Question

Describe how shuttle systems transport reducing equivalents into the mitochondria.

Short answer

Answer

The cytoplasmic reducing equivalents from NADH are transported to the inner mitochondrial membrane through the malate-aspartate shuttle and glycerophosphate shuttle.

Step-by-step solution

Transfer of electrons from the cytoplasmic NADH to mitochondrial ETC.

NADH produced during glycolysis in the cytoplasm cannot pass through intact mitochondrial membranes for aerobic oxidation. R ather than NADH itself, reducing equivalents (i.e., electrons) from NADH are transported across mitochondrial membranes by malate (Mal) or glycerol 3-phosphate.

Step 2: Malate Aspartate Shuttle

In the malate aspartate shuttle, the reducing equivalents of NADH are transferred in the cytoplasm to oxaloacetate, which is then converted to malate.Malate is carried to the mitochondrial matrix, where it is oxidized to oxaloacetate, along with the synthesis of NADH, which yields three molecules of ATP via respiratory chain oxidation. As oxaloacetate cannot pass through mitochondrial membrane, oxaloacetate is first converted to aspartate-by-aspartate aminotransferase and then transported out of the mitochondria via the glutamate-aspartate translocase.

Glycerophsphate Shuttle

In glycerophosphate shuttle, electrons from cytosolic NADH are transferred to the mitochondrial electron-transport chain in three steps.In first step, 3-phosphoglycerol dehydrogenase catalyzes the cytosolic oxidation of NADH by dihydroxyacetone phosphate. Secondly, FAD is reduced to FADH₂ while 3-phosphoglycerol is oxidized by flavoprotein dehydrogenase. Finally, FADH₂ is reoxidized as electrons enter the electron-transport chain.