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J. Bacteriol., Apr 1996, 2060-2064, Vol 178, No. 7
MJ Brumlik and JT Buckley
Aeromonas hydrophila secretes a lipolytic enzyme that has several
properties in common with the mammalian enzyme lecithin-cholesterol
acyltransferase. We have recently shown that it is a member of a newly
described group of proteins that contain five similar blocks of sequence
arranged in the same order in their primary structures (C. Upton and J. T.
Buckley, Trends Biochem. Sci. 233:178-179, 1995). Assuming that, like other
lipases, these enzymes have a Ser-Asp-His catalytic triad, we used these
blocks to predict which aspartic acid and histidine would be at the active
site of the Aeromonas enzyme. Targeted residues were replaced with other
amino acids by site-directed mutagenesis, and the effects on secretion and
activity were assessed. Changing His-291 to asparagine completely abolished
enzyme activity, although secretion by the bacteria was not affected. Only
very small amounts of the D116N mutant appeared in the culture supernatant,
likely because it is sensitive to periplasmic proteases it encounters en
route. Assays of crude preparations containing this variant showed no
detectable enzyme activity. We conclude that, together with Ser-16, which
we have identified previously, Asp-116 and His-291 compose the catalytic
triad of the enzyme.
Copyright © 1996, American Society for Microbiology
Identification of the catalytic triad of the lipase/acyltransferase from Aeromonas hydrophila
Department of Biochemistry and Microbiology, University of Victoria, Victoria, British Columbia, Canada.
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