Question

The net gain of ATP in the malate – aspartate shuttle is
(A)3 ATP
(B)2 ATP
(C)1 ATP
(D)NO ATP

Answer 1

The malate-aspartate shuttle is a process in which electrons produced during glycolysis are transferred across the semipermeable membrane of the mitochondria, specifically the inner membrane for oxidative phosphorylation.

Complete answer:
The important enzyme in the malate-aspartate shuttle is malate dehydrogenase. So this process starts in the cytosol by malate dehydrogenase. This enzyme catalyzes the reaction of oxaloacetate and NADH to produce malate and ${ NA{ D }^{ + } }$ . When NADH which is present in the cytosol is oxidized by the respiratory chain, 3 ATP are formed.

Additional Information: Once the malate is formed, malate-alpha-ketoglutarate imports the malate from the cytosol into the mitochondrial matrix and it also exports alpha-ketoglutarate from the matrix into the cytosol.
The mitochondrial malate dehydrogenase converts malate to oxaloacetate once it reaches the mitochondrial matrix.
In this process, ${ NA{ D }^{ + } }$ is reduced with two electrons to form NADH.
Oxaloacetate is transformed into aspartate by mitochondrial aspartate aminotransferase.
The glutamate-aspartate imports glutamate from the cytosol into the matrix. It also exports aspartate from the matrix to the cytosol.
NADH which is present in the cytosol is oxidized to ${ NA{ D }^{ + } }$, and ${ NA{ D }^{ + } }$ which is present in the matrix is reduced to NADH.
The net effect of the malate-aspartate shuttle is a redox reaction.
The ${ NA{ D }^{ + } }$ which is present in the cytosol can further be reduced again by another cycle of glycolysis, and the NADH in the matrix can be further used for ATP synthesis.
The number of ATP molecules formed via the malate-aspartate shuttle is 38 ATP.
So, the correct answer is '3 ATP'.

Note: There is a difference between the net gain of ATP and the number of ATP molecules produced.
Net gain of ATP means: produced ATP - consumed ATP.
Total ATP produced is the sum of all ATP produced in different steps of any cycle or pathway.