What is Glycerol phosphate shuttle?

The glycerol-3-phosphate shuttle is a mechanism that regenerates NAD+ from NADH, a by-product of glycolysis. The shuttle consists of the sequential activity of two proteins: GPD1 which transfers an electron pair from NADH to dihydroxyacetone phosphate , forming glycerol-3-phosphate and regenerating NAD+ needed to generate energy via glycolysis. The mitochondrial inner membrane protein GPD2 catalyzes the oxidation of G3P, regenerating DHAP in the cytosol and forming FADH2 in the mitochondrial matrix. In mammals, its activity in transporting reducing equivalents across the mitochondrial membrane is considered secondary to the malate-aspartate shuttle.