Purification and Properties of the Arginine-specific Carbamoyl-phosphate Synthase from Saccharomyces cerevisiae

Abstract

The arginine-specific carbamoyl-phosphate synthase [EC 2.7.2.5] of yeast was stabilized sufficiently to allow partial purification of the enzyme (30- to 40-fold). The synthase (MW 115,000) comprised 2 unequal subunits: a heavy subunit (MW 80,000) capable of catalyzing synthesis of carbamoyl phosphate with ammonia as a nitrogen donor and a light subunit conferring upon the holoenzyme the ability to utilize glutamine. The enzyme had unusually high affinity for ATP (Km = 0.2 mM) and atypical negative cooperativity for glutamine binding ([S]0.5 = 0.25 mM). Glutamine activity was not modulated by possible effectors such as arginine, ornithine or N-acetylglutamate. Thus, although the yeast arginine enzyme physically and functionally resembles the single enteric synthase, the systems differ substantially both in kinetic properties and in regulation of activity.