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Abstract:
Mesaconyl-CoA transferase (Mct) is one of the key enzymes of the
3-hydroxypropionate (3HP) bi-cycle for autotrophic CO2 fixation. Mct is
a family III/Frc family CoA transferase that catalyzes an unprecedented
intra-molecular CoA transfer from the C1-carboxyl group to the
C4-carboxyl group of mesaconate at catalytic efficiencies >106 M-1 s-1.
Here, we show that the reaction of Mct proceeds without any significant
release of free CoA or the transfer to external acceptor acids. Mct
catalyzes intra-molecular CoA transfers at catalytic efficiencies that
are at least more than 6 orders of magnitude higher compared to
inter-molecular CoA transfers, demonstrating that the enzyme exhibits
exquisite control over its reaction. To understand the molecular basis
of the intra-molecular CoA transfer in Mct, we solved crystal structures
of the enzyme from Chloroflexus aurantiacus in its apo form, as well as
in complex with mesaconyl-CoA and several covalently enzyme-bound
intermediates of CoA and mesaconate at the catalytically active residue
Asp165. Based on these structures, we propose a reaction mechanism for
Mct that is similar to inter-molecular family III/Frc family CoA
transferases. However, in contrast to the latter that undergo opening
and closing cycles during the reaction to exchange substrates, the
central cavity of Mct remains sealed ("corked-up") by the CoA moiety,
strongly favoring the intra-molecular CoA transfer between the C1 and
the C4 position of mesaconate.
