CHAPTER 6 © Chris L Jones/PhotoLibrary Nutrition in Your Life

26 Jun CHAPTER 6 © Chris L Jones/PhotoLibrary Nutrition in Your Life

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CHAPTER

6

© Chris L Jones/PhotoLibrary

Nutrition in Your Life
The versatility of proteins in the body is impressive. They help your muscles to conThroughout this chapter, the
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tract, your blood to clot, and your eyes to see. They keep you alive and well by facilitating chemical reactions and defending against infections. Without them, your
bones, skin, and hair would have no structure. No wonder they were named proteins,
meaning “of prime importance.” Does that mean proteins deserve top billing in your
diet as well? Are the best sources of protein beef, beans, or broccoli? Learn which
foods will supply you with enough, but not too much, high-quality protein.

173

CHAPTER OUTLINE
The Chemist’s View of Proteins

Protein:
Amino Acids

Amino Acids
Proteins

Digestion and Absorption
of Proteins
Protein Digestion
Protein Absorption

Proteins in the Body
Protein Synthesis
Roles of Proteins
A Preview of Protein Metabolism

Protein in Foods
Protein Quality
Protein Regulations for Food Labels

A few misconceptions surround the roles of protein in the body and the importance of protein in the diet. For example, people who associate meat with protein
and protein with strength may eat steak to build muscles. Their thinking is only
partly correct, however. Protein is a vital structural and working substance in all
cells—not just muscle cells. To build strength, muscles cells need physical activity and all the nutrients—not just protein. Furthermore, protein is found in milk,
eggs, legumes, and many grains and vegetables—not just meat. By overvaluing
protein and overemphasizing meat in the diet, a person may mistakenly crowd
out other, equally important nutrients and foods. As this chapter describes the
various roles of protein in the body and food sources in the diet, keep in mind that
protein is one of many nutrients needed to maintain good health.

Health Effects and
Recommended Intakes
of Protein
Protein-Energy Malnutrition
Health Effects of Protein
Recommended Intakes of Protein
Protein and Amino Acid Supplements

Highlight 6

Nutritional Genomics

The Chemist’s View of Proteins
Chemically, proteins contain the same atoms as carbohydrates and lipids—carbon
(C), hydrogen (H), and oxygen (O)—but proteins also contain nitrogen (N) atoms.
These nitrogen atoms give the name amino (nitrogen containing) to the amino
acids—the links in the chains of proteins.

Amino Acids

All amino acids have the same basic structure—a central
carbon (C) atom with a hydrogen atom (H), an amino group (NH2), and an acid
group (COOH) attached to it. However, carbon atoms need to form four bonds,
♦ so a fourth attachment is necessary. This fourth site distinguishes each amino
acid from the others. Attached to the central carbon at the fourth bond is a distinct atom, or group of atoms, known as the side group or side chain (see Figure 6-1
on p. 174).
Unique Side Groups The side groups on the central carbon vary from one

amino acid to the next, making proteins more complex than either carbohydrates
or lipids. A polysaccharide (starch, for example) may be several thousand units
long, but each unit is a glucose molecule just like all the others. A protein, on the

♦ Reminder:
•
•
•
•

H forms one bond.
O forms two bonds.
N forms three bonds.
C forms four bonds.

proteins: compounds composed of carbon, hydrogen,
oxygen, and nitrogen atoms, arranged into amino acids
linked in a chain. Some amino acids also contain sulfur
atoms.
amino (a-MEEN-oh) acids: building blocks of proteins.
Each contains an amino group, an acid group, a hydrogen
atom, and a distinctive side group, all attached to a
central carbon atom.
• amino = containing nitrogen

CHAPTER 6

174
FIGURE 6-1

Amino Acid Structure

All amino acids have a central carbon
with an amino group (NH2), an acid group
(COOH), a hydrogen (H), and a side group
attached. The side group is a unique chemical structure that differentiates one amino
acid from another.

TABLE 6-1

A mino Acids

Proteins are made up of about 20 common amino acids. The first column lists the essential
amino acids for human beings (those the body cannot make—that must be provided in the
diet). The second column lists the nonessential amino acids. In special cases, some nonessential amino acids may become conditionally essential (see the text). In a newborn, for example,
only five amino acids are truly nonessential; the other nonessential amino acids are conditionally essential until the metabolic pathways are developed enough to make those amino acids in
adequate amounts.
Essential Amino Acids

Nonessential Amino Acids

Histidine

C

COH

H

H

Acid
group

(ARJ-ih-neen)

Leucine

(LOO-seen)

Asparagine

(ah-SPAR-ah-geen)

Lysine

(LYE-seen)

Aspartic acid

(ah-SPAR-tic acid)

Methionine

(meh-THIGH-oh-neen)

Cysteine

(SIS-teh-een)

Phenylalanine

(fen-il-AL-ah-neen)

Glutamic acid

(GLU-tam-ic acid)

(THREE-oh-neen)

Glutamine

(GLU-tah-meen)

(TRIP-toe-fan, TRIP-toe-fane)

Glycine

(GLY-seen)

Valine

HN

(AL-ah-neen)

Arginine

Tryptophan

Amino
group

Alanine

(eye-so-LOO-seen)

Threonine

O

(HISS-tuh-deen)

I soleucine

Side group
varies

(VAY-leen)

Proline

(PRO-leen)

Serine

(SEER-een)

Tyrosine

(TIE-roe-seen)

other hand, is made up of about 20 different amino acids, each with a different
side group. Table 6-1 lists the amino acids most common in proteins.*
The simplest amino acid, glycine, has a hydrogen atom as its side group. A
slightly more complex amino acid, alanine, has an extra carbon with three hydrogen atoms. Other amino acids have more complex side groups (see Figure 6-2 for
examples). Thus, although all amino acids share a common structure, they differ
in size, shape, electrical charge, and other characteristics because of differences
in these side groups.
Nonessential Amino Acids More than half of the amino acids are nonessential,
meaning that the body can synthesize them for itself. Proteins in foods usually
deliver these amino acids, but it is not essential that they do so. The body can
make all nonessential amino acids, given nitrogen to form the amino group and
fragments from carbohydrate or fat to form the rest of the structure.
*Besides the 20 common amino acids, which can all be components of proteins, others do not occur in proteins
but can be found individually (for example, taurine and ornithine). Some amino acids occur in related forms
(for example, proline can acquire an OH group to become hydroxyproline).

FIGURE 6-2

Examples of Amino Acids

Note that all amino acids have a common chemical structure but that each has a different side group. Appendix C presents the chemical structures of the 20 amino acids most
common in proteins.
O
H
H

H

C

HN

C

H

H

O
HN
H

nonessential amino acids: amino acids that the body
can synthesize (see Table 6-1).

C
H

C

Glycine

C

O

H

H

C

H

HN

C

H

H

O
OH

C

Alanine

H
H

C

HN

C

H

H

O
OH

C

H
O

OH

Aspartic acid

C

OH

Phenylalanine

FIGURE 6-3

Condensation of Two Amino Acids to Form a Dipeptide

H
H

C

HN

C

H
H

H

C

HN

C

O

H

PROTEIN: AMINO ACIDS

175

C

H

H

C

HN

C

H

H

Amino acid

H

+

C

H

C

N

C

H

H

O
OH

H

H

H

O
OH

H

Amino acid

An OH group from the acid end of one amino
acid and an H atom from the amino group of
another join to form a molecule of water.

HOH

Water

C

O
C

OH

Dipeptide
A peptide bond (highlighted in
red) forms between the two amino
acids, creating a dipeptide.

Essential Amino Acids There are nine amino acids that the human body either

cannot make at all or cannot make in sufficient quantity to meet its needs. These
nine amino acids must be supplied by the diet; they are essential. ♦ The first column in Table 6-1 presents the essential amino acids.

♦ Some researchers refer to essential amino acids
as indispensable and to nonessential amino acids as dispensable.

Conditionally Essential Amino Acids Sometimes a nonessential amino acid
becomes essential under special circumstances. For example, the body normally
uses the essential amino acid phenylalanine to make tyrosine (a
nonessential amino acid). But if the diet fails to supply enough
FIGURE 6-4 Amino Acid Sequence of Human Insulin
phenylalanine, or if the body cannot make the conversion
for some reason (as happens in the inherited disease phenylHuman insulin is a relatively small protein that consists of 51 amino
acids in two short polypeptide chains. (For amino acid abbreviaketonuria), then tyrosine becomes a conditionally essential
tions, see Appendix C.) Two bridges link the two chains. A third
amino acid.

Proteins Cells link amino acids end-to-end in a variety of
sequences to form thousands of different proteins. A peptide
bond unites each amino acid to the next.
Amino Acid Chains Condensation reactions connect amino

bridge spans a section within the short chain. Known as disulfide
bridges, these links always involve the amino acid cysteine (Cys),
whose side group contains sulfur (S). Cysteines connect to each
other when bonds form between these side groups.
Leu Ala Glu Val Leu His Ser Gly Cys Leu His Gln
Pro Lys Ala

Thr
Leu
acids, just as they combine two monosaccharides to form a diVal
Tyr
saccharide and three fatty acids with a glycerol to form a trigPhe Phe Gly Arg Glu Gly Cys
lyceride. Two amino acids bonded together form a dipeptide
S
(see Figure 6-3). By another such reaction, a third amino acid
S
can be added to the chain to form a tripeptide. As additional amino acids join
the chain, a polypeptide is formed. Most proteins are a few dozen to several hunAsn Cys
dred amino acids long. Figure 6-4 illustrates the protein insulin.

Amino Acid Sequence—Primary Structure The primary structure of a pro-

tein is determined by the sequence of amino acids. If a person could walk along
a carbohydrate molecule like starch, the first stepping stone would be a glucose.
The next stepping stone would also be a glucose, and it would be followed by a
glucose, and yet another glucose. But if a person were to walk along a polypeptide
chain, each stepping stone would be one of 20 different amino acids. The first stepping stone might be the amino acid methionine. The second might be an alanine.
The third might be a glycine, the fourth a tryptophan, and so on. Walking along
another polypeptide path, a person might step on a phenylalanine, then a valine,
then a glutamine. In other words, amino acid sequences within proteins vary.
The amino acids can act somewhat like the letters in an alphabet. If you had
only the letter G, all you could write would be a string of Gs: G–G–G–G–G–G–G.
But with 20 different letters available, you can create poems, songs, and novels.
Similarly, the 20 amino acids can be linked together in a variety of sequences—
even more than are possible for letters in a word or words in a sentence. Thus the
variety of possible sequences for polypeptide chains is tremendous.

Asn

Tyr

Val

S

Phe
S
Gly Ile Val Glu Gln Cys Cys
Ala
S
S

Ser
Val

Tyr Asn Glu Leu Gln Tyr Leu Ser Cys

essential amino acids: amino acids that the body
cannot synthesize in amounts sufficient to meet
physiological needs (see Table 6-1).
conditionally essential amino acid: an amino acid
that is normally nonessential, but must be supplied by
the diet in special circumstances when the need for it
exceeds the body’s ability to produce it.
peptide bond: a bond that connects the acid end of one
amino acid with the amino end of another, forming a link
in a protein chain.
dipeptide (dye-PEP-tide): two amino acids bonded
together.
• di = t wo
• peptide = amino acid
tripeptide: three amino acids bonded together.
• tri = three
polypeptide: many (ten or more) amino acids bonded
together.
• poly = many

Polypeptide Shapes—Secondary Structure The secondary structure of
FIGURE 6-5

The Structure of Hemoglobin

Four highly folded polypeptide chains
form the globular hemoglobin protein.

proteins is determined not by chemical bonds as between the amino acids
but by weak electrical attractions within the polypeptide chain. As positively
charged hydrogens attract nearby negatively charged oxygens, sections of
the polypeptide chain twist into a helix or fold into a pleated sheet, for example. These shapes give proteins strength and rigidity.
Polypeptide Tangles—Tertiary Structure The tertiary structure of
proteins occurs as long polypeptide chains twist and fold into a variety of
complex, tangled shapes. The unique side group of each amino acid gives
it characteristics that attract it to, or repel it from, the surrounding fluids
and other amino acids. Some amino acid side groups are attracted to water
molecules; they are hydrophilic. Other side groups are repelled by water; they
are hydrophobic. As amino acids are strung together to make a polypeptide,
the chain folds so that its hydrophilic side groups are on the outer surface
near water; the hydrophobic groups tuck themselves inside, away from water. Similarly, the disulfide bridges in insulin (see Figure 6-4) determine
its tertiary structure. The extraordinary and unique shapes of proteins enable them to perform their various tasks in the body. Some form globular
or spherical structures that can carry and store materials within them, and
some, such as those of tendons, form linear structures that are more than
ten times as long as they are wide. The intricate shape a protein finally assumes gives it maximum stability.

Iron

Heme, the
nonprotein
portion of
hemoglobin,
holds iron.

The amino acid sequence
determines the shape
of the polypeptide chain.

Multiple Polypeptide Interactions—Quaternary Structures Some polypeptides are functioning proteins just as they are; others need to associate with other
polypeptides to form larger working complexes. The quaternary structure of proteins involves the interactions between two or more polypeptides. One molecule
of hemoglobin—the large, globular protein molecule that, by the billions, packs
the red blood cells and carries oxygen—is made of four associated polypeptide
chains, each holding the mineral iron (see Figure 6-5).
Protein Denaturation When proteins are subjected to heat, acid, or other condi-

tions that disturb their stability, they undergo denaturation—that is, they uncoil
and lose their shapes and, consequently, also lose their ability to function. Past
a certain point, denaturation is irreversible. Familiar examples of denaturation
include the hardening of an egg when it is cooked, the curdling of milk when acid
is added, and the stiffening of egg whites when they are whipped. In the body,
proteins are denatured when they are exposed to stomach acid.
Chemically speaking, proteins are more complex than
carbohydrates or lipids; they are made of some 20 different amino acids, 9 of
which the body cannot make (the essential amino acids). Each amino acid contains an amino group, an acid group, a hydrogen atom, and a distinctive side
group, all attached to a central carbon atom. Cells link amino acids together in
a series of condensation reactions to create proteins. The distinctive sequence
of amino acids in each protein determines its unique shape and function.
I N S U M M A RY
© Matthew Farruggio

CHAPTER 6

176

Cooking an egg denatures its proteins.

Digestion and Absorption of Proteins
hemoglobin (HE-moh-GLO-bin): the globular protein of
the red blood cells that carries oxygen from the lungs to
the cells throughout the body.
• hemo = blood
• globin = globular protein
denaturation (dee-NAY-chur-AY-shun): the change in
a protein’s shape and consequent loss of its function
brought about by heat, agitation, acid, base, alcohol,
heavy metals, or other agents.

Proteins in foods do not become body proteins directly. Instead, they supply
the amino acids from which the body makes its own proteins. When a person
eats foods containing protein, enzymes break the long polypeptide strands into
shorter strands, the short strands into tripeptides and dipeptides, and, fi nally, the
tripeptides and dipeptides into amino acids.

Protein Digestion

Figure 6-6 illustrates the digestion of protein through
the GI tract. Proteins are crushed and moistened in the mouth, but the real action
begins in the stomach.

FIGURE 6-6

Protein Digestion in the GI Tract
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PROTEIN
Mouth and salivary glands
Chewing and crushing moisten
protein-rich foods and mix them with
saliva to be swallowed
Mouth

Salivary
glands

Stomach
Hydrochloric acid (HCl) uncoils protein
strands and activates stomach
enzymes:
Pepsin,
HCI
Protein

Smaller
polypeptides

Stomach
(Esophagus)
Pancreatic
duct
(Liver)
Pancreas

(Gallbladder)

HYDROCHLORIC ACID
AND THE
DIGESTIVE ENZYMES
In the stomach:
Hydrochloric acid (HCl)
• Denatures protein structure
• Activates pepsinogen to pepsin
Pepsin
• Cleaves proteins to smaller
polypeptides and some free
amino acids
• Inhibits pepsinogen synthesis

Small intestine and pancreas

In the small intestine:

Pancreatic and small intestinal
enzymes split polypeptides further:

Enteropeptidasea
• Converts pancreatic trypsinogen
to trypsin

Polypeptides

Pancreatic
and
intestinal
proteases

Tripeptides,
dipeptides,
amino acids

Then enzymes on the surface of the
small intestinal cells hydrolyze these
peptides and the cells absorb them:
Intestinal
tripeptidases
and
dipeptidases
Peptides

Amino acids
(absorbed)

Trypsin
• Inhibits trypsinogen synthesis
• Cleaves peptide bonds next to
the amino acids lysine and
arginine
• Converts pancreatic
procarboxypeptidases to
carboxypeptidases
• Converts pancreatic
chymotrypsinogen to
chymotrypsin
Chymotrypsin
• Cleaves peptide bonds next to
the amino acids phenylalanine,
tyrosine, tryptophan, methionine,
asparagine, and histidine
Carboxypeptidases
• Cleave amino acids from the acid
(carboxyl) ends of polypeptides

Small
intestine

Elastase and collagenase
• Cleave polypeptides into smaller
polypeptides and tripeptides
Intestinal tripeptidases
• Cleave tripeptides to dipeptides
and amino acids
Intestinal dipeptidases
• Cleave dipeptides to amino acids
Intestinal aminopeptidases
• Cleave amino acids from the
amino ends of small polypeptides
(oligopeptides)
aEnteropeptidase was formerly known
as enterokinase.

In the Stomach The major event in the stomach is the partial breakdown (hydrolysis) of proteins. Hydrochloric acid uncoils (denatures) each protein’s tangled
strands so that digestive enzymes can attack the peptide bonds. The hydrochloric
acid also converts the inactive form ♦ of the enzyme pepsinogen to its active form,
pepsin. Pepsin cleaves proteins—large polypeptides—into smaller polypeptides
and some amino acids.
In the Small Intestine When polypeptides enter the small intestine, several

pancreatic and intestinal proteases hydrolyze them further into short peptide

♦ The inactive form of an enzyme is called a
proenzyme or a zymogen (ZYE-moh-jen).
pepsin: a gastric enzyme that hydrolyzes protein. Pepsin
is secreted in an inactive form, pepsinogen, which is
activated by hydrochloric acid in the stomach.

PROTEIN: AMINO ACIDS

177

CHAPTER 6

178

♦ A string of four to nine amino acids is an oligopeptide (OL-ee-go-PEP-tide).
• oligo = few

chains, ♦ t ripeptides, dipeptides, and amino acids. Then peptidase enzymes on
the membrane surfaces of the intestinal cells split most of the dipeptides and tripeptides into single amino acids. Only a few peptides escape digestion and enter
the blood intact. Figure 6-6 includes names of the digestive enzymes for protein
and describes their actions.

Protein Absorption A number of specific carriers transport amino acids
(and some dipeptides and tripeptides) into the intestinal cells. Once inside the
intestinal cells, amino acids may be used for energy or to synthesize needed compounds. Amino acids that are not used by the intestinal cells are transported across
the cell membrane into the surrounding fluid where they enter the capillaries on
their way to the liver.
Consumers lacking nutrition knowledge may fail to realize that most proteins
are broken down to amino acids before absorption. They may be misled by advertisements urging them to “Eat enzyme A. It will help you digest your food.” Or
“Don’t eat food B. It contains enzyme C, which will digest cells in your body.” In
reality, though, enzymes in foods are digested, just as all proteins are. Even the digestive enzymes—which function optimally at their specific pH—are denatured
and digested when the pH of their environment changes. The enzyme pepsin, for
example, which works best in the low pH of the stomach becomes inactive and
digested when it enters the higher pH of the small intestine.
Another misconception is that eating predigested proteins (amino acid supplements) saves the body from having to digest proteins and keeps the digestive system from “overworking.” Such a belief grossly underestimates the body’s abilities.
As a matter of fact, the digestive system handles whole proteins better than predigested ones because it dismantles and absorbs the amino acids at rates that are
optimal for the body’s use. (The last section of this chapter discusses amino acid
supplements further.)
Digestion is facilitated mostly by the stomach’s acid and
enzymes, which first denature dietary proteins, then cleave them into smaller
polypeptides and some amino acids. Pancreatic and intestinal enzymes split
these polypeptides further, to oligo-, tri-, and dipeptides, and then split most
of these to single amino acids. Then carriers in the membranes of intestinal
cells transport the amino acids into the cells, where they are released into the
bloodstream.
I N S U M M A RY

Proteins in the Body
♦ The study of the body’s proteins is called
proteomics.

♦ The human genome is the full set of chromosomes, including all of the genes and associated
DNA.

The human body contains an estimated 30,000 different kinds of proteins. Of
these, about 3000 have been studied, ♦ although this number is growing rapidly
w ith the recent surge in knowledge gained from sequencing the human genome.
♦ Only about 10 are described in this chapter—but these should be enough to illustrate the versatility, uniqueness, and importance of proteins. As you will see,
each protein has a specific function, and that function is determined during protein synthesis.

Protein Synthesis
peptidase: a digestive enzyme that hydrolyzes peptide
bonds. Tripeptidases cleave tripeptides; dipeptidases
cleave dipeptides. Endopeptidases cleave peptide bonds
within the chain to create smaller fragments, whereas
exopeptidases cleave bonds at the ends to release free
amino acids.
• tri = three
• di = t wo
• endo = w ithin
• exo = outside

Each human being is unique because of small differences in the body’s proteins. These differences are determined by the amino acid
sequences of proteins, which, in turn, are determined by genes. The following
paragraphs describe in words the ways cells synthesize proteins; Figure 6-7 provides a pictorial description. Protein synthesis depends on a diet that provides
adequate protein and essential amino acids.
The instructions for making every protein in a person’s body are transmitted
by way of the genetic information received at conception. This body of knowledge,
which is filed in the DNA (deoxyribonucleic acid) within the nucleus of every cell,
never leaves the nucleus.

FIGURE 6-7

Protein Synthesis
Cell

DNA

Nucleus
DNA

mR

NA

1 The DNA serves as a template to make strands
of messenger RNA (mRNA). Each mRNA
strand copies exactly the instructions for
making some protein the cell needs.
Ribosomes
(protein-making
machinery)

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3 The mRNA attaches itself to the protein-making
machinery of the cell, the
ribosomes.
Ribosome

2 The mRNA leaves
the nucleus through the
nuclear membrane. DNA
remains inside the nucleus.

A

mRN

4 Another form of RNA, transfer RNA (tRNA), collects
amino acids from the cell fluid. Each tRNA carries
its amino acids to the mRNA, which dictates the
sequence in which the amino acids will be
attached to form the protein strands. Thus the
mRNA ensures the amino acids are lined
up in the correct sequence.
Amino acid
tRNA

A

mRN

5 As the amino acids are lined up in the right
sequence, and the ribosome moves along
the mRNA, an enzyme bonds one amino
acid after another to the growing protein
strand. The tRNA are freed to return for
more amino acids. When all the amino
acids have been attached, the
completed protein is released.

Protein strand

A

mRN

6 Finally, the mRNA and ribosome separate. It takes
many words to describe these events, but in the cell,
40 to 100 amino acids can be added to a growing
protein strand in only a second. Furthermore, several
ribosomes can simultaneously work on the same
mRNA to make many copies of the protein.

PROTEIN: AMINO ACIDS

179

Delivering the Instructions Transforming the information in DNA…