Galactokinases: Potential Biotechnological Applications as Biocatalysts
David J. Timson.
Galactokinase, a member of the GHMP (galactokinase, homoserine kinase, mevalonate kinase,
phosphomevalonate kinase) family of kinases, catalyses the ATP-dependent phosphorylation of galactose at position 1 on
the sugar. This reaction is important in the Leloir pathway of galactose catabolism. The need to produce monosaccharides
phosphorylated at position 1 for the synthesis of complex molecules, including aminoglycoside antibiotics, has stimulated
interest in exploiting the catalytic potential of galactokinases. However, the enzyme is quite specific, generally only
catalysing the phosphorylation of D-galactose and closely related molecules. Directed evolution strategies have identified
a key tyrosine residue (Tyr-371 in the Escherichia coli enzyme) which, although distant from the active site, influences
the specificity of the enzyme. Alteration of this residue to histidine in E. coli and Lactococcus lactis galactokinases
dramatically expanded the substrate range to include both D- and L-sugars. Similar experiments with the human enzyme
demonstrated that alteration of the equivalent tyrosine (Tyr-379) to cysteine, lysine, arginine, serine or tryptophan
increased the catalytic promiscuity of the enzyme. It has been hypothesised that these specificity changes arise because of
alterations in the flexibility of the polypeptide chain. This hypothesis has yet to be tested experimentally. The
biotechnological potential of galactokinases is clearly considerable and exploitation of closely related enzymes such as Nacetylgalactosamine
kinase and arabinose kinase would expand that potential still further.
Keywords: Galactokinase, N-acetylgalactosamine kinase, Sugar 1-phosphate, in vitro glycorandomisation, enzyme
engineering, protein flexibility.
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