Methionine (Met) residues in either free or protein-based forms are a major
target of reactive oxygen species (ROS)-induced oxidation and are converted to
S and R epimers of methionine sulfoxide (MetSO). The oxidative modification of
Met leads to changes in structure and function of many proteins. Living
organisms evolved methionine sulfoxide reductase (MSR), which reverses MetSO
back to Met to counteract Met oxidation by ROS. The two major types of MSR are
MsrA and MsrB, which are specific for methionine S-sulfoxide (Met-S-SO) and
methionine R-sulfoxide (Met-R-SO), respectively. These two classes of enzymes
display neither sequence nor structural homology, but share a 'mirrored
catalytic mechanism'. Hence, this represents an compelling case of convergent
evolution. Functions of these proteins include repair of oxidatively damaged
proteins, regulation of protein function and elimination of oxidants through
reversible formation of methionine sulfoxides. Most organisms contain MsrA and
MsrB typically as separate enzymes. However, MsrA and MsrB exist as domains in
a single fused protein (MsrAB) in some bacteria such as Streptococcus
pneumonia, Neisseria gonorrhoea and Haemophilus influenza. MsrB proteins have
been identified and characterized in various organisms including bacteria,
yeast, fruit fly, and mammals
[3][6][4][2][5][1].
The MsrB core domain is composed of two antiparallel beta-sheets generated by
strands beta1, beta2 and strands beta9 and beta3-beta7, respectively. Alignment of MSRB sequences reveals only a single Cys that is
conserved in all family members. The Cys is located in the C-terminal part of
the protein and is the catalytic residue that directly attacks MetSO. The MsrB
domain activates the cysteine or selenocysteine nucleophile through a unique
Cys-Arg-Asp/Glu catalytic triad. The collapse of the reaction intermediate
most likely results in the formation of a sulfenic or seleneic acid moiety.
Regeneration of the active site occurs through a series of thiol-disulfide
exchange steps involving another active site Cys residue and thioredoxin. The
first step in the catalytic cycle (reductase step) leads to the formation of a
sulphenic acid intermediate on the catalytic Cys with the concomitant release
of reduced Met. Immediately, an intramolecular disulphide bond is formed via
the attack of a second Cys (recycling Cys) on the sulphenic acid intermediate
and the release of a water molecule. Finally, this disulphide bond is reduced
via an inter-molecular thiol-disulphide exchange by thioredoxin to regenerate
the active-site Cys-thiol function. The second Cys that is involved in the
recycling of the active site ('recycling Cys') is not universally conserved.
Many MSRB sequences carry four additional conserved Cys residues that are
organized in two CXXC motifs, which co-ordinate a zinc ion required for the
enzymatic activity
[3][2][5].
The profile we developed covers the entire MsrB domain.