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Figure 4.
Fig. 4. Postulated mechanism of acetyl-CoA synthesis by PFOR.
Only the thiazolium ring moiety of ThDP is fully depicted (R and
R' as in Fig. 1, B and C). (A) Deprotonated carbanion species
(see Fig. 1A). The proton is putatively bound to
4'-iminopyridimine (not shown). (B) Pyruvate decarboxylation and
hypothetical enamine formation; the CO[2] reaction product stays
in the active site. (C) One electron transfer from the active
site to one of the [4Fe4S] clusters. Hypothetical n cation
radical formation. (D) Observed  /n cation
radical with a long C2-C2  bond (27)
and a bent thiazole ring (Fig. 2). Note that (i) ketonization of
the enamine (B) upon radical formation (C) and (ii)
tautomerization of the C5-C4 double bond to a C4-C4 double
bond, in going from (B) to (C), are required to explain the
observed stereochemistry of the adduct. The net result of these
two rearrangements is a significant reduction in the aromaticity
of the thiazole ring. Because this process is generally
considered to be unfavorable, the protein environment is thought
to play a key role in the stabilization of (C) and (D). The loss
of one electron from the active site and the bending of the
thiazole ring are shown here as a single step because we do not
know the detailed sequence of events. (E) Hypothetical
fragmented C-C bond resulting in carbocation and acetyl radical
species (28, 29). Upon fragmentation, the aromaticity of the
thiazole ring is thought to be restored (A), closing the cycle.
(F) Acetyl-CoA synthesis through condensation of a thiyl CoA
radical with the acetyl radical. Although the reaction is shown
in the direction of acetyl-CoA synthesis, PFORs are capable of
catalyzing the reverse reaction.
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