Organic Chemistry

1040 CHAPTER 25 The Organic Mechanisms of the Coenzymes • Metabolism

The heterocyclic component of one of the nucleotides of is nicotinamide, and

the heterocyclic component of the other is adenine. This accounts for the coenzyme’s

name (nicotinamide adenine dinucleotide). The positive charge in the abbrevi-

ation indicates the positively charged nitrogen of the substituted pyridine ring.

The only way in which differs structurally from is in the phosphate

group bonded to the group of the ribose of the adenine nucleotide; this

explains the addition of “P”to the name. and NADH are generally used as

coenzymes in catabolic reactions; and NADPH are generally used as coen-

zymes in anabolic reactions.

The adenine nucleotide for the coenzymes is provided by ATP. The nicotinamide

nucleotide is not synthesized by biological systems but is derived instead from the

vitamin known as niacin.

HO OH

O− O− O−

−O OOO O

P P P

OOO

N

N N

N

NH 2

adenosine triphosphate

ATP

NADP+

NAD+

2 ¿-OH

NADP+ NAD+

NAD+

NAD+

N

N H N

−O O O N

O−

HO OH

NH 2

N

O O

P heterocycle CNH 2

N

O

COH

a nucleotide adenine niacinamide

nicotinamide

niacin

nicotinic acid

3-D Molecules:

Niacin;

Nicotinamide;

Reduced form of nicotinamide

NIACIN DEFICIENCY

A deficiency in niacin causes pellagra, a disease

that begins with dermatitis and ultimately causes

insanity and death. More than 120,000 cases of pellagra were

reported in the United States in 1927, mainly among poor peo-

ple with unvaried diets. A factor known to be present in prepa-

rations of vitamin B prevented pellagra, but it was not until

1937 that the factor was identified as nicotinic acid.

When bread companies started adding nicotinic acid to their

bread, they insisted that its name be changed to niacin because

they thought that nicotinic acid sounded too much like nicotine

and they did not want their vitamin-enriched bread to be associ-

ated with a harmful substance. Niacinamide is a nutritionally

equivalent form of the vitamin.

The oxidation of the secondary alcohol groupof malate to a ketone groupis one of the

reactions in the citric acid cycle (Figure 25.2). is the oxidizing reagent in this

reaction. Many enzymes that catalyze oxidation reactions are called dehydrogenases.

Recall that the number of bonds decreases in an oxidation reaction (Section 20.0).

In other words, dehydrogenases remove hydrogen.

-semialdehyde is reduced to homoserine in an anabolic pathway, with

NADPH as the reducing agent.

b-Aspartate

O HO O

+

malate

−OCCH

2 CHCO

− NAD+

O OO

+

oxaloacetate

−OCCH

2 CCO

− NADH + H+

malate

dehydrogenase

C¬H

NAD+