TrpD

trpD is a bacterial gene encoding anthranilate phosphoribosyltransferase (APRT), an enzyme in the tryptophan biosynthesis pathway. The enzyme is typically cytosolic and participates in the early steps of constructing the indole-containing amino acid tryptophan. TrpD is usually annotated as such in bacterial genomes and has the Enzyme Commission number EC 2.4.2.18. In many bacteria, trpD is located within the tryptophan operon (often trpEDCBA), where it is co-transcribed with other genes involved in tryptophan synthesis.

Functionally, TrpD catalyzes the phosphoribosylation of anthranilate using 5-phosphoribosyl-1-pyrophosphate (PRPP) to yield N-(5'-phosphoribosyl)-anthranilate (PRA) and pyrophosphate. This reaction provides the PRA intermediate that proceeds through subsequent steps catalyzed by TrpC (indole-3-g glycerol phosphate synthase) and other enzymes to form tryptophan. The reaction typically requires divalent metal ions such as Mg2+ and occurs within the cytosol of the cell.

Genetically, expression of trpD is generally coordinated with other tryptophan biosynthesis genes. In many bacteria, the trp operon is subject to feedback regulation by tryptophan through a repressor protein (TrpR) and transcriptional attenuation mechanisms that respond to intracellular tryptophan levels, thereby modulating the flux through the pathway.

Structural and evolutionary notes indicate that TrpD is conserved across bacteria and archaea, with homodimeric or higher-order quaternary structures reported in various species. As a key early enzyme in de novo tryptophan biosynthesis, TrpD is a focus of interest in antimicrobial research and in metabolic engineering aimed at optimizing tryptophan production.