Introduction
Tryptophan synthase (TrpS) is an enzyme mainly found in bacteria, plants, and fungi [1]. TrpS must be functional for pathogenic bacteria in macrophages to survive, and it is used as a way to avoid host immune response. Tryptophan is necessary for bacteria and lower eukaryotic organisms for protein synthesis. Salmonella typhimurium and Escherichia coli serve as model organisms for the understanding of TrpS processes and structure [1].
Function
TrpS is a pyridoxal 5’-phosphate (PLP)-dependent enzyme. TrpS has an alpha and beta chain that form a linear alpha-beta-beta-alpha heterotetrameric complex [2]. This is termed as the alpha2beta2 complex, and each subunit is also referred to as TrpA and TrpB for the alpha and beta subunits respectively. The alpha active site contains catalytic residues Glu and Asp, and a hydrophobic intramolecular tunnel allows for the transport of indole from the alpha subunit active site to the beta subunit active site [3]. The alpha and beta chain are encoded by trpA and trpB genes that are involved in TrpS regulatory operon. In bacteria and plants, the alpha and beta chains are separate, but in fungi the two chains are fused into one protein (Tryptophan synthase, alpha chain, active site). The alpha2beta2 tetramer complex contains pyridoxal-phosphate, glycerol-3-phosphate, Na+ ions. Another key site in tryptophan synthase is the monovalent cation (MVC) site, which is made up of cations like Na+ and K+, along with Cs+ [4]. For regulation, TrpA and TrpB cycle between low-activity open conformation and a high-activity closed state. This is done by the binding of an IGP substrate to TrpA which promotes the high activity closed state which activates the TrpB high activity closed state. The states are reset back to low activity open conformation by the production of L-Trp external aldimine [2].
Disease
Relevance
Structural highlights
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