Organic Chemistry

1042 CHAPTER 25 The Organic Mechanisms of the Coenzymes • Metabolism

and releasing cysteine. Notice that at the end of the reaction the holoenzyme is exactly

as it was at the beginning, so the catalytic cycle can be repeated. The NADH produced

in the reaction is reoxidized to in the fourth stage of catabolism (Figure 25.1).

The mechanism for reduction by NADH (or by NADPH) is the reverse of the mech-

anism for oxidation by (or by ). If a substrate is being reduced, the di-

hydropyridine ring donates a hydride ion from its 4-position to the substrate. An acidic

side chain of the enzyme aids the reaction by donating a proton to the substrate.

Because NADH and NADPH reduce compounds by donating a hydride ion, they can

be considered biological equivalents of or —hydride donors used as

reducing reagents in nonbiological reactions (Section 20.1).

Why are the structures of biological redox (reducing and oxidizing) reagents so

much more complicated than the structures of the redox reagents used to carry out the

same reactions in the laboratory? NADH is certainly more complicated than

although both reagents reduce compounds by donating a hydride ion. Much of the

structural complexity of a coenzyme is for molecular recognition—to allow it to be

LiAlH 4 ,

NaBH 4 LiAlH 4

O

N+

reduction of substrate

oxidation of coenzyme

CNH 2

R

OH B–

H

H

4

3

6 2

5

1

C

O

OHB

N

CNH 2

R

HH

H

C

NAD+ NADP+

NAD+

S

H

B−

NAD+

S

H

NAD+

S

H

B

NAD+

S

H

B−

NAD+

S

H

B

NAD+

NADH

S

H

B

NAD

GAP CO

R

H C

R

O−

O−

H

C

C

O

−OO−OH

R

R

C

O

O

R

P

O O

O

O−

+ P

NAD+

S

H

B

C O−

R

enzyme

B−

O−

O O

−O

P

NAD+

product

substrate