Question

both with reasons!

D1. The reducing power of cytosolic NADH can enter the electron transfer chain via the malate-aspartate shuttle or the glycerophosphate shuttle. Describe why the ATP yield from these two routes differs. (10 marks) OR D2. An inherited mutation can lead to the loss of glucose-6-phosphatase activity Describe how this will affect the functioning of glycolysis and gluconeogenesis, and glycogen storage (10 marks)

Answer 1

D1. The glycerophosphate shuttle and malate shuttle help in the transport of reducing equivalent of NADH across the mitochondrial membrane as NADH itself cannot cross the inner mitochondrial membrane.

In the case of glycerophosphate shuttle, flavoprotein dehydrogenase helps in the conversion of Glycerol-3- phosphate to dihydroxyacetone phosphate (DHAP). This enzyme is an FAD- dependent enzyme located in the inner mitochondrial membrane and it donates electrons directly to Coenzyme Q without pumping protons, similar to complex II of electron transport chain. So, the electrons can only be used to synthesize a maximum of two ATPs.

In the malate aspartate shuttle, there is net movement of the electrons from NADH from the cytosol side to inside of mitochondria. Thus, the energy of the NADH electrons is conserved and could be used to make three ATPs.

Both the shuttles result in the net movement of electrons across the inner mitochondrial membrane, with no net movement of the metabolites.

Malate-aspartate shuttle Mitochondrial matri Cytoso Malate NAD Malate NAD NADH NADH Oxaloacetate oxaloacetate Aspartate Aspartate

CYTOPLASM Glycerol 3-phosphate NAD Glycerol 3-phosphate FAD Glycerol 3-phosphate dehydrogen ase Glycerol 3-phosphate dehydrogen ase Dihydroxyacetone phosphate NADH H Dihydroxyacetone phosphate FADH Outer mito chondrial Inner mitochondria - membrane membrane Glycolysis Electron trans port chain