Regulation of Crassula argentea phosphoenolpyruvate carboxylase in relation to temperature.

The effect of temperature on the kinetic parameters of phosphoenolpyruvate carboxylase purified from Crassula argentea was such that both the Vmax and Km(MgPEP) values tended upward over the range from 11 to 35 degrees C. The increased rate at low temperatures due to the low Km is at least partially offset by the increased Vmax at higher temperatures, potentially leading to a broad plateau of enzyme activity and a relatively small effect of temperature on the enzyme. The cooperativity was negative at 11 degrees C, but above 15 degrees C it became positive. The presence of 5 mM glucose-6-phosphate has relatively little effect on Vmax but it clearly reduces Km and overcomes any effect of temperature on this parameter in the range studied. Positive cooperativity is observed only at temperatures above 25 degrees C. The size of the native enzyme, as determined by dynamic light scattering, was strongly toward the tetrameric form. At a temperature of 40 degrees C and above, a considerable oligomerization takes place. No loss of activity can be observed in this range of temperature. In the presence of either glucose-6-phosphate or magnesium phosphoenolpyruvate, at temperatures under 25 degrees C, the equilibrium is displaced toward higher levels of aggregation. Maximal accumulation of lead malate occurred at 10 to 12 degrees C in vivo with reduction to about 25% at 35 degrees C. Glucose-6-phosphate followed a similar curve in response to temperature, but the overall difference was about 50%. The sum of phosphoenolpyruvate plus pyruvate is level at night temperatures below 25 degrees C, doubling at 35 degrees C. Calculated concentrations of malate, glucose-6-phosphate, and phosphoenolpyruvate plus pyruvate indicate that the concentrations present are equal to or greater than Ki, Ka, and Km values for these metabolites, respectively.