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Aspartokinase-Homoserine Dehydrogenase I
Background:
The
phosphorylation of the ß-carboxyl group of aspartic acid is the
commitment step toward the biosynthesis in plants and microorganisms of 4
of the 20 amino acids that are found in proteins. In most microorganisms
there are several isofunctional enzymes that
catalyze this step, with each enzyme differentially regulated by an end
product amino acid. Aspartokinase- homoserine dehydrogenase I (AK-HSD I) is
a bifunctional enzyme, subject to feed back inhibition by threonine, that
catalyzes the first and the third steps in this pathway.
Kinetic
Studies:
The kinetic
mechanism of each of the catalytic activities of AK-HSD I has been
determined (Angeles & Viola, 1990) and chemical modification
studies have identified a histidyl and tyrosyl residues that are essential for catalysis (Angeles, et al., 1989). Specificity studies have
established the essential role of the substrate α-amino group as a
binding determinant, and have identified a number of alternative substrates
with derivatized -carboxyl groups. Unexpectedly, blocking the β-carboxyl
group, the site of phosphorylation, does not prevent the reaction (Angeles, et al., 1992). These β-derivatized
analogs are capable of productive binding to the enzyme through a reversal
of regioselectivity to make the α-carboxyl group available as the
phosphoryl acceptor. Many of the resulting α-acyl phosphates have also
been found to be substrates for the next two enzymes in this biosynthetic
pathway.
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