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Radicals in Enzyme Catalysis 247
H
N
O
N
H
H
N
H
H
O
N
H
H
N
H
H
O
N
H
H
S
SH
S
O
CO 2
−
SH
H 3 C
O
S
CO 2
−
SH
Cys 419
Cys 418
Cys 419
Cys 418
Cys 419
Cys 418
CH 3 COSCoA
H
N
SH
O
N
H
H 3 C
S
− SCoA
O
HCO 2
−
H
N
S
O
N
H
H
O
H 3 C O O−
SH
H
N
H
H
S
O
N
H
O
H 3 C O O−
SH
Cys 418
Cys 419
Cys 419
Cys 418
Cys 419
Cys 418
Figure 11.10 Mechanism for pyruvate formate lyase.
required SAM for activity. Until this point, the only known role for SAM was
for methyl group transfer (see Section 5.8), thus it was surprising to Wnd it
involved in redox chemistry.
The molecular details of the involvement of SAM in radical chemistry
were Wrst established for the enzyme lysine 2,3-aminomutase, from Clostridium
subterminale. This enzyme catalyses the interconversion of l-lysine with
b-lysine (see Figure 11.11), a 1,2-rearrangement similar in kind to the vitamin
B 12 -dependent reactions described in Section 11.2. The enzyme did not, however,
utilise vitamin B 12 as a coenzyme, but instead required pyridoxal
5 0 -phosphate (PLP) and SAM for activity, and a reduced [4Fe4S] cluster.
Detailed studies of this enzyme by rapid-quench EPR spectroscopy have
established the existence of radical intermediates in the catalytic mechanism.
Following the attachment of l-lysine to the PLP cofactor (see Section 9.2),
hydrogen atom abstraction at the b-position generates a C-3 radical, which
attacks the imine linkage to give a three-membered aziridine intermediate.
+
H 3 N
H
H*
H
CO −
2
NH 3
+
lysine
2,3-aminomutase
PLP
SAM, [4Fe4S] red
H 3 N +
+
H 3 N
*H
H
H
CO 2
−
Figure 11.11 Reaction catalysed by lysine 2,3-aminomutase.