Chemistry Reference
In-Depth Information
methyltransferases involve three protein components, each of which is localised on a different polypeptide or
domain. The first, (MT1) binds the methyl donor (CH
3
-X) and transfers it to the B
12
-containing protein, leading to
the formation of an organometallic methylcobalt intermediate. The third component (MT2) catalyses the transfer
of the Co-bound methyl group to the acceptor Y
. The methyl donor can be any one of a number of molecules like
methyltetrahydrofolate, while the methyl acceptor can be, for example, homocysteine to give methionine, or the
CODH/ACS bifunctional complex to form acetyl-CoA. The MT2 enzymes all appear to contain Zn, which both
coordinates and activates the thiolate methyl acceptor (
Figure 15.10
(b)). However, in the transfer of a methyl
group within the CODH/ACS system, a different type of reaction is involved in which the methyl group is
transferred from Co to Ni.
The best characterised B
12
-dependent methyltransferase is methionine synthase (
Figure 15.11
)
from E. coli,
which catalyses the transfer of a methyl group from methyltetrahydrofolate to homocysteine to form methionine
Co
I
Methionine
CH
3
-H
4
folate
Inactive
enzyme
Co
II
Fld
o
MSR
AdoMet
e
-
Homocysteine
H
4
folate
CH
3
Co
III
N-His
FIGURE 15.11
Reactions catalysed by cobalamin-dependent methionine synthase.
(From
Banerjee & Ragsdale, 2003
.
Reprinted with
permission from Annual Reviews.)
and tetrahydrofolate. During the catalytic cycle, B
12
cycles between CH
3
-Co(III) and Co(I). However, from time
to time, Co(I) undergoes oxidative inactivation to Co(II), which requires reductive activation. During this process,
the methyl donor is S-adenosylmethionine (AdoMet) and the electron donor is flavodoxin (Fld) in E. coli,or
methionine synthase reductase (MSR) in humans. Methionine synthase is a modular enzyme, with separate
domains for binding of homocysteine, methyltetrahydrofolate, B
12
, and AdoMet (
Figure 15.12
)
. The B
12
domain
in its different oxidation states must interact with each of the other three domains: the Co(I) form with methyl-
tetrahydrofolate , the inactive Co(II) form with the AdoMet binding domain, and the CH
3
-Co(III) form with the
homocysteine-binding domain. When cobalamin binds, the lower axial Dmb ligand is replaced by His to generate
the His-on conformation (
Figure 15.8
B). This His residue is part of a catalytic triad which controls the coordi-
nation state of cobalt (His-on/His-off) by modulating the protonation state of the histidine.
As pointed out earlier, a third class of B
12
-dependent enzymes, present in anaerobic microbes, carry out
reductive dehalogenation reactions, which play an important role in the detoxification of chlorinated aliphatic and
aromatic compounds, among which are many important man-made pollutants (
El Fantroussi et al., 1998
)
. The role
of B
12
in this class of enzymes is not clear
possibly by formation of an organocobalt adduct, as in the case of
methyltransferases or alternatively by the corrinoid serving as an electron donor.
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