Abstract
Vitamin B6 functions primarily as pyridoxal 5′-phosphate (PLP) in body processes. PLP is the coenzyme of various enzymes involved in the metabolism of nitrogen-containing molecules, namely amino acids. PLP can catalyze a variety of reactions such as transamination, decarboxylation, elimination, replacement, and aldol condensation/cleavage. However, in the presence of enzyme proteins, PLP catalyzes a specific type of reaction and simultaneously enhances its catalytic efficiency. Analyses on the interactions between PLP and enzyme proteins are, therefore, of great importance for understanding the function of vitamin B6 enzymes. Aspartate aminotransferase imposes strain on the Schiff base between PLP and the active site Lys and increases the energy level of E + S, thereby enhancing kcat/Km by 103 folds. In threonine synthase, the phosphate ion released from the substrate O-phospho-L-homoserine remains at the active site and directs the reaction to catalyze the β-elimination/γ-addition reaction. Knowledge on the reaction specificity of decarboxylases is also important for elucidating the physiological functions of these enzymes in mammalian cells.
