Enzymatic Synthesis of 5′-Phosphoribosylamine from Ribose 5-Phosphate and Ammonia, an Alternate First Step in Purine Biosynthesis

Abstract

Abstract An enzyme activity responsible for the formation of 5'-phosphoribosylamine from ribose 5-phosphate and a nitrogen donor was prepared from livers of ducks, chickens, and pigeons. This enzyme activity is distinct from ribosylpryophosphate 5-phosphate amidotransferase (EC 2.4.2.14), which catalyzes the formation of 5'-phosphoribosylamine from ribosylpyrophosphate 5-phosphate and glutamine. The enzyme was partially purified and completely separated from ribosylpyrophosphate 5-phosphate amidotransferase. For maximal synthesis of 5'-phosphoribosylamine, the reaction required ribose 5-phosphate, ammonium chloride, ATP, and magnesium ions. Prolonged heating at 80° destroyed the activity of the enzyme. It is proposed that an alternate pathway for the initial step of purine synthesis exists, namely, the direct conversion of ribose 5-phosphate to 5'-phosphoribosylamine. This reaction is enzymatic and is sensitive to inhibition by ribonucleotides and by ribose 5-phosphate.