Human Physiology, 14th edition (2016)

42 Chapter 2

numerous amino acids are joined in this way, a chain of amino
acids, or a polypeptide, is produced.
The lengths of polypeptide chains vary widely. A hormone
called thyrotropin-releasing hormone, for example, is only
three amino acids long, whereas myosin, a muscle protein,
contains about 4,500 amino acids. When the length of a poly-
peptide chain becomes very long (containing more than about
100 amino acids), the molecule is called a protein.

The structure of a protein can be described at four different

levels. The first level of structure describes the sequence of amino

acids in the particular protein; this is the primary structure of

the protein. Each type of protein has a different primary structure.

All of the billions of copies of a given type of protein in a person

have the same structure, however, because the structure of a given

protein is coded by the person’s genes. The primary structure of a

protein is illustrated in figure 2.28 a.

Figure 2.28 The structure of proteins. The primary structure ( a ) is the sequence of amino acids in the polypeptide chain.
The secondary structure is the conformation of the chain created by hydrogen bonding between amino acids; this can be either an
alpha helix ( b ) or a beta pleated sheet ( c ). The tertiary structure ( d ) is the three-dimensional structure of the protein. The formation of
a protein by the bonding together of two or more polypeptide chains is the quaternary structure ( e ) of the protein. Hemoglobin, the
protein in red blood cells that carries oxygen, is used here as an example.

(a) Primary structure

(polypeptide strand)

(b) Secondary structure

(α helix)

(d) Tertiary structure

(c) Secondary structure (β pleated sheet)

(e) Quaternary structure

(hemoglobin)

Heme

group

Amino acid 3 Amino acid 2 Amino acid 1

O

R

H

N N

H

R

O H

R H

H

α helix

C C C

C C

Figure 2.27 The formation of
peptide bonds by dehydration synthesis
reactions. Water molecules are split off as the
peptide bonds (highlighted in red) are produced
between the amino acids.

H

H

R

N

H

O

CC

H

H

R

N

H

O

CC

H

H

N

R

O

CC

OH

OH

H

R H

N

H

N

H O

CC

H

N

R

O

CC

H H

N

R

O

CC

R

H O

CC

H OH

H

H

N

R

O

CC

OH

H

+ 3H 2 O