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Enzymatic Redox Chemistry 125
NH 2
N
N
N N
O
O
HO OH
(OPO 3
2− for NADPH)
NADH
O O
P
O
P
O
O− O−
H S H R
N
O
HO OH
CONH 2
H
H
CONH 2
+ 2e − /H + (or H − )
CONH 2
+
N
− 2e − /H +
N
R
R
NAD +
(oxidised form)
Figure 6.3 Structures of NAD þ and NADH.
NADH
(reduced form)
in the reduced form. Dehydrogenase enzymes which utilise this cofactor are
usually speciWc either for NAD þ =NADH or for NADP þ =NADPH. NAD þ and
NADH tend to be utilised for catabolic processes (which break down cellular
metabolites into smaller pieces), whereas NADP þ and NADPH tend to be used
for biosynthetic processes.
The redox potential for NAD þ is 0.32 V, consequently NADH is the most
powerful of the commonly available biological reducing agents. NAD þ is,
therefore, a relatively weak oxidising agent. NADH is most commonly used
for the reduction of ketones to alcohols, although it is also sometimes used for
reduction of the carbon–carbon double bond of an a,b-unsaturated carbonyl
compound, as shown in Figure 6.4.
H
alcohol dehydrogenase
H S
H R
O
OH
NADH
NAD +
O
enoyl reductase
O
R
SCoA
R
SCoA
NADH NAD +
Figure 6.4 Examples of NAD þ -dependent dehydrogenases.