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Enzymatic Redox Chemistry 123
O
L-lactate dehydrogenase
H
OH
CO 2
−
NADH NAD +
CO 2
−
(1)
NAD + + H + + 2e −
NADH
− 0.32 V
(2)
CH 3 COCO 2 H + 2H + + 2e −
CH 3 CH(OH)CO 2 H
− 0.19 V
(2) - (1)
CH 3 COCO 2 H + NADH CH 3 CH(OH)CO 2 H + NAD +
+ 0.13 V
Figure 6.2 Redox potentials in the lactate dehydrogenase reaction.
potential of 0.19 V. We simply subtract the redox potentials as shown in
Figure 6.2, giving an overall redox potential diVerence for the pyruvateto-lactate
reaction of þ0.13 V. If the redox potential diVerence is above zero,
then the reaction is thermodynamically favourable. The more positive the redox
potential diVerence is, the more favourable the reaction is and the greater the
equilibrium constant for the reaction (the equilibrium constant can be calculated
from the redox potential diVerence using the Nernst equation – see
physical chemistry texts). Remember that a highly positive redox potential
means a strong oxidising agent, whilst a highly negative redox potential
means a strong reducing agent.
One Wnal point is that an enzyme cannot change the equilibrium constant of a
chemical reactionthat is thermodynamically unfavourable, howeveritcan modify
the redox potential of a cofactor bound at its active site by selectively stabilising
either the oxidised or the reduced form of the cofactor. For example, the redox
potential for free oxidised riboXavin is 0.20 V, whereas redox potentials of
between 0.45 and þ0.15 V have been measured for Xavo-enzymes in general.
Justas with amino acid side chain pK a values,enzymes areableto modify chemical
reactivity at their active sites using subtle changes in micro-environment.
6.2 Nicotinamide adenine dinucleotide-dependent
dehydrogenases
The Wrst class of redox enzymes that we shall meet are the dehydrogenases
(Table 6.1). These enzymes transfer two hydrogen atoms from a reduced
substrate, which is usually an alcohol, to an electron acceptor. The electron
acceptor in these enzymes is the coenzyme NAD, whose oxidised and reduced
forms are shown in Figure 6.3.
The redox-active part of this coenzyme is the nicotinamide heterocyclic ring.
In the oxidised form NAD þ this is a pyridinium salt, which is reduced to a 1,4-
dihydro-pyridine in the reduced form NADH. A phosphorylated version of the
cofactor is also found, bearing a 2 0 -phosphate on the adenosine portion of
the structure, which is written as NADP þ in the oxidised form and NADPH