Mechanism of arginine biosynthesis in Chlamydomonas reinhardti: II. Purification and properties of N-acetylglutamate 5-phosphotransferase, the allosteric enzyme of the pathway

Abstract

1. 1. The allosteric enzyme of the arginine pathway, N-acetylglutamate 5-phosphotransferase (ATP: N- acetyl- l- glutamate 5-phosphotransferase), which catalyzes the formation of N-acetylglutamate 5-phosphate from N-acetylglutamate and ATP, has been purified 120-fold from cells of the fresh water alga Chlamydomonas reinhardti. 2. 2. The pH optimum of the enzymic reaction is 5.5. The enzyme requires the bivalent ions Mg 2+ or Co 2+ for activity. The initial velocity data obtained obey the normal Michaelis-Menten kinetics, and do not change in form with variation of pH between 5.5 and 7.5. The apparent K m value of the enzyme at pH 5.5 is 1.5 · 10 −2 M for N- acetyl- l-glutamate and 1.6 · 10 −3 M for ATP. 3. 3. l-Arginine inhibits the activity of the enzyme, and pH optimum for inhibition is 7.5. The initial velocity data in the presence of l-arginine are compatible with normal Michaelis-Menten kinetics. The kinetic relationship between N- acetyl- l-glutamate and l-arginine is competitive; between ATP and l-arginine it is non-competitive and does not change in form with variation of pH between pH 5.5 and 7.5. The inhibition of enzyme activity by l-arginine is apparently a bimolecular reaction at any pH between pH 5 5 and 7 5. 4. 4. Structural analogs of arginine such as l-canavanine and l-citrulline, in higher concentration as compared with arginine, also inhibit the enzyme activity. 5. 5. Urea at low concentrations reversibly suppresses the inhibitory effect of l-arginine and has no effect on enzyme activity. The kinetic relationship between urea and l-arginine is competitive. 6. 6. l-Arginine protects the enzyme against the inactivating effects of heat and high concentration of urea.