Human Physiology, 14th edition (2016)

Chemical Composition of the Body 41

The enormous diversity of protein structure results from the
fact that there are 20 different building blocks—the amino
acids —that can be used to form a protein. These amino acids,
as will be described in the next section, are joined together
to form a chain. Because of chemical interactions between
the amino acids, the chain can twist and fold in a specific
manner. The sequence of amino acids in a protein, and thus
the specific structure of the protein, is determined by genetic
information. This genetic information for protein synthesis
is contained in another category of organic molecules, the
nucleic acids, which includes the macromolecules DNA and
RNA. The structure of nucleic acids is described in the next
section, and the mechanisms by which the genetic informa-
tion they encode directs protein synthesis are described in
chapter 3.

2.3 Proteins

Proteins are large molecules composed of amino acid

subunits. There are about 20 different types of amino

acids that can be used in constructing a given protein,

so the variety of protein structures is immense. This vari-

ety allows each type of protein to perform very specific

functions.

Structure of Proteins

Proteins consist of long chains of subunits called amino

acids. As the name implies, each amino acid contains an amino

group (NH 2 ) on one end of the molecule and a carboxyl group

(COOH) on another end. There are about 20 different amino

acids, each with a distinct structure and chemical properties,

that are used to build proteins. The differences between the

amino acids are due to differences in their functional groups.

R is the abbreviation for the functional group in the general

formula for an amino acid ( fig. 2.26 ). The R symbol actually

stands for the word residue, but it can be thought of as indicat-

ing the “ rest of the molecule.”

When amino acids are joined together by dehydration

synthesis, the hydrogen from the amino end of one amino

acid combines with the hydroxyl group in the carboxyl end

of another amino acid. As a covalent bond is formed between

the two amino acids, water is produced ( fig. 2.27 ). The bond

between adjacent amino acids is called a peptide bond, and

the compound formed is called a peptide. Two amino acids

bound together are called a dipeptide; three, a tripeptide. When

Figure 2.26 Representative amino acids. The figure

depicts different types of functional (R) groups. Each amino acid

differs from other amino acids in the number and arrangement of

atoms in its functional groups.

Carboxyl group

Functional group

Amino group

Nonpolar amino acids

Polar amino acids

Valine Tyrosine

Basic

H 2 N

Sulfur-containing Acidic

Arginine Cysteine Aspartic acid

H

H

H

H H

H H

H H

N

R

O

O O

OH

OH OH

OH

C

CC CC

C

N

C

N

C

H 3 C

CH 2

CH 3

CH

H

H

H

O

OH

NH

NH

C

C

CN

(CH 2 ) 3 CH 2 CH 2

H

H

H

O

N CCOH

H

H

H

O

O

OH

OH

C

C

CN

CH

HC CH

HC

SH

| CHECKPOINTS

5a. Describe the structural characteristic of all

carbohydrates, and distinguish between

monosaccharides, disaccharides, and

polysaccharides.

5b. Describe the characteristics of a lipid, and discuss

the different subcategories of lipids.

6. Explain, in terms of dehydration synthesis and
   hydrolysis reactions, how disaccharides and
   mono saccharides can be interconverted and how
   triglycerides can be formed and broken down.


7. Relate the functions of phospholipids to their
   structure, and explain the significance of the
   prostaglandins.

LEARNING OUTCOMES

After studying this section, you should be able to:

8. Describe amino acids and explain how peptide
   bonds between them are formed and broken.


9. Describe the different orders of protein structure,
   the different functions of proteins, and how protein
   structure grants specificity of function.