-!- An essential membrane enzyme, functioning at the central junction of glycolysis, respiration and phospholipid biosynthesis. -!- In bacteria, the enzyme is localized to the cytoplasmic membrane, while in eukaryotes it is tightly bound to the outer surface of the inner mitochondrial membrane. -!- In eukaryotes, this enzyme, together with the cytosolic enzyme EC 1.1.1.8 forms the glycerol-3-phosphate shuttle by which NADH produced in the cytosol, primarily from glycolysis, can be reoxidized to NAD(+) by the mitochondrial electron-transport chain. -!- This shuttle plays a critical role in transferring reducing equivalents from cytosolic NADH into the mitochondrial matrix. -!- Insect flight muscle uses only CoQ(10) as the physiological quinone whereas hamster and rat mitochondria use mainly CoQ(9). -!- The enzyme is activated by calcium. -!- Formerly EC 1.1.2.1 and EC 1.1.99.5.