FIGURE 12-17 The malate-aspartate shuttle. This cyclical series of reactions transfers electrons from NADH in the cytosol (via the intermembrane space) across the inner mitochondrial membrane, which is impermeable to NADH itself, to NAD⁺ in the matrix. The net result is the replacement of cytosolic NADH with NAD⁺ and matrix NAD⁺ with NADH. Step 1: Cytosolic malate dehydrogenase transfers electrons from cytosolic NADH to oxaloacetate, forming malate. Step 2: An antiporter (blue oval) in the inner mitochondrial membrane transports malate into the matrix in exchange for α-ketoglutarate. Step 3: Mitochondrial malate dehydrogenase converts malate back to oxaloacetate, reducing NAD⁺ in the matrix to NADH in the process. Step 4: Oxaloacetate, which cannot directly cross the inner membrane, is converted to aspartate by addition of an amino group from glutamate. In this transaminase-catalyzed reaction in the matrix, glutamate is converted to α-ketoglutarate. Step 5: A second antiporter (red oval) exports aspartate to the cytosol in exchange for glutamate. Step 6 A cytosolic transaminase converts aspartate to oxaloacetate and α-ketoglutarate to glutamate, completing the cycle. The blue arrows reflect the movement of the α-ketoglutarate, the red arrows the movement of glutamate, and the black arrows that of aspartate/malate. It is noteworthy that as aspartate and malate cycle clockwise, glutamate and α-ketoglutarate cycle in the opposite direction.