Aspartate aminotransferase.

Aspartate aminotransferase (EC 2.6.1.1; AspAT) is present in all cellular life on the earth. It catalyses the removal of an amino group from an amino acid, followed by the subsequent amination of a keto acid, to create glutamate from aspartate or vice versa. AspAT is a vitamin B6-dependent enzyme that plays a pivotal role in carbon and nitrogen metabolism. While bacteria have only one AspAT isoenzyme, eukaryotic organisms have multiple isoenzymes that are located in different sub-cellular compartments. The structure of the AspAT enzyme was determined decades ago, and has since been refined. Most of this work was done using either the chicken (Gallus gallus), pig (Sus scrofa) or Escherichia coli versions of the enzyme. Many residues of the enzyme are 100% conserved and, as a convention, to make things comparable between species, they are numbered based on the pig version of the enzyme. AspAT functions as a homodimer, with each monomer binding to one molecule of pyridoxal-5′-phosphate (PLP), the active form of vitamin B6. The binding of the PLP cofactor to the AspAT monomer is required for both formation of the dimer and the enzymatic activity of AspAT. PLP is a cofactor for more than 100 enzymes in biology, and acts as an excellent electron sink for modifying the substrates of the PLP-dependent enzymes in many biochemical reactions, including decarboxylation, transamination, racemization and elimination. Recent studies suggest that PLP-dependent AspAT may play a role in PLP homeostatic sensing. The transgenic overexpression of various AspAT isoforms results in altered nitrogen metabolism and amino acid content in rice and Arabidopsis. Additional roles of AspATs associated with other physiological functions are also discussed in the chapter.