How to recycle back the surplus formation of NADH?

TCA CYCLE

cytoplasm mitochondria membrane

Most efficient shuttle mechanism found in mammalian kidney, liver and heart. Make use of the fact that malate can cross the membrane while oxaloacetate do not. VIP is that the NADH in the cytosol produces NADH in the mitochondrion. Aspartate can cross the membrane, then converted to oxalacetate in the cytosol, completing the cycle of the reactions. The NADH that is produced in the mitochondrion thus passes electrons to the elctron transport chain. With the malate-aspartate shuttle, 3 molecules of ATP are produced for each molecule of cytosolic NADH rather than 2 molecules of ATP in the glycerol phosphate shuttle, which uses FADH 2 as a carrier.

Glycerol phosphate shuttle Glycerol phosphate Dihydroxyacetone phosphate FAD FADH 2 ATP Mitochondrial glycerol phosphate dehydrogenase mitochondria Glycerol phosphate Dihydroxyacetone phosphate NADH + H + NAD + CYTOSOL GLYCOLYSIS Cytosol glycerol phosphate dehydrogenase membrane

Produced during Glycolysis/ TCA cycle Needed in Glycolysis step 6, TCA cycle, anaerobic condition

NADH is produced by glycolysis, which occurs in the cytosol, but NADH in the cytosol cannot cross the mitochondrial membrane to enter the electron transport chain. However the electrons can be transferred to a carrier that can cross the membrane. The number of ATP molecules generated depends on the nature of the carrier, which varies according to the type of cell in which it occurs. One carrier that has been extensively studied is Glycerol phosphate shuttle in insect flight muscle. Glycerol phosphate and dihydroxyacetone can cross over the membrane. Oxidising agent is FAD and the product is FADH 2, which then passes electrons through the electron transport chain, leading to the production of 2 molecules of ATP for each molecule of cytosolic NADH. This mechanism has also been observed in muscle and membrane.