Difference between uracilylalanine synthases and cysteine synthases in Pisum sativum

Abstract

Purification of cysteine synthase from seedlings of pea (Pisum sativum) reveals the presence of three forms of this enzyme, separated by chromatography on DEAE-Sephadex A-50, and also differences between the cysteine- and uracilylalanine-synthases. Isoenzymes A and B of pea cysteine synthase were purified about 1200-fold and had specific activities of 933 U/mg protein and 892 U/mg protein, respectively. Both isoenzymes were found to have the same M r (52 000) and to dissociate into two identical subunits ( M r 26 000). The K m value of isoenzyme A is 2.1 mM for O-acetyl- l-serine (OAS) and 36 μM for sulphide, while that of isoenzyme B is 2.3 mM for OAS and 38 μM for sulphide. None of the three isoenzymes from pea seedlings catalyses the formation of the uracilylalanines l-willardiine and l-isowillardiine from OAS and uracil, although isoenzyme A catalyses the formation of β-cyano- l-alanine, and isoenzyme C catalyses the formation of l-quisqualic acid and l-mimosine. Other significant differences occur in the substrate specificity of the three isoenzymes. Several properties, including the amino acid composition of the purified cysteine synthase isoenzymes, are also described.