Alanine aminotransferase: amino acid metabolism in higher plants.

Alanine aminotransferase (AlaAT, EC 2.6.1.2) catalyses the reversible transfer of an amino group from alanine (Ala) to 2-oxoglutarate to form pyruvate and glutamate. AlaAT is a pyridoxal phosphate-dependent enzyme that serves as the main route of alanine biosynthesis and degradation. This chapter provides a comprehensive review of the biochemical characteristics of AlaAT and provides insight into the physiological and metabolic functions of the enzyme as reported from several elegant studies across the world. The diversity in cellular and sub-cellular localizations of multiple AlaAT homologues indicates their involvement in multifaceted metabolic processes, including amino acid biosynthesis and primary carbon metabolism. The enzyme is fairly ubiquitous, occurring across different phyla - fungi, algae, mycorrhizae and higher plants, and existing as various isoforms. Detailed kinetic characterization of the enzyme from several laboratories provides interesting details about its plethora of biochemical functions. The recent elucidation of the crystal structure of plant AlaAT paves opportunities for expanding our understanding of the AlaAT reaction mechanism and catalysis. Additionally, the roles of AlaAT in hypoxia, photorespiration and nitrogen use efficiency are reviewed in terms of stress amelioration and overall plant physiology.