Question

3. Explain why and how the malate-aspartate shuttle moves nicotinamide adenine dinucleotide reducing equivalents from the cytosol to the mitochondrial matrix.

Answer 1

Malate aspartate shuttle means a biochemical system of electron translocation from glycolysis to inner mitochondrial membrane for oxidative phosphorelation in eukaryotes.This system is needed because the inner mitochondrial membrane is impermeable to NADH,it is the primary reducing equivalent of electron transport chain.malate carries reducing equvalent through the membrane.

Malate dehydrogenase is the main enzyme responsible for this shuttle system.It present in two forms like mitochondrial malate dehydrogenase and cystolic malate dehydrogenase.

Initially, in cystol, oxalocaetate and NADH produces malate and NAD+, reaction is catalysed by malate dehydrogenase.In this two electrons will be generated from NADH and an H⁺ are attached to oxaloacetate to form malate.

Then importing of malate from cystol into the matrix of mitochondria and also exports of alpha-ketoglutarate  from the matrix into cystol occurs by the antiporter(malate-alpha-ketoglutarate) after the formation of malate.When malate reaches mitochondrial matrix it is converted to oxaloacetate by mitochondrial malate dehydrogenase,during this NAD⁺ will form NADH with reduction of two electrons.By mitochondrial aspartate aminotransferase,oxaloacetate is transformmed to aspartate(since oxaloacetate cannot be transported into cystol).Aminoacid radical is supplied by glutamate in aspartate.

NADH in cystol is oxidised to NAD⁺ and NAD⁺ in the matrix is reduced NADH,from this we can say that the net effect of malate-aspartate shutle is purely redox reaction.Another round of glycolysis can reduce again the NAD⁺ in the cystol and NADH in the matrix can be used for passing of electrons to electron transport chain,which can produce ATP