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Figure 5.
Figure 5. A possible mechanism for the ring expansion catalyzed
by DAOCS. The mechanism is based on the mode of penicillin
and cephalosporin binding shown in Figures 1d -f and 2. The
presumed oxidation states of the iron are marked. In the
oxidative half reaction, one of the oxygen atoms of dioxygen is
incorporated into succinate while the other one remains on the
iron. This oxygen can remove two electrons and two protons from
the five-membered thiazolidine ring to form the six-membered
dihydrothiazine ring of the cephalosporin product in the
reductive half reaction. Note ligation of the penicillin sulfur
to the iron (Fe -S distance: 2.1 -2.0 Å in the various
complexes) and that both  -
and  -methyl
groups are in van der Waals contact with the iron (iron -methyl
distances: 2.1 -2.5 Å).
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