HUMAN BIOLOGY

32 Chapter 2

valine (val) tryptophan (trp) methionine (met)

Amino Group Carboxyl Group

R Group (20 kinds, each

with distinct properties)

R

+H

H

H

N

H

C C

O

O

CH 3

CH 3

+H

H

H H

HC

NCC

O

O +H

H

H H

NCC

O

O

N

H

C

+H

H

H H

NCC

O

O

C

C

S

CH 3

proteins: Biological molecules with many roles

n    Proteins are the most diverse biological molecules.

A protein is an organic compound built of one or more

chains of amino acids. Biochemists estimate that the

human body contains roughly 30,000 proteins that can

be sorted into categories based on their general function

(Table 2.4). For instance, proteins called enzymes speed

up chemical reactions. Structural proteins are building

blocks of cells and tissues in bones, muscles, and other

parts. Transport proteins move substances. A variety of

regulatory proteins, including some hormones, adjust

cell activities. They help make possible activities such as

waking, sleeping, and engaging in sex, to cite just a few.

Other proteins are important in body defenses.

proteins are built

from amino acids

Amazingly, our body cells build

thousands of different proteins from

only twenty kinds of amino acids.

An amino acid is a small organic

compound that consists of an amino

group, a carboxyl group (an acid), an

atom of hydrogen, and one or more

atoms called its R group. As you can

see from the structural formula in

Figure 2.24A, these parts generally

are covalently bonded to the same

carbon atom. R groups include func-

tional groups, which help determine

an amino acid’s chemical properties.

valine (val) tryptophan (trp) methionine (met)

Amino Group Carboxyl Group

R Group (20 kinds, each

with distinct properties)

R

+H

H

H

N

H

C C

O

O

CH 3

CH 3

+H

H

H H

HC

NCC

O

O +H

H

H H

NCC

O

O

N

H

C

+H

H

H H

NCC

O

O

C

C

S

CH 3

A

B

PhotoDisc/Getty Images;

inset: © matin/Shutterstock.com

2.11

Figure 2.24 All amino acids have the same basic chemical parts. As

you can see in A, these building blocks are an amino group, a carboxyl

group, an R group, and a hydrogen atom, all connected to a carbon

atom by covalent bonds. A variety of foods provide these small organic

compounds. B shows structural formulas for three common amino

acids human cells use. (© Cengage Learning)

Type of Protein Examples

Structural Serves as building materials for cells

and tissues. Examples: Girderlike

support fibers inside cells; collagen

fibers that strengthen skin.

Enzyme Speeds up chemical reactions.

Example: Digestive enzymes that

speed the breakdown of complex

carbohydrates, fats, and dietary

proteins in the digestive system.

Transport Carries substances in body fluids or

moves them into or out of cells.

Examples: The protein hemoglobin,

which carries oxygen to cells; proteins

that pump ions into or out of cells.

Movement Produces movements of cells and cell

parts. Examples: Contraction of muscle

cells; swimming by sperm cells.

Regulator Adjusts cell activities. Examples:

Hormones such as sex hormones that

govern puberty; insulin, which regulates

blood sugar.

Receptor Binds molecules to or inside cells.

Example: Receptors that bind hormones

to target cells.

Defense Assists in immune responses and other

bodily defenses. Examples: Antibodies

that attach to invading organisms and

molecules; proteins that identify cells as

“self” (belonging to a given person’s body).

Table 2.4 Some Roles Proteins Play in the Body

amino acid Small organic

compounds that are the

building blocks of proteins.

peptide bond Covalent

bond that joins the amino

group of one amino acid to

the carboxyl group of a sec-

ond amino acid.

polypeptide chain Chain of

three or more amino acids

joined by peptide bonds.

primary structure Sequence

of amino acids that makes

up a given protein.

protein Organic compound

composed of one or more

chains of amino acids.

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