins

in a given tissue or food can be collectively called

‘nonprotein’ amino acids or ‘nonprotein’ nitrogen.

In spite of the fact that nonprotein amino acids are

present in relatively small amounts, their importance

in food science and nutrition can be significant.

Table 3 lists the common protein amino acids, as

free species, their solubility, genetic code, pK and pI

values.

0051Less common, nonprotein amino acids, some with

very peculiar properties, are listed in Table 4. Worthy

of mention are theanine, typical of green tea, which

has a relaxing effect in humans (Figure 11) and taur-

ine in human milk. Taurine (b-aminoethanesulfonic

acid) is an unusual amino acid, normally found in the

HOOC

COOH

CH CH

COOH

N COOH

(CH

)

(CH

)

(CH

)

(CH

)

fig0010 Figure 10 The structure of desmosine.

AMINO ACIDS/Properties and Occurrence 189

tbl0004 Table 4 Natural nonprotein amino acids found in plant and animal tissues

Name Occurrence, function, andimportance

b-Alanine (b-aminopropionic acid) A component of pantothenic acid, coenzyme A, constituent of carnosine and

anserine. Also occurs in the free state

L-a-Aminobutyric acid Normal human urine, animal and plant tissues. Also as a component of the

tripeptide ophthalmic acid

b-Aminoisobutyric acid Human urine and iris bulb in the free state

g-Aminobutyric acid Occurs in the free state in several plants, animal tissues, and certain bacteria.

Has been found in the brain of mammals, some amphibia, and birds. This

amino acid appears to function in the transmission of nerve impulses

a-Aminoadipic acid Maize, pea and other plants, human urine

a-Aminopimelic acid Isolated from Asplenium septentrionale

O-acetylhomoserine This amino acid has been isolated from pea plants

O-Phosphohomoserine Isolated from extracts of Lactobacillus casei

L-Azaserine (O-diazetoacetil-L-serine) Isolated from cultures of a strain of Streptomyces. This amino acid inhibits the

growth of certain experimental tumors

L-Albizzine (L-2-amino-3-ureidopropionic acid) It has been found in the seeds of Albizzia julibrissin and in other species of

Mimosaceae

Alliin (S-allyl-

L-cysteine) A constituent of garlic. Alliin is enzymatically converted by alliinase to allicin,

which exhibits an odor characteristic of garlic

Betaine A constituent of plant and animal tissues, an intermediate in the metabolism

of lipids

L-Baikiain Rhodesian teak (Baikiaea plurijuga)

Canavanine Soy and jack bean meals and also in a number of other plants

m-Carboxy-

L-tyrosine Seeds of Reseda odorata L.

b-Cyano-

L-alanine Seeds of the vetch (Vicia sativa)

L-Citrulline Fish, juice of watermelon, other plants and animal tissues, important

intermediate in urea cycle; precursor of arginine

L-Djenkolic acid Djenkol bean (Pithecolobium lobatum, from Java), Acacia farnesiana

Dichrostathinic acid Seeds of Dichrostachys glomerata

b,g-Dihydroxyglutamic acid Found in lettuce seeds, rhubarb, and other plants

1,3-Dimethylhistidine Reported in the Australian seaweed Gracilaria secundata

2-(b-

D-Glucopyranosyl)-4-alanyl-3-isoxazolin-5-one Found in peas (Pisum sativum)

Glycine In the free form plays an important role in the sweet flavor of shrimp

L-Homoarginine Seeds of legumes of the genus Lathyrus and Lotus helleri

L-Homomethionine Present in cabbage

Homoserine Peas and other plants

Homocysteine Found in adrenal extracts

Hypusine A basic amino acid reported in bovine brain and other mammalian tissues

3-Hydroxypipecolic acid Halophyte leaves, flowers, and fruits

5-Hydroxy-

L-tryptophan Found (6–10%, fresh weight) in the West African legume Griffonia simplicifolia

Lanthionine Antibiotics (subtilin, penicillins)

Lathyrine Seeds of Lathyrus tingitanus

Lisopine Crown-gall tissues of salsify, tobacco, and normal plant tissues

1-Methylhistidine Anserine, human urine

b-Methylene-

L-norvaline Found in the mushroom Lactarius helvus

b-Methylene-

L-norleucine Found in the mushroom Amanita vaginata

S-Methylmethionine Methylsulfonium derivative of methionine found in cabbage and asparagus

N-Methyltryptophan Methyl ester found in the legume Aotus subglauca

Ornithine Tyrocidine, urea cycle, as ornithuric acid in excreta of birds, some fishes

Octopine Octopus muscle

Pipecolic acid Occurs in plant tissues and in mammalian livers as a product of lysine

metabolism

L-Saccharopine Bakers’ and brewers’ yeasts

Taurine Sulfur amino acid essential to infants fed cows’ milk

Theanine Green tea. Causes a specific relaxation effect in humans

Tricholomic acid Responsible for the good taste of mushrooms. Toxic to flies

L-Willardiin Seeds of several Acacia species, including Acacia willardiana

190 AMINO ACIDS/Properties and Occurrence

free form, manufactured by the body from cysteine or

methionine and essential for the production of conju-

gated bile salts (taurocholic acid). Since cows’ milk

has low concentrations (1–3 mmol 100 ml

1

) of taur-

ine in relation to human milk (27–35 mmol 100 ml

1

there is concern that infants fed formulas based on

cows’ milk may develop a deficiency of taurine.

Amino Acids, Neurotransmitters, and Hormones

0052 The amino acids glutamate, glutamine, asparate, gly-

cine, and g-aminobutyric acid (GABA) will act as

neurotransmitters for specific types of neurones or

regions of the brain. Glutamate is one important

transmitter in the central nervous system of inverte-

brates and possibly in humans. GABA is found in

relatively high concentrations (around 0.8 mmol

1

), almost exclusively in brain tissue. It acts as an

inhibitory transmitter by making it more difficult for

a nerve to fire impulses. The structure of GABA is

shown in Figure 12.

0053 Thyroxine and triiodothyronine (Figure 13) are

hormones derived from the amino acid tyrosine.

Both these hormones stimulate metabolism in tissues.

(See Hormones: Thyroid Hormones.)

Chemical and Physicochemical

Properties Relevant to Foods

0054 The participation of amino acids in the many changes

that occur during ripening, storage, or processing of

raw materials and foodstuffs is a direct consequence

of their capacity to react chemically or interact

physicochemically with other food components.

Amino acids, per se, for example have little direct

relevance on flavor, while proteins have even less.

Proteins and peptides however have an important

role as emulsifiers, fat and odor binders and thus

can affect the flavor and texture characteristics sig-

nificantly, while some amino acids or their derivatives

have direct bearings on food flavor and aroma.

Examples are the sweet taste of naturally grown

shrimp because of the high concentrations of glycine

and the characteristic flavor of mushrooms imparted

by tricholomic acid (Table 4, Figure 11).

0055The shifting of the pK

s of a free amino acid to-

wards lower pH values makes the amine groups more

reactive against carbonyl compounds. Side chain

reactivity is also enhanced through the effect of the

a-amino/a-carboxylic system. Amino acids with aro-

matic or alifatic side chains are also not as hydro-

phobic as they would be without the a-amino/

a-carboxylic system. The following are reactions

responsible for the development of some food attri-

butes.

Deamination

0056Amino acids undergo deamination during fermenta-

tion. This is important in the production of flavor

compounds in bread, some cheeses, and beers. If dea-

mination is allowed to proceed extensively, diamines

and biogenic amines may form, giving the food a fishy

or spoiled odor.

0057During the ripening of bananas l-leucine can give

rise to 13 volatile alcohols, aldehydes and esters,

while l-phenylalanine will generate seven and

l-valine produce four, some of which contribute to

the characteristic banana aroma. Sulfur amino acids

can be precursors of such volatile compounds as

S, CH

SH, CH

SH, propanethiol, dimethyl-

sulfoxide, methylisothiocyanate, allylthiocyanate,

CH(NH

)CO

CH CH COOHS

L-Theanine (γ-glutamylethylamide)

HOOC C

Tricholomic acid Alliine

fig0011 Figure 11 The structures of L-theanine, tricholomic acid and

alliine.

−

OOC (CH

)

fig0012Figure 12 The structure of g-aminobutyric acid (GABA).

Thyroxine

OCH

CH COO

−

Triiodothyronine

OCH

CH COO

−

fig0013Figure 13 The structures of thyroxine and triiodothyronine.

AMINO ACIDS/Properties and Occurrence 191

allylisothiocyanate, thiophene, acetylthiophene, ben-

zothiophene. Garlic, onions chives, leeks, and shallots

are rich in alliins (trans-S(þ)-1-propenyl-l-cysteine-

sulfoxide is a major alliin), important sulfur com-

pounds that are released when the bulbs are crushed

(Figure 11). (See Flavor (Flavour) Compounds: Struc-

tures and Characteristics.)

Maillard Browning

0058 Under a wide range of moisture and temperature

conditions, amino acids and carbonyl compounds

can undergo a complex series of condensation,

deamination, decarboxylation, and polymerization

reactions called ‘Maillard browning.’ The products

of the reaction of amino acids or proteins with alde-

hydes, ketones, and reducing sugars are numerous.

They can be volatile, low-molecular-weight or com-

plex brown-colored, high-molecular-weight, fluores-

cent compounds referred to collectively as

‘melanoidins.’ The correct amount of Maillard prod-

ucts is very important in determining the acceptability

of baked and roasted foods. Even though an extensive

part of the classical nutritive value of the reacted

amino acids will be lost and some biogenic amines

may form, these products add to the desired aroma,

flavor, and color of cakes, pies, powder and con-

densed milk, roasted groundnuts, chocolate, and

coffee.

Processing Aids

0059 l-Cysteine, l-glutamyl-l-cysteine, l-cysteinylglycine

and glutathione (g-l-glutamyl-l-cysteinyl-glycine)

are naturally present in wheat flour. In certain

dough processes the flour can be enriched with these

molecules or with mixtures of these compounds with

oxidizers to produce desired properties in the dough

or to reduce mixing times. MSG, naturally present in

cabbage and other products of either vegetable or

animal origin, has a ‘meaty’ taste and can be added

in the crystalline form as a flavor enhancer. (See

Flour: Dietary Importance.)

Amino Acids in Artificial Sweeteners

0060 Aspartame is the dipeptide l-aspartyl-l-phenylalan-

ine methylester. This molecule is about 200 times

sweeter than sucrose with almost no aftertaste. It

degrades slowly under acid conditions at room tem-

perature and rapidly at higher temperatures, making

it unsuitable for baked foods. Phenylketonurics

cannot tolerate this sweetener because of the phenyl-

alanine content. (See Sweeteners: Intensive.)

See also: Amino Acids: Metabolism; Browning:

Nonenzymatic; Enzymes: Functions and Characteristics;

Exercise: Muscle; Flavor (Flavour) Compounds:

Structures and Characteristics; Flour: Dietary

Importance; Hormones: Thyroid Hormones; Migraine

and Diet; Niacin: Physiology; pH – Principles and

Measurement; Protein: Chemistry; Food Sources;

Synthesis and Turnover; Scurvy; Selenium: Physiology