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Biochemistry

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CAM Photosynthesis: Regulation of PEP Carboxylase

BIOCHM-ZRLDRW

The first biochemical reaction in the CAM (Crassulacean Acid Metabolism) pathway of photosynthesis is the carboxylation of phosphoenolpyruvate (PEP) using bicarbonate to form oxaloacetate (OAA). This cytosolic reaction is catalyzed by the enzyme PEP carboxylase, as shown in the reaction below. This process occurs at night when temperatures are lower, and the opening of leaf stomates allows ${ CO }_{ 2 }$ to enter without excess water loss.

$$\text{PEP} + {HC }{ O }_{ 3 }^{ - }{ \rightarrow P }_{ i }+ \text{OAA }$$

In many CAM plants (e.g., pineapple, cacti, jade) the OAA produced is, in turn, converted to malate in the cytosol by the enzyme NAD-malate dehydrogenase, as shown in the reaction below.

$$\text{OAA + NADH} + {H }^{ + } \rightarrow \text{Malate} + {\text{NAD}}^{ + }$$

However, the product of this second reaction, malate, is an allosteric inhibitor of PEP carboxylase.

Which of the following statements BEST explains how/why PEP carboxylase remains uninhibited during this dark carboxylation phase in CAM plants?

A

PEP carboxylase is a cytosolic enzyme, and malate is produced in the cytosol, but the malate is quickly shuttled into and stored in the vacuole, away from the enzyme.

B

PEP carboxylase is a cytosolic enzyme, and malate is produced in the cytosol, but the malate is quickly shuttled into and stored in the mitochondria, away from the enzyme.

C

PEP carboxylase is a cytosolic enzyme, and malate is produced in the cytosol, but the malate is quickly shuttled into the chloroplast where it is rapidly decarboxylated near the enzyme RuBisco (ribulose-1,5-bisP carboxylase/oxygenase).

D

The equilibrium of the reaction catalyzed by NAD-malate dehydrogenase lies far to the left (OAA synthesis) such that malate never accumulates to high enough levels in the cytosol to inhibit the enzyme (under normal conditions).

E

The malate produced is rapidly shuttled into the mitochondria where it is completely metabolized by the citric acid cycle reactions, generating ${ CO }_{ 2 }$ and ATP needed for the Calvin cycle (dark reactions catalyzed by RuBisco).