 Glycerol-3-phosphate dehydrogenases are located both in the cytosol and the intermembrane face of mitochondrial inner membrane.Glycerol 3-phosphate (G3P) and dihydroxyacetone phosphate (DHAP) are molecules so small that they can permeate the mitochondrial outer membrane through porins and shuttle between two dehydrogenases.Using this shuttle system, NADH generated by cytosolic metabolisms including glycolysis is reoxidized to NAD+ reducing DHAP to G3P, and the reducing equivalent can be used for generating a proton gradient across the mitochondrial inner membrane by coupling and oxidizing G3P and reducing quinone.