increasing the rate of reaction. Function The pyruvate dehydrogenase complex is responsible for the oxidative decarboxylation of pyruvate, with the final product being Acetyl CoA. Overall the complex catalyzes five reactions, with the overall reaction being: Pyruvate + CoA + NAD+ → acetyl-CoA + CO2 There are three different coenzymes required throughout the 5 steps that this complex carries out: thiamine pyrophosphate (TPP), lipoamide, and coenzyme A. This step is only one of the central metabolic pathway carried out by eukaryotes, in which glucose is oxidized to form carbon dioxide, water, and ATP. The E1 complex specifically uses the TPP cofactor to cleave the Calpha-C(=O) bond of pyruvate, and then transfer the acetyl group to the TPP coenzyme, thus resulting in an intermediate, hydroxylethyl-Tpp*E1, and producing CO2. The thiazolium ring on the TPP is ideal for adding to carbonyl groups and acting as an electron sink, or a group that can pull electrons from a reaction and stabilize an electron-deficient intermediate. Regulation The activity of the PDH complex in mammalian tissues is largely determined by the phosphorylation of certain subunits within the complex. As such, the absolute amounts of site-specific kinases and phosphates expressed in the mitochondria directly affect PDH activity. Specifically, a tyrosine reside, Tyr-301, can be phosphorylated on PDHA1 and therefore inhibit its activity; this is done by blocking the substrate binding of pyruvate. This phosphorylation is in tandem with phosphorylation of the Ser-293 residue. Phosphorylation of another site, Ser-264, can regulate function. A phosphorylated serine residue at this position prevents TPP from ordering two loops within E1 that contain three phosphorylation sites. The disorder is the direct result of steric clash between the new phosphate group at Ser-264 and another serine, Ser-266, which destabilizes the hydrogen-bonding network that keeps the loops in place. Overall, this disorder results in the inactivation of the catalytic machinery. The PDH complex can be regulated in a variety of conditions. The PDHA1 subunit has been shown to be regulated by free fatty acids during bouts of exercise. The presence of free fatty acids increases the level of phosphorylation, thereby decreasing PDH activity. During exercise, however, these effects are overruled, and there is a much higher level of dephosphorylated PDHA1 in the cells. In certain muscles, such as the triceps, the metabolic enzyme profile seems to directly affect the level of PDH activity, which can result in higher levels of lactate in muscles