Human Physiology, 14th edition (2016)

Chemical Composition of the Body 43

Weak hydrogen bonds may form between the hydrogen
atom of an amino group and an oxygen atom from a different
amino acid nearby. These weak bonds cause the polypeptide
chain to assume a particular shape, known as the secondary
structure of the protein ( fig. 2.28 b,c ). This can be the shape of
an alpha ( a ) helix, or alternatively, the shape of what is called
a beta ( b ) pleated sheet.
Most polypeptide chains bend and fold upon themselves to
produce complex three-dimensional shapes called the tertiary
structure of the protein ( fig. 2.28 d ). Each type of protein has its
own characteristic tertiary structure. This is because the folding
and bending of the polypeptide chain is produced by chemical
interactions between particular amino acids located in different
regions of the chain.
Most of the tertiary structure of proteins is formed and
stabilized by weak chemical interactions between the func-
tional groups of amino acids located some distance apart along
the polypeptide chain. In terms of their strengths, these weak
interactions are relatively stronger for ionic bonds, weaker
for hydrogen bonds, and weakest for van der Waals forces
( fig. 2.29 ). The natures of ionic bonds and hydrogen bonds
have been previously discussed. Van der Waals forces are weak
forces between electrically neutral molecules that come very
close together. These forces occur because, even in electri-
cally neutral molecules, the electrons are not always evenly
distributed but can at some instants be found at one end of the
molecule.
Because most of the tertiary structure is stabilized by weak
bonds, this structure can easily be disrupted by high temper-
ature or by changes in pH. Changes in the tertiary structure
of proteins that occur by these means are referred to as dena-
turation of the proteins. The tertiary structure of some pro-
teins, however, is made more stable by strong covalent bonds
between sulfur atoms (called disulfide bonds and abbreviated
S—S) in the functional group of an amino acid known as cys-
teine ( fig. 2.29 ).
Denatured proteins retain their primary structure (the pep-
tide bonds are not broken) but have altered chemical proper-
ties. Cooking a pot roast, for example, alters the texture of the
meat proteins—it doesn’t result in an amino acid soup. Dena-
turation is most dramatically demonstrated by frying an egg.
Egg albumin proteins are soluble in their native state in which
they form the clear, viscous fluid of a raw egg. When denatured

by cooking, these proteins change shape, cross-bond with each

other, and by this means form an insoluble white precipitate—

the egg white.

Hemoglobin and insulin are composed of a number of

polypeptide chains covalently bonded together. This is the

quaternary structure of these molecules. Insulin, for exam-

ple, is composed of two polypeptide chains—one that is

21 amino acids long, the other that is 30 amino acids long.

Hemoglobin (the protein in red blood cells that carries oxy-

gen) is composed of four separate polypeptide chains (see

fig. 2.28 e ). The composition of various body proteins is

shown in table 2.4.

Protein Number of Polypeptide Chains Nonprotein Component Function

Hemoglobin 4 Heme pigment Carries oxygen in the blood

Myoglobin 1 Heme pigment Stores oxygen in muscle

Insulin 2 None Hormonal regulation of metabolism

Blood group proteins 1 Carbohydrate Produces blood types

Lipoproteins 1 Lipids Transports lipids in blood

Table 2.4 | Composition of Selected Proteins Found in the Body

Figure 2.29 The bonds responsible for the tertiary

structure of a protein. The tertiary structure of a protein is

held in place by a variety of bonds. These include relatively weak

bonds, such as hydrogen bonds, ionic bonds, and van der Waals

(hydrophobic) forces, as well as the strong covalent disulfide

bonds.

C

2

HO C O

Ionic bond

S

S

C

H

H 2 C CH 3

H

C

CH 2

CH 3

H 3 C

H 3 C

OH

• O OC

+NH 3

Hydrogen

bond

Disulfide bond

(covalent)

van der Waals

forces