Introduction to Enzyme and Coenzyme Chemistry - E-Library Home

Radicals in Enzyme Catalysis 245
Tyr 122
H 2 O
Asp 84
O
His 118
long range electron transfer ~35 Å
O
5.3 Å O Glu 238
O
H 2 O
O
O
Glu
Fe
Fe
204
O O
O
O
N
N
N Glu 115 N His 241
PPO
Glu 441
Cys 439
CO 2
−
HO
S
O
OH
B
SH
Cys 462
SH
Cys 225
R2 sub-unit
Figure 11.7 Tyrosyl and cysteinyl radicals in ribonucleotide reductase.
R1 sub-unit
shown in Figure 11.8. Protonation of the C-2 0 hydroxyl group by Cys-462, and
one-electron transfer from C-3 0 to C-2 0 , generates a radical at C-2 0 . Abstraction
of a hydrogen atom from a second cysteine, Cys-225, gives the reduced centre at
C-2 0 , and a Cys–Cys disulphide radical anion, which then transfers one electron
to C-3 0 to generate a further radical at C-3 0 . Abstraction of a hydrogen atom
from Cys-439 regenerates the cysteinyl radical and completes the catalytic cycle.
Cys 439
Cys 439
Cys 439
PPO
S
H
O
B
PPO
SH
O
B
1e − transfer PPO
from C-3 to C-2
SH
O
B
Glu 441
CO 2
−
HO
OH
SH SH
Cys 462 Cys 225
Glu 441
H
CO −
2
O OH
H
S SH
Cys 462 Cys 225
H 2 O
Glu 441
O
CO 2 H
H
H
S − S
Cys 462 Cys 225
PPO
Cys 439
S
H
B
H
O
PPO
Cys 439
S
H
H
B
O
PPO
Cys 439
SH
H
B
O
Glu 441
CO 2
−
HO
H
S S
Cys 462 Cys 225
Glu 441
HO
CO 2
−
H
S S
Cys 462 Cys 225
H O
O
O
Glu 441
1e −
H
S S
Cys 462 Cys 225
Figure 11.8 Mechanism for ribonucleotide reductase.