Glycerol-3-phosphate dehydrogenase (quinone)

Glycerol-3-phosphate dehydrogenase (quinone), also known as sn-glycerol-3-phosphate dehydrogenase (quinone) or G3PDH(quinone), is an enzyme that catalyzes the reversible reaction between glycerol-3-phosphate and a quinone, producing dihydroxyacetone phosphate and a reduced quinone (quinol). It belongs to the family of oxidoreductases, specifically those acting on the CH-OH group of donors with a quinone or similar compound as acceptor.

This enzyme plays a crucial role in linking glycolysis and mitochondrial oxidative phosphorylation. It allows the transfer of electrons from cytosolic NADH (via dihydroxyacetone phosphate and glycerol-3-phosphate) into the mitochondrial electron transport chain. Specifically, glycerol-3-phosphate dehydrogenase (quinone) is located on the outer surface of the inner mitochondrial membrane. It oxidizes glycerol-3-phosphate to dihydroxyacetone phosphate, reducing ubiquinone (coenzyme Q) to ubiquinol. Ubiquinol then enters the electron transport chain complex III, bypassing complex I and feeding electrons directly into the ubiquinone pool.

The enzyme's activity is important in tissues with high rates of glycolysis and oxidative metabolism, such as skeletal muscle and brown adipose tissue. In these tissues, it contributes to energy production and thermogenesis, respectively. The G3PDH(quinone) pathway is one of two major mechanisms for transferring cytosolic reducing equivalents into the mitochondria; the other is the malate-aspartate shuttle. The relative importance of these two pathways varies depending on the tissue and physiological conditions.

Different isoforms of glycerol-3-phosphate dehydrogenase exist, and they may utilize different electron acceptors, though the (quinone) variant specifically refers to the enzyme that interacts directly with ubiquinone.