Organic Chemistry

1052 CHAPTER 25 The Organic Mechanisms of the Coenzymes • Metabolism

Notice the similar function of TPP in all TPP-requiring enzymes. In each reaction,

the nucleophilic coenzyme attacks a carbonyl group of the substrate and allows a bond

to that carbonyl group to be broken because the electrons left behind can be delocal-

ized into the thiazolium ring. The resulting two-carbon fragment is then transferred—

to a proton in the case of pyruvate decarboxylase, to coenzyme A (via lipoate) in the

pyruvate dehydrogenase system, and to a carbonyl group in Problems 8, 9, and 10.

PROBLEM 11

An unfortunate effect of drinking too much alcohol—a hangover—is attributable to the

acetaldehyde formed when ethanol is oxidized. There is some evidence that vitamin can

cure a hangover. How can the vitamin do this?

25.5 Biotin: Vitamin H

Biotin(vitamin H) is an unusual vitamin because it can be synthesized by bacteria that

live in the intestine. Consequently, biotin does not have to be included in our diet and

deficiencies are rare. Biotin deficiencies, however, can be found in people who main-

tain a diet high in raw eggs. Egg whites contain a protein (avidin) that binds biotin

tightly and thereby prevents it from acting as a coenzyme. When eggs are cooked,

avidin is denatured, and the denatured protein does not bind biotin. Biotin is attached

to its enzyme by forming an amide with the amino group of a lysine side chain.

HN NH

CH 2 CH 2 CH 2 CH 2 COH

O

O

S

HN NH

CH 2 CH 2 CH 2 CH 2 CNH(CH 2 ) 4 E

O

O

S

biotin enzyme-bound biotin

B 1

B−

H B

B

COH

H

R

CH 2 OH

N

C

−

+

C

S N+

−

HO

C

H

H OH

H OH

H OH

O

CH 2 OH

CH 2 OPO 32 − CH^2 OPO^32 −

CH 2 OPO 32 −

HO

H O C CH 2 OH

H OH

H

H OH

H OH

H OH

HC O

H OH

H OH

CH 2 OPO 32 −

H OH

R

C

S N+ R

HC O

+

+

S