4
1 Redox Proteins and Bioelectrocatalysis
is thermodynamically unfavorable (uphill) and requires sunlight energy. The electron
transport cycle is regarded as a kind of battery; photosynthesis is a charging process
and a coupled process of metabolism and respiration is a discharging process.
1.2 Redox Components
Redox enzymes use a palette of redox components called coenzymes, prosthetic
groups, or cofactors: β-nicotine amide dinucleotides (phosphate) (NAD(P)
+ ), flavins,
quinones, hemes, ion-sulfur clusters, copper, molybdenum, nickel, etc. β-NAD- and
β-NADP-dependent enzymes catalyze hydride ion transfers in the main streams of
catabolism and anabolism, respectively. Core redox enzymes are ancient and highly
diverse in amino acid sequence, and usually require specific transition metal(s) in
their active site to catalyze (multi-step) single-electron transfers. On the other hand,
flavoproteins and quinoproteins can catalyze both (two-step) single-electron transfer
and hydride ion transfer and link the electron transfer between several organic redox
substances (including NAD(P)(H)) and inorganic redox-centers in metal-containing
redox enzymes. In the following, we describe the structures and redox reactions of
these redox components.
1.2.1 NAD(P)(H) [1, 5]
Pyridine nucleotide coenzymes NAD(P)(H) undergo a hydride ion (H
− ) transfer
(i.e. a single step two-electron one-proton redox reaction without involving its intermediate radical) in the biological system (Fig. 1.3A). NAD(P)-dependent enzymes
catalyze the reversible transfer of hydride ion from NAD(P)H to carbon atoms on
the oxidized substrate and vice versa (Fig. 1.3B). Thus, NAD(P)(H) are usually
regarded as hydride ion-transferring coenzymes. Many NAD(P)-dependent dehydrogenases utilize ordered mechanisms; NAD(P)(H) is non-covalently and weakly
bound to NAD(P)-dependent dehydrogenases during the enzyme reactions and is
released into solution after the catalytic reactions. Therefore, the enzymatic reactions
are frequently monitored spectrophotometrically at 340 nm due to the absorption of
NAD(P)H in solution (Fig. 1.3C). NAD-dependent dehydrogenases are involved in
catabolic processes, while NADP-dependent ones are involved in anabolic ones.
NAD(P)-dependent enzymes show high specificity toward both the substrate and
NAD(P)(H).
The biological standard redox potential (E
⊕ ) for the NAD
+ /NADH couple is −
0.315 V versus the standard hydrogen electrode (SHE)* and the formal potential
(E
◦ ) shifts by −29.5 mV per pH at 298 K, according to Eq. (1.6):
E
◦
= E
⊕
−
2.303m RT
n F
pH,
(1.6)
1 Redox Proteins and Bioelectrocatalysis
is thermodynamically unfavorable (uphill) and requires sunlight energy. The electron
transport cycle is regarded as a kind of battery; photosynthesis is a charging process
and a coupled process of metabolism and respiration is a discharging process.
1.2 Redox Components
Redox enzymes use a palette of redox components called coenzymes, prosthetic
groups, or cofactors: β-nicotine amide dinucleotides (phosphate) (NAD(P)
+ ), flavins,
quinones, hemes, ion-sulfur clusters, copper, molybdenum, nickel, etc. β-NAD- and
β-NADP-dependent enzymes catalyze hydride ion transfers in the main streams of
catabolism and anabolism, respectively. Core redox enzymes are ancient and highly
diverse in amino acid sequence, and usually require specific transition metal(s) in
their active site to catalyze (multi-step) single-electron transfers. On the other hand,
flavoproteins and quinoproteins can catalyze both (two-step) single-electron transfer
and hydride ion transfer and link the electron transfer between several organic redox
substances (including NAD(P)(H)) and inorganic redox-centers in metal-containing
redox enzymes. In the following, we describe the structures and redox reactions of
these redox components.
1.2.1 NAD(P)(H) [1, 5]
Pyridine nucleotide coenzymes NAD(P)(H) undergo a hydride ion (H
− ) transfer
(i.e. a single step two-electron one-proton redox reaction without involving its intermediate radical) in the biological system (Fig. 1.3A). NAD(P)-dependent enzymes
catalyze the reversible transfer of hydride ion from NAD(P)H to carbon atoms on
the oxidized substrate and vice versa (Fig. 1.3B). Thus, NAD(P)(H) are usually
regarded as hydride ion-transferring coenzymes. Many NAD(P)-dependent dehydrogenases utilize ordered mechanisms; NAD(P)(H) is non-covalently and weakly
bound to NAD(P)-dependent dehydrogenases during the enzyme reactions and is
released into solution after the catalytic reactions. Therefore, the enzymatic reactions
are frequently monitored spectrophotometrically at 340 nm due to the absorption of
NAD(P)H in solution (Fig. 1.3C). NAD-dependent dehydrogenases are involved in
catabolic processes, while NADP-dependent ones are involved in anabolic ones.
NAD(P)-dependent enzymes show high specificity toward both the substrate and
NAD(P)(H).
The biological standard redox potential (E
⊕ ) for the NAD
+ /NADH couple is −
0.315 V versus the standard hydrogen electrode (SHE)* and the formal potential
(E
◦ ) shifts by −29.5 mV per pH at 298 K, according to Eq. (1.6):
E
◦
= E
⊕
−
2.303m RT
n F
pH,
(1.6)