Interaction of Acetyl Phosphate and Carbamyl Phosphate with Plant Phosphoenolpyruvate Carboxylase

D.H. GONZÁLEZ, A.A. IGLESIAS, C.S. ANDREO

BIOCHEMICAL JOURNAL

Año: 1987 vol. 241 p. 543 - 548

0264-6021

1. Acetyl phosphate produced an increase in the maximum velocity (Vmax. for the carboxylationof phosphoenolpyruvate catalysed by phosphoenolpyruvate carboxylase. The limiting Vmax. was22.2 ,tmol min-' * mg-' (185% of the value without acetyl phosphate). This compound also decreased theKm for phosphoenolpyruvate to 0.18 mm. The apparent activation constants for acetyl phosphate were1.6 mm and 0.62 mm in the presence of 0.5 and 4 mM-phosphoenolpyruvate respectively. 2. Carbamylphosphate produced an increase in Vmax and Km for phosphoenolpyruvate. The variation of max. /Km withcarbamyl phosphate concentration could be described by a model in which this compound interacts withthe carboxylase at two different types of sites: an allosteric activator site(s) and the substrate-binding site(s).3. Carbamyl phosphate was hydrolysed by the action of phosphoenolpyruvate carboxylase. The hydrolysisproduced Pi and NH4+ in a 1:1 relationship. Values of Vmax. and Km were 0.11+0.01,umol ofPi - min-' mg-' and 1.4 + 0.1 mm, respectively, in the presence of 10 mM-NaHCO3. If HCO3- was not added,these values were 0.075 + 0.014 #tmol of Pi. min-' mg-1 and 0.76 + 0.06 mm. Vmax /Km showed no variationbetween pH 6.5 and 8.5. The reaction required Mg2+; the activation constants were 0.77 and 0.31 mm atpH 6.5 and 8.5 respectively. Presumably, carbamyl phosphate is hydrolysed by phosphoenolpyruvatecarboxylase by a reaction the mechanism of which is related to that of the carboxylation ofphosphoenolpyruvate.