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Aspartokinase III
Background:
Aspartokinase
III (AK III) from Escherichia coli is one of three enzymes that
catalyze the first step in the biosynthetic pathway from L-aspartate
leading to the amino acids lysine, threonine, methionine, and isoleucine.
Each of these isofunctional enzymes are differentially regulated to control the relative
amounts of the end product amino acid. In E. coli, AK III is a dimer
composed of identical subunits of 50,000 daltons,
and the synthesis and the activity of this enzyme are regulated by lysine (Richaud et al., 1974). The amino acid sequence of the
enzyme has been determined, and there is high sequence homology among these
isofunctional enzymes (Zakin
et al., 1983).
Results:
AK III has been
purified from a plasmid-containing strain of E. coli. The enzyme
shows broad specificity for the phosphoryl acceptor substrate. Structural
analogs of aspartic acid with either a derivatized ß-carboxyl or an α-amino group are accepted as alternative
substrates by the enzyme. As has been previously observed with
aspartokinase I (Angeles and Viola, 1992), derivatization of the
ß-carboxyl group, which serves as the normal phosphoryl acceptor in
this reaction, does not prevent catalytic activity. These
ß-derivatized analogs are capable of productive binding to these
enzymes through a reversal of regiospecificity, making the α-carboxyl
group available as the phosphoryl acceptor. Chemical modification and pH
profile studies have identified the functional groups of cysteine and
histidine as being involved in the catalytic activity of AK III (Keng
& Viola, 1996).
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