trpA

Gene Description

trpA encodes the alpha subunit of tryptophan synthase (EC 4.2.1.20), the enzyme catalyzing the final step of L-tryptophan biosynthesis. The alpha subunit carries out the retro-aldol (aldol) cleavage of (1S,2R)-1-C-(indol-3-yl)glycerol 3-phosphate (indole-3-glycerol phosphate) to yield indole and D-glyceraldehyde 3-phosphate. The indole intermediate is channeled through an internal intersubunit tunnel to the beta subunit (TrpB), where it is condensed with L-serine in a pyridoxal 5'-phosphate-dependent reaction to form L-tryptophan. The functional enzyme is a tetramer of two alpha and two beta chains (alpha-beta-beta-alpha), and the alpha and beta subunits mutually allosterically activate one another, with the alpha subunit having very low catalytic activity in isolation. In Pseudomonas putida KT2440, trpA (PP_0082) lies in a trpBA operon and is required for tryptophan prototrophy; disruption produces a tryptophan auxotroph. The protein is a soluble, cytosolic enzyme of the aromatic amino acid biosynthetic pathway, adopting a TIM-barrel (ribulose-phosphate-binding barrel) fold.

Existing Annotations Review

GO Term	Evidence	Action	Reason
GO:0000162 L-tryptophan biosynthetic process	IEA GO_REF:0000120	ACCEPT	Summary: Tryptophan synthase alpha subunit catalyzes the final (step 5/5) of L-tryptophan biosynthesis from chorismate. This biological process annotation is strongly supported by the conserved enzymology and by P. putida KT2440 genetics, where trpA disruption produces a tryptophan auxotroph. Reason: Core biological process of the gene; supported by experimental auxotrophy data (Molina-Henares et al. 2009) and UniPathway/UniProt pathway assignment.
GO:0004834 tryptophan synthase activity	IEA GO_REF:0000120	ACCEPT	Summary: TrpA enables the alpha reaction of tryptophan synthase, the aldol cleavage of indole-3-glycerol phosphate to indole and glyceraldehyde 3-phosphate (EC 4.2.1.20, RHEA:10532). This is the canonical, conserved molecular function captured by HAMAP rule MF_00131 and InterPro family signatures. Reason: Core molecular function, well supported by family/domain assignment (TrpA family, Pfam PF00290, TIGR00262) and EC/RHEA mapping. GO:0004834 is the standard term applied to both subunits of tryptophan synthase.
GO:0005829 cytosol	IEA GO_REF:0000118	ACCEPT	Summary: Tryptophan synthase is a soluble cytosolic enzyme complex; cytosolic localization is the expected compartment for this amino acid biosynthetic enzyme in bacteria and is consistent with the lack of any signal/membrane features in the sequence. Reason: Consistent with the soluble nature of the tryptophan synthase complex and the cytosolic localization of aromatic amino acid biosynthesis. Phylogeny-based (TreeGrafter) inference is reasonable for this conserved cytosolic enzyme.