Effect of limited tryptic modification of a bacterial poly(3-hydroxybutyrate) depolymerase on its catalytic activity

The extracellular poly(3-hydroxybutyrate) depolymerase of Alcaligenes faecalis T1, which hydrolyzes both hydrophobic poly(3-hydroxybutyrate) and water-soluble oligomers of D(-)-3-hydroxybutyrate, lost its hydrolyzing activity toward the hydrophobic substrate on mile trypsin treatment, but retained its activity toward water-soluble oligomers. The molecular mass of the trypsin-treated enzyme was 44 kDa, as estimated by polyacrylamide gel electrophoresis in the presence of sodium dodecyl sulfate, which was 6 kDa smaller than that of the native enzyme (50 kDa). The trypsin-treated enzyme seemed to be less hydrophobic than the native one, because it was rather weakly adsorbed to a hydrophobic butyl-Toyopearl column compared with the native enzyme, and showed no ability to bind to poly(3-hydroxybutyrate), to which the native enzyme tightly bound. These results suggest that, in addition to a catalytic site, the enzyme has a hydrophobic site, which is not essential for the hydrolysis of water-soluble oligomers, but is necessary for the hydrolysis of hydrophobic substrates, and this hydrophobic site is removed from the enzyme by the action of trypsin.