Kelter P., Mosher M., Scott A. Chemistry. The Practical Science

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our cells is quite fast; some are so fast that they must pause and wait for molecules

to arrive at the reaction site. What type of compound makes these reactions fast?

Speciﬁc types of proteins in the body, known as enzymes, catalyze these reactions.

A few key principles govern enzyme catalysis. This process is summarized in

Figure 22.23.

■

Enzymes contain speciﬁc sites to which certain chemicals can bind.

■

The structure of the sites enhances the binding of speciﬁc molecules and ions

through the same type of intermolecular forces of attraction that we discussed

in Chapter 10.

■

One of the binding sites on an enzyme includes the active site where the sub-

strate

binds.

■

The enzyme next catalyzes the reaction of the substrate and produces the

product, which is then released from the enzyme.

Cofactors, Coenzymes, and Protein Modiﬁcations

Regulation and control of the activity of an enzyme is often necessary. Imagine,

for instance, what would happen if all of the sucrose you just consumed at lunch

were immediately converted into energy. That energy would be used up quite

quickly in the biological processes that keep you alive. After a very short time,

your body would be out of energy, and the basic reactions that keep you

functioning would stop. This suggests that the reactions within a cell must be

regulated—turned on and off—so that they are performed only when needed.

This regulation often involves chemical modiﬁcation of the enzyme. The major

modiﬁcations are as follows:

■

Incorporation of speciﬁc metal ions at appropriate sites, to form metallopro-

teins, in which metal ions are tightly bound within the protein structure, or

metal-activated proteins, which are activated by loosely bound metal ions

picked up from the surrounding solution

■

Incorporation of loosely bound small chemical groups called coenzymes at

speciﬁc sites within the protein

■

Incorporation of tightly bound prosthetic groups, often held to the protein by

covalent bonds

■

Covalent linking of phosphates to speciﬁc sites to form phosphoproteins

■

Covalent linking of carbohydrates at speciﬁc sites to form glycoproteins

■

Covalent bonding of lipids at speciﬁc sites to form lipoproteins

The need for many enzymes and other proteins to be modiﬁed in these ways ex-

plains many of our nutritional needs for speciﬁc minerals and vitamins. For in-

stance, much of the iron that we must consume in our diet is needed to provide

948 Chapter 22 The Chemistry of Life

FIGURE 22.23

Substrates bind to the active site of an enzyme. The enzyme then catalyzes the reaction and

releases the products. The enzyme is then primed for the next reaction.

Free

enzyme

SES

Free

substrate

EP EP

Free

products

iron ions for incorporation into metalloproteins such as hemoglobin. Many of

the vitamins we must consume in tiny amounts are needed to form the coen-

zymes that combine with speciﬁc enzymes and allow them to function.

The General Classes of Enzymatic Reactions

Enzymes catalyze many thousands of different chemical reactions. Fortunately,

these reactions can be classiﬁed into just six categories on the basis of the general

type of reaction that is being catalyzed.

■

Oxidoreductases are enzymes that catalyze redox reactions.

■

Transferases are enzymes that catalyze the transfer of groups from one mole-

cule to another.

■

Hydrolases are enzymes that catalyze hydrolysis reactions.

■

Lyases are enzymes that catalyze elimination reactions that remove hydrogen

atoms or functional groups and form alkenes (CPC) in the substrate.

■

Isomerases are enzymes that catalyze the interconversion between isomers.

■

Ligases are enzymes that catalyze the formation of new bonds linking sub-

strates together.

EXERCISE 22.4 What Is It Doing?

Examine the following reaction and classify the enzyme’s activity into one of the six

types of reactions common among enzymes.

Solution continues on page 952.

Benzyl alcohol Benzoic acid

Enzyme

22.4 Enzymes 949

Vitamin B1 (thiamine) binds within the

structure of yeast transketolase (a trans-

ferase enzyme). The thiamine is shown

as a spaceﬁlling model.

950 Chapter 22 The Chemistry of Life

Vitamins are small molecules that we need

for proper health; most either are not nor-

mally synthesized in our bodies or are

made in insufﬁcient amounts. We humans

obtain most of our needed vitamins from

food. The exception to this is that our bod-

ies can make vitamin D and vitamin K. In

all, we need 13 vitamins to sustain our life:

vitamins A, C, D, E, and K and the B

vitamins (thiamine, riboﬂavin, niacin, pan-

tothenic acid and biotin, vitamin B6, vita-

min B12, and folate). Each of these vita-

mins plays an interesting role in the

biochemistry of life. Details of these vita-

mins are shown in Table 22.2.

Vitamin D

Vitamin D (calciferol) promotes retention

and absorption of calcium and phospho-

rus, primarily in the bones. Too much vita-

min D in the body may have the opposite

effect of taking calcium from the bones and

depositing it in the heart or lungs, making

them function less efﬁciently. Because vita-

min D is essential for the body’s utilization

of calcium, a deﬁciency may result in severe

loss of calcium and, consequently, a soften-

ing and weakening of bones (osteomala-

cia). Extreme vitamin deﬁciency gives rise

to a disease known as rickets.

Like most vitamins, vitamin D may be

obtained in the recommended amount

with a well-balanced diet, including some

enriched or fortiﬁed foods such as milk. In

addition, the liver and kidneys manufac-

ture vitamin D when the skin is exposed to

sunshine. Deﬁciencies in this vitamin arise

primarily from insufﬁcient exposure to

sunlight. That is why it is recommended

that we get at least 10 to 15 minutes of sun-

shine three times a week.

Vitamin A

Vitamin A (retinol) is supplied by many

foods of both animal and plant origin. Veg-

etables sources, such as carrots, pumpkin,

and brocolli, actually contain a precursor

of retinol called beta-carotene, which is

NanoWorld / MacroWorld

Big effects of the very small:

Vitamins and disease

Selected Vitamins and Their Sources in Food

Vitamin Deﬁciency Disease Sources in Food

TABLE 22.2



HOCH



Vitamin A

B1 (thiamine)

B2 (riboﬂavin)

Niacin

Vitamin B6

Night blindness,

xerophthalmia

Beriberi

Ariboﬂavinosis

Pellagra

Vitamin B6 deﬁciency

Eggs, whole milk, cream, cheese,

liver, green and yellow vegetables

(carrots, squash, sweet potatoes,

spinach, kale, and broccoli)

Ham, pork, milk, fortiﬁed cereals,

peanuts, liver, yeasts

Liver and other organ meats,

milk, green vegetables, fortiﬁed

cereals, yeast

Peanuts, lean meats, poultry, ﬁsh,

bran, yeast, liver

Whole-grain cereal, ﬁsh, legumes,

liver and other organ meats, yeast

22.4 Enzymes 951

Vitamin B12

Vitamin C

(ascorbic acid)

Vitamin D

Vitamin E

Vitamin K

OHHO

HOCH

(

)

CH C

(CH

CH)

converted into retinol by the body. In the

body, vitamin A is used in the rods and

cones within your eyes to absorb light and

enable you to see. The vitamin also plays an

important role in bone growth, reproduc-

tion, and cell division. Vitamin A has been

implicated in the regulation of your im-

mune system by facilitating the action of

lymphocytes, a type of white blood cell that

ﬁghts infection.

Deﬁciencies in vitamin A rarely occur

in the United States but are very common

in poor and developing countries. Night

blindness and xerophthalmia (damage to

the cornea of the eye) are the results of this

deﬁciency. These conditions lead to as

many as a half-million cases of blindness

each year and, according to the World

Health Organization, account for 70% of

all childhood blindness in developing

countries. Recent advances in treating

this preventable disease have included the

development of so-called golden rice, a

genetic hybrid of rice that produces beta-

carotene. Its use in countries where the

staple food is rice has been quite suc-

cessful in reducing the number of cases of

blindness.

Vitamin E

Vitamin E (alpha-tocopherol) is found in

corn, nuts, seeds, olives, spinach, asparagus,

and other green leafy vegetables and in

products made from them, such as mar-

garine. This vitamin is often touted as

“the body’s antioxidant.” Considerable evi-

dence suggests that it prevents the natural

Eggs, milk, liver

Fresh fruits and vegetables

(oranges and other citrus

fruits, Brussels sprouts,

cabbage, etc.)

Fortiﬁed milk, ﬁsh liver oil

Vegetable seed oil, egg yolk,

cereals, beef liver

Leafy green vegetables, liver

(Continued)

Vitamin Deﬁciency Disease Sources in Food

TABLE 22.2

Pernicious anemia

Scurvy

Rickets

Vitamin E deﬁciency

Vitamin K deﬁciency

See drawing at right.

HOCH

–

continued

952 Chapter 22 The Chemistry of Life

oxidation of lipoproteins, which play an

important role in the development of

atherosclerosis, the disease process that

leads to heart attacks and strokes.

Vitamin C

Vitamin C (ascorbic acid) has many

functions in the body, including assist-

ing in the absorption of iron. This vita-

min is found in citrus fruits, such as lemons and limes,

and in most other vegetables. It is an component of en-

zymes involved in the synthesis of collagen. Without vita-

min C, the enzymatic preparation of collagen still pro-

ceeds, but the resulting collagen protein doesn’t include

the modiﬁed amino acid known as hydroxyproline. Col-

lagen with hydroxyproline forms strong intermolecular

forces of attraction between strands of the collagen pro-

tein. Without hydroxyproline, the collagen cannot form

these strong attractive forces. The result is nothing short

of the breakdown of the human body. Deﬁciency in vita-

min C, known as scurvy, is evidenced by sore joints, loss

of teeth, and aching muscles. Severe deﬁciencies and un-

treated deﬁciencies result in respiratory failure.

Abnormally large doses of vitamin

C appear to be harmless at the least,

and they may be helpful. Because vita-

min C is water soluble, excess vitamin

C can be easily excreted from the

body. Linus Pauling (remember him

from Chapter 8) suggested that

vitamin C may play a crucial role in

the maintenance of a healthy immune

system. He proposed that massive

doses of vitamin C (as high as 1000 mg/day) may even

reduce the frequency and severity of the common cold.

Recent research suggests that this may, in fact, be true.

Vitamin B1

Vitamin B1 (thiamine) is found in cereals, breads, and

pasta. This vitamin, like vitamin C, is one of the water sol-

uble vitamins. Deﬁciency in vitamin B1 leads to fatigue,

psychosis, and even nerve damage. Extremely severe deﬁ-

ciencies result in beriberi, a condition that is character-

ized by nerve degeneration and muscle disease and that

affects the function of the heart. This disease is prevalent

in developing countries such as those in eastern and

southern Asia. In developed countries, it is rarely found.

4-Hydroxyproline

Solution

At ﬁrst glance, it appears that the substrate (benzyl alcohol) might be converted into

the product (benzoic acid) by a lyase, because there is a double bond formed in the

product. However, this double bond is not an alkene. Further examination of the

oxidation states of the carbon atoms in the molecule reveals that one of them is un-

dergoing oxidation. Therefore, this reaction is catalyzed by an oxidoreductase.

PRACTICE 22.4

Indicate what type of enzyme is responsible for each of these transformations.

See Problems 19 and 20.

Enzyme

Acetal CoA

Enzyme

CH C



CH C

NanoWorld/MacroWorld (continued)

Insulin

22.5 The Diversity of Protein Functions

One way of summarizing the relationship between genes and proteins—the two

central categories of chemicals that sustain life—is to say that “genes hold the in-

structions while proteins do the work.” In a human, our 20,000 to 30,000 genes

encode the structure of proteins, and each protein has its own specialized task, its

own little bit of “chemical work” to do. There is often simplicity at the heart of

complexity, however, and proteins are no exception. Almost everything they

achieve can be explained in terms of three fundamental activities. Selective bind-

ing, catalysis, and conformational change are the three keys to understanding the

activities of proteins. Here is a brief list of the main tasks these three operational

principles enable proteins to achieve.

Proteins as Enzymes

Some proteins contain active sites that bind substrates and catalyze reactions to

make products, the main subject of Section 22.4.

Proteins as Transporters

Many proteins bind to speciﬁc chemicals at particular sites and then release their

cargo at other sites. The protein hemoglobin, for example, binds to oxygen mol-

ecules in the lungs, transports the oxygen through the blood, and then releases it

in the tissues of the body that need oxygen to survive.

Proteins as “Movers and Shakers”

What in your body makes you move? You move because you have muscles. Mus-

cles are composed of ﬁlamentous contractile proteins. These are proteins that re-

spond to stimuli from nerves to undergo conformational changes that result in

their being ratcheted past one another and causing the whole structure of a mus-

cle to contract. Reversal of the process enables the muscles to relax.

Proteins as Scaffolding and Structure

Much of the shape and structure of cells, tissues, organs, and the body as a whole

is maintained by strong ﬁbrous structural proteins that form ﬁbers and pillars and

sheets of material. Hair, skin, connective tissue, and an intricate skeleton of

tubules within all cells are composed of structural proteins.

Proteins as Messengers

Proteins that are made at one place in the body can be transported

through the blood to other parts of the body, where they bind to other

chemicals to initiate speciﬁc biochemical responses. Some such pro-

teins are known as

hormones. The protein insulin, for example, is a

hormone that is released from the pancreas and then binds to cells

and assists in the uptake of glucose into these cells. A deﬁciency in this

hormone or in its production causes diabetes.

Proteins as Receptors

The outermost portion of the cell, known as the cell membrane, is studded

with proteins that act as receptor molecules. These receptors bind to speciﬁc mes-

senger molecules, some of which are proteins like insulin, and then participate in

acts of catalysis or conformational change that initiate the effect that the messen-

ger elicits. Protein receptors also exist inside of cells. The interactions between re-

ceptors and messengers are vital in controlling cell growth. This makes disorders

in these systems very signiﬁcant in the onset of serious diseases such as cancer.

22.5 The Diversity of Protein Functions 953

Proteins as Gates and Pumps

Cell membranes act as very selective barriers, through which only speciﬁc chem-

icals can pass, and only at appropriate rates and at appropriate times. Much of

this selective movement of chemicals into and out of cells is maintained by pro-

teins spanning the cell membrane. Some of these proteins can simply open or

close a hole in the membrane, allowing selected chemicals to pass. Others act

more like pumps; that is, they transport a chemical across a membrane in such a

way that a signiﬁcant difference can build up between the concentration of the

chemical on one side of the membrane and its concentration on the other side.

Proteins as Controllers

A whole battery of proteins known as regulatory proteins serve the function of

controlling other proteins, and controlling genes, by binding to them and switch-

ing them on or off. In most cells of the body, for example, only a small proportion

of the genes in the cell nucleus must be active at any one time. Switching the right

genes on and off at the correct times is largely achieved by the activities of regu-

latory proteins.

Proteins as Defenders

The immune system, which defends us against disease, is a network of interacting

cells and chemicals, and many of its most signiﬁcant chemicals are proteins. Most

famous of all are the

antibodies—proteins that selectively bind to foreign chemi-

cals and assist in their elimination from the body. Allergies such as hay fever result

when this system goes wrong by eliciting too strong a reaction against relatively

harmless foreign chemicals. When it goes wrong by attacking the body’s own tis-

sues, serious autoimmune diseases such as rheumatoid arthritis can result.

Glorious Complexity

The actions of genes and proteins are part of a very complex picture of the bio-

chemistry of life. All of that complexity is, at heart, simply chemistry, driven by

the interactions of energy, charge, and force. However, genes and proteins aren’t

the only players. There are many other compounds involved in the smooth oper-

ation of a living being. Carbohydrates and lipids are two important classes of the

compounds of life.

22.6 Carbohydrates

Most people love carbohydrates. Runners eat copious amounts before a very long

race. Snackers munch on carbohydrates in front of the TV. Toddlers often will

drop everything else to get a small taste of a

carbohydrate. What are carbohydrates

and why do our bodies require them?

This class of compounds shares the general

954 Chapter 22 The Chemistry of Life

Extracellular protein receptors.

Video Lesson: Carbohydrates

formula C

—historically, they were known as hydrates of carbon, hence

their name. The simplest carbohydrates are

sugars, which are known as triose sug-

ars if they contain three carbon atoms per molecule, tetrose sugars with four car-

bons, pentose sugars with ﬁve carbon atoms per molecule, hexose sugars with six

carbon atoms per molecule, and so on. In addition, the carbohydrates can be clas-

siﬁed on the basis of the type of functional group (see Chapter 12). For example,

glucose is an aldose because it contains an aldehyde functional group. Fructose is

a ketose because it contains a ketone functional group. The glucose used in med-

ical drip-feeding is an aldohexose (from aldose and hexose) and is the main form

in which carbohydrate circulates in our blood.

The familiar table sugar we may add to coffee is sucrose and is formed when

two simpler hexose sugars, glucose and fructose, become linked together in a con-

densation reaction, as shown in Figure 22.24. You’ll immediately notice a differ-

ence between the sugars in Figure 22.24 and those from our previous discussion.

22.6 Carbohydrates 955

Glyceraldehyde

CHO

Erythrose

CHO

HCOH

Ribose

CHO

HCOH

Glucose

Triose Tetrose Pentose Hexose

HCOH

HO C

CHO

Classiﬁcation of saccharides by number

of carbon atoms.

Erythrulose

C O

Fructose

HCOH

Glucose

Aldose

HCOH

HO C

CHO

HO C H

Ketose

Classiﬁcation of saccharides by type of

functional group.

FIGURE 22.24

The condensation of glucose with fruc-

tose gives sucrose. Monosaccharides in-

clude glucose and fructose. Sucrose is a

disaccharide.

HO OH

HOH

Glucose Fructose Sucrose



OHH

HOCH

H CH

HOH

HOCH

HOH

Monosaccharides Disaccharide

Why? Most carbohydrates undergo a condensation reaction within their

own structures when they are placed in aqueous solution. The result is a group

of cyclic structures that are more stable than the straight-chain carbohydrates.

Sugars such as glucose and fructose are also known as

monosaccharides (for

“single sugars”), whereas those like sucrose, made when two monosaccharides

combine, are

disaccharides. Note that two cyclic sugars, one glucose and one fruc-

tose, have come together to form the disaccharide, as shown in Figure 22.24.

One major role for carbohydrates in living things is to serve as energy storage

compounds. For long-term storage, monosaccharides are linked up into giant

polymers known as

polysaccharides. The major storage polysaccharide in plants is

starch, composed of many glucose monomers linked in the manner shown in

Figure 22.25. Foods such as rice or potatoes supply us with energy largely thanks

to the deposits of starch within them, whose role in the plant is to provide the en-

ergy needed to sustain its growth. Animals, including humans, contain a slightly

different polymer of glucose called glycogen, in which the glucose monomers are

linked together in a slightly different fashion, as shown in Figure 22.26. Another

important polysaccharide is cellulose, which is a polymer of glucose that forms

much of the framework of plant cell

walls. The glucose monomers are

linked together in a completely differ-

ent manner, as shown in Figure 22.27.

Humans lack the enzyme capable of

hydrolyzing cellulose into glucose, so

cellulose contains no nutritional value

to us.

956 Chapter 22 The Chemistry of Life

FIGURE 22.25

Starch is a polymer of glucose units

linked together as shown.

OOO

OH OH OH OH

OH OH

FIGURE 22.26

Glycogen is a polysaccharide.

OH OH

OOO

OH OH OH OH

OH OH

FIGURE 22.27

Cellulose is a polymer of glucose units linked to-

gether as shown. What is the difference be-

tween cellulose and starch (Figure 22.25)? Note

the position of the OH on the glucose monomer

at the right. It is pointed in a direction different

from the direction it would point in starch.

OH OH

22.7 Lipids 957

FIGURE 22.28

Triacylglycerides result from the condensation of three fatty acids and glycerol.

HO  3H

Some Common Fatty Acids

Common Name Formula Melting Point

Lauric acid CH

(CH

)

COOH 46°C

Myristic acid CH

(CH

)

COOH 55°C

Palmitic acid CH

(CH

)

COOH 64°C

Stearic acid CH

(CH

)

COOH 69°C

Oleic acid CH

(CH

)

CHPCH(CH

)

COOH 13°C

Linoleic acid CH

(CH

)

CHPCHCH

CHPCH(CH

)

COOH −5°C

TABLE 22.3

22.7 Lipids

Lipids are a very broad class of compounds that differ from most other classes of

compounds. For example, the molecules known as carbohydrates exhibit similar-

ity in structure and function. Lipids do not have similarity in structure or func-

tion. A compound is characterized as a

lipid solely on the basis of its solubility in

nonpolar solvents. Lipids include fats, oils, some hormones, and some vitamins.

Lipids function as important energy storage molecules, act as intercellular signal-

ing molecules, and also play many structural roles in living things. For example,

the thin membranes around every living cell are composed of lipids.

Some of the simplest lipids are the

fatty acids, which are long-chain carboxylic

acids that may contain one or more CPC double bonds. Table 22.3 lists some of

the common fatty acids. When three fatty acids combine with the tri-alcohol

glycerol in an esteriﬁcation reaction, a

triglyceride (triacylglycerol) is formed.

Figure 22.28 shows an example of this reaction. Triglycerides that are solid at

room temperature are called

fats, whereas those that are liquid at room tempera-

ture are

oils. We explored this distinction in Chapter 10 when examining the

effects of CPC double bonds on the melting points of fats and oils.

Triglycerides can be modiﬁed in various ways to form compound lipids, such

as the

phospholipids that are important components of cell membranes (see Fig-

ure 22.29). Living things also contain a great many derived lipids, which are fatty

substances that can be made from simpler lipids but have their own unique struc-

tures. One of the major classes of derived lipids consists of the

steroids, which

share a characteristic four-ring structure and include cholesterol and various

Application

FIGURE 22.29

Sphingomyelin is one of the components

of cellular membranes.

Video Lesson: Lipids