What is the function of the malate-aspartate shuttle?

The malate-aspartate shuttle plays a crucial role in cellular metabolism by allowing the transfer of reducing equivalents, specifically electrons carried by NADH, from the cytosol into the mitochondria. This is essential because the inner mitochondrial membrane is impermeable to NADH itself. In the shuttle, malate is synthesized from oxaloacetate in the cytosol, allows for the reduction of NAD+ to NADH, which can then cross into the mitochondria, where malate is converted back to oxaloacetate, regenerating NADH in the mitochondrial matrix.

This process effectively maintains the balance of NAD+/NADH between the cytosol and the mitochondria, which is vital for numerous metabolic pathways including the citric acid cycle. By ensuring the regeneration of NAD+ in the cytosol, the shuttle allows glycolysis, which depends on NAD+, to continue functioning efficiently. Hence, the principal function is to maintain the gradient of NADH/NAD+ and support cellular respiration by allowing the re-oxidation of NADH, making option B the most accurate description of the shuttle's role in cellular metabolism.