Temperature Effects on Phosphoenolpyruvate Carboxylase from a CAM and a C4 Plant 1: A Comparative Study

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The effect of temperature in the range from 10 to 35°C on various characteristics of phosphoenolpyruvate carboxylase from the leaves of a CAM plant, Crassula argentea and a C4 plant Zea mays shows a number of different effects related to the environment in which these distinct types of metabolic specialization normally operate. The Arrhenius plot of Vmax for the two enzyme forms shows that the CAM enzyme has a linear increase with temperature while the C4 enzyme has an inflection at 27°C implying a conformational or aggregational change in the enzyme or a shift in reaction mechanism to one requiring a lower activation energy. The Arrhenius plot of Km for the two enzymes reveals the startling fact that at temperatures above 20°C an increasing temperature causes an increase in KmPEP for the CAM enzyme while the C4 enzyme displays a decreased Km as the temperature increases. The inhibitory effect of 5 millimolar malate also shows opposite trends for the two enzymes. For the CAM enzyme the percent inhibition by malate increases from essentially none at 15°C to 70% at 35°C. For the C4 enzyme the percent inhibition drops from about 60% at 20°C to 2% at 30°C. Similar opposite behavior of the two enzymes is found with the Ki for malate. Pretreatment at high temperatures for periods up to 2 hours was found to result in differences similar to those described above if the treated enzyme were subsequently assayed at 25°C.

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