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Enzymatic Redox Chemistry 129
can be explained by a puckering of the dihydropyridine ring, causing one of the
two hydrogens to adopt a pseudo-axial orientation. This C2H bond is aligned
more favourably with the p-orbitals of the dihydropyridine ring and is, therefore,
stereoelectronically better aligned for hydride transfer. It seems likely that
dehydrogenase enzymes also use this stereoelectronic eVect to assist the reaction.
6.3 Flavin-dependent dehydrogenases and oxidases
RiboXavin was Wrst isolated from egg white in 1933 as a vitamin whose
deWciency causes skin lesions and dermatitis. The most striking property of
riboXavin is its strong yellow–green Xuorescence, a property conveyed onto
those enzymes which bind this cofactor. The structure of riboXavin consists of
an isoalloxidine heterocyclic ring system, which is responsible for its redox
activity. Attached to N-10 is a ribitol side chain, which can be phosphorylated
in the case of Xavin mononucleotide (FMN) or attached through a diphosphate
linkage to adenosine in Xavin adenine dinucleotide (FAD), as shown in
Figure 6.11.
The Wrst important diVerence between NAD and Xavin is that enzymes
which use Xavin bind it very tightly, sometimes covalently (attached to Cys or
His through C-8a), such that it is not released during the enzymatic reaction but
remains bound to the enzyme throughout. Consequently the active form of the
cofactor must be regenerated at the end of each catalytic cycle by external redox
reagents. The next important diVerence is that Xavin can exist either as oxidised
FAD, or reduced FADH 2 , or as an intermediate semiquinone radical species
FADH . , as shown in Figure 6.12. Therefore Xavin is able to carry out
HO
N
N
OH
OR R = H riboflavin
Numbering
Scheme
R = PO 2−
3 FMN
OH
R
N O
O O
8a 9
1
8 N 10a N O
R = P O P OAd FAD
10 2
NH
N 4 NH
O− O−
7
4a 3
6 5
O
O
- occasionally covalently attached to Cys or His via C-8a.
Figure 6.11 Structures of Xavin redox cofactors.
R
R
R
N N O
H
+1e − N N O
+1e −
N N O
NH
NH
N
+ H + N
+ H +
NH
N
O
H
O
H
O
oxidised flavin (FAD) flavin semiquinone reduced flavin (FADH 2 )
Figure 6.12 Redox states of riboXavin.