HUMAN BIOLOGY

Chemistry of Life 33

What are proteins?

• Proteins are organic molecules built of one or more chains of
  amino acids.


• DNA determines the order of amino acids in the chain. The
  sequence is unique for each type of protein.


• The sequence of amino acids that makes up a protein is the
  protein’s primary structure.


• Proteins perform many functions in the body. Some are building
  blocks, others transport substances, and still others regulate
  body functions or serve in defense.

taKe-home message

the sequence of amino acids is
a protein’s primary structure

When a cell makes a protein, amino acids become linked,
one after the other, by peptide bonds. As Figure 2.25
shows, this is the type of covalent bond that forms between
one amino acid’s amino group (NH 31 ) and the carboxyl
group (— COO^2 ) of the next amino acid.
When peptide bonds join two amino acids together,
we have a dipeptide. When they join three or more amino
acids, we have a polypeptide chain.
Each type of polypeptide chain, and therefore each
type of protein, has its own unique sequence of amino
acids. The sequence forms as different amino acids are
added in a specific order, one at a time, from the twenty
kinds available to body cells. Figure 2.24B gives you
an idea of how different amino acids can vary in their
chemical structure.
As Chapter 21 describes, DNA determines the order in
which amino acids are added to the growing chain. Every
kind of protein in the body will have its own sequence of
amino acids, linked one to the next like the links of a chain.
This sequence is called the primary structure of a protein.
This is a representation of the primary structure of a small
but vital protein in humans, the hormone insulin, which
consists of just fifty-one amino acids:

Figure 2.25 Animated! A protein is built as peptide bonds form between amino acids. DNA determines the order of amino
acids in a polypeptide chain. A When the chain starts to form, a peptide bond forms between the first two amino acids—here,
methionine and serine. Notice that the bond forms between the carboxyl group of the methionine and the amino group of the serine.
Peptide bonds are formed during condensation reactions, so as each one joins amino acids, water forms as well. B More amino
acids are added to the chain, with a peptide bond linking each one to the next in line. (© Cengage Learning)

H OH OH

methionine

CH 2

CH 2

CH 2

HOH

CH 3

S

OH

serine

CH 2

OH

+

methionine

methionine serine arginine glutamine

CH 2

CH 2

H

CH 3

S

serine

A B

CysLeuHisGln

Asn

Val

Phe

LeuAlaGluValLeuHisSerGly

Ty r

Leu

Val

PhePheGlyArgGluGlyCys

Ty r

Thr

ProLysAla

CysTy rAsnGluLeuGlnTy rLeuSerCys

Val

S

Asn

GlyIleValGluGlnCysCys

Ala

Ser

S

S

S

S

S

© Cengage Learning

Dietary supplements of some amino acids sometimes

are promoted to consumers as helpful for “curing”

herpes infections or providing relief from depression

or some other health problem. Visit the website of

the federal Food and Drug Administration (FDA) at

http://www.fda.gov and check out information there on

food supplements. Does the FDA regulate dietary

supplements? Who is responsible for ensuring the

safety of amino acid supplements?

THInk OuTSIDE THE BOOk

A large number of amino acids can

be linked up this way. The primary struc-

ture of the largest known protein, which

is a building block of human muscle, is a

string of 27,000 amino acids!

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