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yet clear what limits turnover rates in Cu,Mo-CODHs, however two observations
may give a hint. First, Hille and coworkers showed that the reductive half-reaction
is likely limiting the overall rate, therefore the chemistry at the Cu-S-Mo site and
not the electron flow from the FAD to external electron acceptors is probably the
limiting catalytic factor [ 55 ]. Second, Mo-hydroxylases have typically turnover
numbers in the range of 1-100 s 1 [ 148 ] and there may be an intrinsic barrier in one
of the key steps of the Mo chemistry that limits turnover in this enzyme family
including Cu,Mo-CODHs.
3 Concluding Remarks and Future Directions
Our understanding how the oxidation of CO is catalyzed by enzymes greatly
increased during the last 10-15 years. The composition and architecture of the
responsible metal clusters have been firmly established and mechanistic proposals
have been refined by additional spectroscopic and computational studies. Of course,
several questions about the enzyme mechanisms remain unanswered and we expect
to see further progress in the next years.
One especially promising area of research not covered in this text is the assembly
of the metal sites and, although some progress has been made in the last years, we
still know considerably less about how the active sites of CODH and ACS are
assembled, when compared to other metalloenzymes, such as nitrogenases
[ 149 - 152 ], hydrogenases [ 153 - 155 ], and ureases [ 156 ].
A deeper understanding of common principles and differences between CODHs
and their related enzymes will probably advance the field, once we better under-
stand their evolution. Judged by the employed metals, use by aerobic versus
anaerobic microbes and the phylogenetic distribution of homologous proteins,
one would guess that ancestral Ni,Fe-CODHs existed already very early in the
evolution, likely in pre-LUCA times, whereas Cu,Mo-CODHs are evolutionary
young inventions. Further phylogenetic analysis may yield clues how the catalytic
strategies of the enzymes developed over time.
The complete genome sequences of prokaryotes show a vast amount of enzymes,
whose function we do not know and cannot even guess at the moment. Given the
rich chemistry of CO, it would be surprising if organisms living with CO would not
use more than the two reactions described in this chapter to harness the energy
stored in CO. One especially attractive way is C-C coupling of CO in the presence
of H 2 similar to the Fischer-Tropsch reaction, which has recently been shown as a
side-reaction of V- and Mo-containing nitrogenases [ 157 , 158 ]. We are therefore
optimistic that our knowledge on the use of CO by microbes will further expand in
the years to come.
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