Organic Chemistry

ornithine

H 3 NCH 2 CH 2 CH 2 CHCO−

O

+

+NH

3

L-Val

L-Val

L-Pro L-Orn

L-Phe L-Leu

L-Leu D-Phe

D-Orn L-Pro

gramicidin S

976 CHAPTER 23 Amino Acids, Peptides, and Proteins

PROBLEM 20

a. How many different octapeptides can be made from the 20 naturally occurring amino

acids?

b. How many different proteins containing 100 amino acids can be made from the 20 nat-

urally occurring amino acids?

PROBLEM 21

Which bonds in the backbone of a peptide can rotate freely?

23.8 Some Interesting Peptides

Enkephalinsare pentapeptides that are synthesized by the body to control pain. They

decrease the body’s sensitivity to pain by binding to receptors in certain brain cells.

Part of the three-dimensional structures of enkephalins must be similar to those of

morphine and painkillers such as Demerol because they bind to the same receptors

(Sections 30.3 and 30.6).

Bradykinin, vasopressin, and oxytocin are peptide hormones. They are all nona-

peptides. Bradykinin inhibits the inflammation of tissues. Vasopressin controls

blood pressure by regulating the contraction of smooth muscle. It is also an antidi-

uretic. Oxytocin induces labor in pregnant women and stimulates milk production in

nursing mothers. Vasopressin and oxytocin both have an intrachain disulfide bond,

and their C-terminal amino acids contain amide rather than carboxyl groups. Notice

that the C-terminal amide group is indicated by writing after the name of the

C-terminal amino acid. In spite of their very different physiological effects, vaso-

pressin and oxytocin differ only by two amino acids.

Gramicidin S is an antibiotic produced by a strain of bacteria. It is a cyclic dec-

apeptide. Notice that it contains the amino acids L-ornithine (L-Orn),D-ornithine

(D-Orn), and also D-phenylalanine. Ornithine is not listed in Table 23.1 because it oc-

curs rarely in nature. Ornithine resembles lysine, but has one less methylene group in

its side chain.

The synthetic sweetener aspartame, or NutraSweet (Section 22.21), is the methyl

ester of a dipeptide of L-aspartate and L-phenylalanine. Aspartame is about 200 times

sweeter than sucrose. The ethyl ester of the same dipeptide is not sweet. If a D-amino

acid is substituted for either of the L-amino acids of aspartame, the resulting dipeptide

is bitter rather than sweet.

COO−

CH 2 CH 2

+

O

H 3 NCHC NHCHCOCH 3

O

aspartame

NutraSweet

®

bradykinin Arg-Pro-Pro-Gly-Phe-Ser-Pro-Phe-Arg

vasopressin Cys-Tyr-Phe-Gln-Asn-Cys-Pro-Arg-Gly-NH 2

SS

oxytocin Cys-Tyr-Ile-Gln-Asn-Cys-Pro-Leu-Gly-NH 2

SS

“NH 2 ”

leucine enkephalin

Tyr-Gly-Gly-Phe-Leu

methionine enkephalin

Tyr-Gly-Gly-Phe-Met

®

Oxytocin was the first small peptide

to be synthesized. Its synthesis was

achieved in 1953 by Vincent du

Vigneaud (1901–1978), who later

synthesized vasopressin. Du Vi-

gneaud was born in Chicago and was

a professor at George Washington

University Medical School and later

at Cornell University Medical

College. For synthesizing these

nonapeptides, he received the Nobel

Prize in chemistry in 1955.

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