Biology of Disease

Vitamin B 1 or thiamin is an essential component of the coenzyme or prosthetic

group, thiamin pyrophosphate (TPP, Figure 10.10). This is necessary for the

actions of some enzymes, for example transketolase activity of the pentose

phosphate pathway for oxidizing glucose and oxidative decarboxylations

catalyzed by pyruvate and 2-oxoglutarate dehydrogenases in carbohydrate

metabolism. Thiamin is therefore necessary for the metabolic formation of ATP,

the major energy carrier in metabolism, and NADPH. Thiamin pyrophosphate

is also known to function in nerve conductance. Vitamin B 2 or riboflavin is

also required to form coenzymes or prosthetic groups, in this case flavin

mononucleotide (FMN, Figure 10.11 (A)) and flavin dinucleotide (FAD, Figure

10.11 (B)), which function as electron carriers in flavoproteins. The flavoproteins

are extremely numerous and include renal L-amino acid oxidase, NADH

reductase, 2-hydroxyacid oxidase (FMN) and D- and L-amino acid oxidases,

succinate dehydrogenase and glutathione reductases (FAD). Hence riboflavin

is essential to many oxidation and reduction reactions in, for example, the TCA

cycle, electron transport and the oxidation of fatty acids in mitochondria.

X]VeiZg&%/ DIET AND DISEASE

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H

CH 2

CH 2 CH 2

N

N

N

NH 2

CH 3

S

H 3 C

O

O P

O-

O

O P O-

O-

Figure 10.10 The structure of TPP.

Figure 10.11 The structures of (A) FMN and (B) FAD.

HH

H 3 C

H 3 C

N

N

O

O

• O P

O-

C

C HH

C

C

H

H

H

HO

HO

HO

C

NH

N O

O

A)

O

H H

HO OH

H H

H

H

HH

H

H

H

N

N N

N

NH 2

CH 2

H 3 C

H 3 C

N

N

O

O

O

P

O

O

• O P


• O

C

C HH

C

C

HO

HO

HO

C

NH

N O

B) O

Nicotinic acid and nicotinamide are vitamers of niacin, which strictly is not

a vitamin since limited amounts of it can be synthesized from tryptophan.

Niacin is required to form coenzymes NAD+ and NADP+ (Figure 10.12) which