What is the role of malate dehydrogenase in the citric acid cycle, and why is its reaction considered reversible under biological conditions?

Malate dehydrogenase converts oxaloacetate to malate, and the reaction is irreversible due to a high positive delta G.

Malate dehydrogenase converts malate to oxaloacetate, and the reaction is reversible due to a delta G close to zero under biological conditions.

Malate dehydrogenase converts malate to fumarate, and the reaction is reversible due to a negative delta G.

Malate dehydrogenase converts oxaloacetate to fumarate, and the reaction is irreversible due to a delta G close to zero.