The problem of regenerating NAD+ from NADH for cytoplasmic processes by using mitochondria is solved in the most energy-efficient manner by which one of the following intercellular shuttle systems?
|A||Citrate → pyruvate shuttle|
|B||Dihydroxyacetone phosphate →α-glycerophosphate shuttle|
|C||Malate → citrate shuttle|
|D||Citrate → citrate shuttle|
a. NADH generated from glycolysis must be relieved of an electron to form nicotinamide adenine dinucleotide (NAD) so that glycolysis may continue.
b. However, mitochondrial membranes are impermeable to both NADH and NAD The solution to this problem is the transfer of electrons from NADH to molecules that traverse the membrane.
c. In the glycerophosphate shuttle, dihydroxyacetone phosphate (DI-TAP) is reduced to glycerol-3-phosphate and thereby regenerates NAD The glycerol-3-phosphate diffuses into mitochondria and is oxidized by flavin adenine dinucleotide (FAD) back to DHAP, which can diffuse back into the cytosol.
d. The reduced form of flavin adenine dinucleotide (FADH produced in the mitochondria yields 2 ATP in the electron transport chain.
e. In the heart and liver, the more energy-efficient malate-aspartate shuttle moves electrons into mitochondria. Cytoplasmic oxaloacetate is reduced to malate, which diffuses into the mitochondria and is oxidized by NAD back to oxaloacetate.
f. The NADH produced in the mitochondria yields 3 ATP on electron transport. The mitochondrial oxaloacetate is converted to aspartate, which diffuses into the cytosol, where it is converted back into cytoplasmic oxaloacetate.