Biosynthesis of galactan by a particulate enzyme preparation from Phaseolus aureus seedlings

Abstract

A particulate cell-free enzyme system was prepared from Phaseolus aureus shoots. This preparation was able to incorporate [(14)C]galactose from UDP-[(14)C]galactose into a water-soluble polysaccharide, which has a probable molecular weight of at least 4600. The only labelled component detectable in the polymer was shown to be [(14)C]galactose; two labelled oligosaccharides containing only [(14)C]galactose were isolated by partial hydrolysis. The galactan-synthesizing activity of this particulate preparation is maximal at 30 degrees and pH7.1 in the presence of 5.0mm-magnesium chloride and 0.2m-sucrose. Although 3-day-old seedlings were used as a source of enzyme, it appears that 4- or 5-day-old beans contain greater synthetase activity. The enzyme system has an apparent Michaelis constant of 5.8x10(-6)m, and will catalyse the polymerization of galactose residues at the rate of 7.5mmumoles/mg. of protein/min. at a substrate concentration of 9.6mm.