A principal function of thiamin in all cells is as the coenzyme cocarboxylase or TPP. The tricarboxylic acid cycle (TCA; citric acid cycle; Krebs cycle) is responsible for production of energy in the body. In this cycle, breakdown products of carbohydrates, fats and proteins are brought together for further breakdown and for synthesis. The vitamins riboflavin, pantothenic acid and niacin, as well as thiamin, play roles in the cycle. Thiamin is the coenzyme for all enzymatic decarboxylations of alpha-keto acids. Thus, it functions in the oxidative decarboxylation of pyruvate to acetate, which in turn is combined with coenzyme A (CoA) for entrance into the TCA cycle.
Thiamin is essential in two oxidative decarboxylation reactions in the TCA cycle that take place in cell mitochondria and one reaction in the cytoplasm of the cells (Figure 1). These reactions are essential for utilization of carbohydrates to provide energy. Decarboxylation in the TCA cycle removes carbon dioxide, and the substrate is converted into the compound having the next lower number of carbon atoms:
Pyruvate ---> acetyl-CoA + CO2
alpha-Ketoglutaric acid ---> succinyl-CoA + CO2