Final answer:
In the presence of oxygen, the malate-aspartate shuttle enables the conversion of lactate back into pyruvate, which enters the Krebs cycle. The shuttle ultimately facilitates the production of ATP by ensuring the regeneration of NAD+ through the action of malate dehydrogenase, enabling continuous acetyl-CoA entry into the cycle.
Step-by-step explanation:
When lactate serves as a precursor, the malate-aspartate shuttle plays a critical role in ensuring that the reducing equivalents from NADH produced in glycolysis are effectively shuttled into the mitochondria, thereby supporting the continuous generation of ATP through oxidative phosphorylation. During anaerobic conditions, lactate dehydrogenase (LDH) catalyzes the conversion of pyruvic acid and NADH to lactic acid and NAD+, freeing the NAD+ to participate in further glycolysis. However, when oxygen is available, the lactate can be shuttled back to the liver and converted into pyruvate, entering the Krebs cycle. At the step governed by malate dehydrogenase, malate is dehydrogenated to oxaloacetate, regenerating NAD+ from NADH + H+, which is essential for the Krebs cycle to continue as oxaloacetate can accept an acetyl group from acetyl-CoA, allowing the cycle to begin again.