Organic Chemistry

Section 20.11 Biological Oxidation–Reduction Reactions 869

rest of the molecule is used to bind or NADH to the proper site on the enzyme

that catalyzes the reaction.

oxidizes a compound by accepting a hydride ion from it. In this way, the

number of carbon–hydrogen bonds in the compound decreases (the compound is oxi-

dized) and the number of carbon–hydrogen bonds in increases ( is re-

duced). can accept a hydride ion at the 4-position of the pyridine ring because

the electrons can be delocalized onto the positively charged nitrogen atom. Although

could also accept a hydride ion at the 2-position, the hydride ion is always de-

livered to the 4-position in enzyme-catalyzed reactions.

N+

R

O

CNH 2

N

R

O

CNH 2

ethanol

NAD+

1

2

3

4

5

6

NADH

acetaldehyde

H

H

CH 3 OH ++

H H

H

CH 3

C O + H+

H

CH 3

C OH

+

NAD+

NAD+

NAD+ NAD+

NAD+

N+

O

CNH 2

a pyridine ring H H

OH

O

P

HO

OH

O

HO

O

O

−O

O

P

O

−O

nicotinamide adenine dinucleotide

NAD+

N N

N N

NH 2

N

O

CNH 2

O

reduced nicotinamide adenine dinucleotide

NADH

OH

O

P

HO

OH

O

HO

O

O

−O

O

P

O

−O

N N

N N

NH 2

O

NAD+

causes violently unpleasant effects when ethanol is consumed,

even when it is consumed a day or two after Antabuse®is taken.

TREATING ALCOHOLICS WITH

ANTABUSE

Disulfiram, most commonly known by one of its

trade names, Antabuse®, is used to treat alcoholics. The drug

O

CH 3 CH 2 OH CH 3 CH

ethanol acetaldehyde

O

CH 3 COH

acetic acid

alcohol

dehydrogenase

aldehyde

dehydrogenase

3-D Molecules:

NAD+;NADH

S S

N C S S C N

CH 3 CH 2 CH 2 CH 3

CH 3 CH 2 CH 2 CH 3

Antabuse

Antabuse®inhibits aldehyde dehydrogenase, the enzyme re-

sponsible for oxidizing acetaldehyde to acetic acid, resulting in a

buildup of acetaldehyde. Acetaldehyde causes the unpleasant

physiological effects of intoxication: intense flushing, nausea,

dizziness, sweating, throbbing headaches, decreased blood pres-

sure, and, ultimately, shock. Consequently, Antabuse®should be

taken only under strict medical supervision. In some people,

aldehyde dehydrogenase does not function properly. Their

symptoms in response to ingesting alcohol are nearly the same

as those of individuals who are medicated with Antabuse®.

BRUI20_841_882r3 01-04-2003 1:11 PM Page 869