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Figure 7.
Figure 7. Schematic drawing
illustrating the reaction mechanism
proposed for tCGS. After formation
of the Michaelis complex (I), trans-
aldimination leads to an external
aldimine (II); via a carbanionic
intermediate (III), a PLP substrate
ketimine (IV) is formed. After
release of the phosphate leaving
group from an a-b-unsaturated
intermediate (V), cysteine enters
the active site and reacts at C
g
of
the partitioning intermediate (qui-
ninoid form of PLP-bound vinyl-
glycine, VI). The resulting a-b-
unsaturated intermediate (VII) is
protonated to form the PLP pro-
duct ketimine (VIII). Finally, the
product PLP aldimine results from
protonation of a carbanionic inter-
mediate (IX) by the active site
lysine.
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