Question

Question: The action of malic enzyme opposes the action of pyruvate carboxylase. (a) Diagram the substrate cycle formed by these enzymes plus a third enzyme. (b) Write the net equation for converting malate to pyruvate and back to malate. What does this accomplish for the cell?

Short answer

Answer

a.

1. Net equation for converting malate to pyruvate and back to malate:

$Pyruvate+ATP+C{O}_2+NADP+H^{+}\rightleftharpoons Malate+ADP+P_i+NA{D}^{+}$

The reversible conversion of pyruvate to malate benefits the cell by regulating the amount of malate and pyruvate produced during this cycle.

Step-by-step solution

Malic enzyme

Malic enzyme is a tetrameric protein with a double dimer structure that catalyzes the reversible oxidative decarboxylation of malate to pyruvate, CO₂, and NAD(P)H in the presence of a divalent metal ion

Action of pyruvate carboxylase, malate dehrogenase and malic enzyme

Pyruvate carboxylase (PC) catalyzes the following reaction to convert pyruvate to oxaloacetate:

$Pyruvate+ATP+C{O}_2\to Oxaloacetate+ADP+P_i$

Malate dehydrogenase (MDH) catalyzes the conversion of malate to oxaloacetate in the mitochondria. On the other hand, it catalyzes the conversion of oxaloacetate to malate in the cytoplasm, allowing for transport. The interconversion between malate and oxaloacetate can be summarized by the following reaction:

$Oxaloacetate+NADH+H^{+}\rightleftharpoons Malate+NA{D}^{+}$

The oxidative decarboxylation of malate is catalyzed by malic enzyme (ME) by the following reaction (which is also the net reaction):

$Malate+NAD{P}^{+}\to C{O}_2+Pyruvate+NADPH$

Diagram of substrate cycle

ME: Malic enzyme

PC: Pyruvate carboxylase

MDH: Malate Dehydrogenase

Net reaction

The net reaction for conversion of pyruvate to malate and back to malate is summarized as follows:

$Pyruvate+ATP+C{O}_2+NADP+H^{+}\rightleftharpoons Malate+ADP+P_i+NA{D}^{+}$