Organic Chemistry

1044 CHAPTER 25 The Organic Mechanisms of the Coenzymes • Metabolism

25.3 Flavin Adenine Dinucleotide and Flavin

Mononucleotide: Vitamin

A flavoproteinis an enzyme that contains either flavin adenine dinucleotide (FAD)

or flavin mononucleotide (FMN)as a coenzyme. FAD and FMN, like and

are coenzymes used in oxidation reactions. As its name indicates, FAD is a

dinucleotide in which one of the heterocyclic components is flavin and the other is

adenine. FMN contains flavin but not adenine—it is a mononucleotide. (Flavin is a

bright yellow compound; flavusis Latin for “yellow.”) Notice that instead of ribose,

the flavin nucleotide has a ribitol group (a reduced ribose). Flavin plus ribitol is called

riboflavin. Riboflavin is also known as vitamin A vitamin deficiency causes

inflammation of the skin.

In most flavoproteins, FAD (or FMN) is bound quite tightly. Tight binding allows

the enzyme to control the oxidation potential of the coenzyme. (The more positive the

oxidation potential, the stronger is the oxidizing agent.) Consequently, some flavopro-

teins are stronger oxidizing agents than others.

PROBLEM 2

FAD is obtained by an enzyme-catalyzed reaction that uses FMN and ATP as substrates.

What is the other product of this reaction?

How can we tell which enzymes use FAD (or FMN) rather than (or )

as the oxidizing coenzyme? A rough guideline is that and are the coen-

zymes used in enzyme-catalyzed oxidation reactions involving carbonyl compounds

(alcohols being oxidized to ketones, aldehydes, or carboxylic acids), while FAD and

FMN are the coenzymes used in other types of oxidations. For example, in the following

reactions, FAD oxidizes a dithiol to a disulfide, an amine to an imine, and a saturated

alkyl group to an unsaturated alkene, and FMN oxidizes NADH to (This is only

an approximate guideline, however, because FAD is involved in some oxidations that

involve carbonyl compounds, and and are involved in some oxidations

that do not involve carbonyl compounds.)

NAD+ NADP+

NAD+.

NAD+ NADP+

NAD+ NADP+

FMN

CH 2

O

O

NH

N N

N

HOH

HOH

HOH

CH 2

H 3 C

H 3 C

O−

OO−

P

O

B 2. B 2

NADP+,

NAD+

B 2

3-D Molecules:

Flavin adenine dinucleotide

(FAD);

Flavin mononucleotide (FMN)

FAD

CH 2

O

O

NH

N N

N

HOH

HOH

HOH

CH 2

H 3 C

H 3 C

9 10 1

8

7

6 5 4

4a

3

2

adenine

riboflavin

flavin

ribitol

N N

N N

NH 2

HO OH

O

O−

O O

P

O

O−

O

P

O