HUMAN BIOLOGY

34 Chapter 2

glycoprotein Protein that

has a sugar, such as an oli-

gosaccharide, attached to it.

lipoprotein Protein that has

a lipid attached to it.

a protein’s shape and function

n    When amino acids have been assembled into a protein, the

protein folds into its final shape.

n    A protein’s final shape determines its function.

proteins fold into complex shapes

that determine their function

As you have just read, a protein’s primary structure is the

first step in the formation of a functioning protein (Figure

2.26A). Secondary structure emerges as the chain twists,

bends, loops, and folds. These shape changes occur as

hydrogen bonds form between different amino acids in

different parts of the chain (Figure 2.26B). Even though

the primary structure of each protein is unique, similar

patterns of coils, sheets, and loops occur in most proteins.

The coils, sheets, and loops of a protein fold up even more,

much like an overly twisted rubber band. This is the third

level of organization, or tertiary structure, of a protein (Figure

2.26C). Tertiary structure is what makes a protein a molecule

that can perform a particular function. For instance, some

proteins fold into a hollow “barrel” that provides a channel

through which substances can move into or out of cells.

a protein may have more than

one polypeptide chain

Some proteins are built of more than one polypeptide

chain. This type of protein has quaternary structure (Fig-

ure 2.26D). Interactions between its

polypeptide chains (such as hydro-

gen bonds) hold the chains together.

In some cases the links include cova-

lent bonds between sulfur atoms

of R groups. These bonds between

two sulfur atoms are called disulfide

bridges (di = t wo).

The hormone insulin is an example

of a protein with quaternary struc-

ture. So is hemoglobin, a protein in

red blood cells that binds oxygen. It has four molecules

of globin, as well as an iron-containing functional group

(called a heme group) near the center of each globin mol-

ecule. Each of the millions of red blood cells in your body is

transporting a billion molecules of oxygen, bound to some

250 million molecules of hemoglobin. You will learn more

the function of hemoglobin in Chapter 8.

Hemoglobin and insulin are globular proteins. So are

most enzymes. Many other proteins with quaternary

structure are fibrous—like heavy-duty thread, they are

elongated and strong. An example is collagen, the most

common protein in the body. Your skin, bones, corneas,

and other body parts depend on its strength. Multiple

polypeptide chains of some proteins may be organized into

coils or sheets. Keratin, a structural protein of hair, is like

this (Figure 2.27).

glycoproteins have sugars attached

and lipoproteins have lipids attached

Some proteins have other organic compounds attached to

their polypeptide chains. For example, lipoproteins form

when certain proteins circulating in blood combine with

cholesterol, triglycerides, and phospholipids that were

consumed in food. Most glycoproteins (from glukus, the

Greek word for “sweet”) have oligosaccharides bonded to

them. Most of the proteins found at the surface of cells are

glycoproteins, as are many proteins in blood and those that

cells secrete (such as protein hormones).

Figure 2.26 Animated! Proteins can have up to four levels of organization. (© Cengage Learning)

lysine glycine glycine arginine

A The primary structure of a
protein is its linear sequence
of amino acids. This string of
amino acids is a polypeptide
chain.

B Secondary structure comes

about as a polypeptide chain

twists or folds. Hydrogen bonds

hold the molecule in this shape.

C More folding of the chain

produces a protein’s tertiary

structure—its overall three-

dimensional shape. The

folding results in pockets or

crevices that establish

how a protein will function

chemically.

D In proteins that have quater-

nary structure, bonds and other

forces hold two or more poly-

peptide chains together in one

molecule. This example shows

hemoglobin, which consists of

four chains (here colored green

or blue). A pocket in each chain

holds a heme group (red) that

contains an iron atom.

Disulfide bridges

2.12

© Cengage Learning

Copyright 2016 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part. Due to electronic rights, some third party content may be suppressed from the eBook and/or eChapter(s).