Histidine catabolism to glutamate

A reusable pathway for degradation of L-histidine to L-glutamate. Three conserved reactions convert histidine through trans-urocanate and 4-imidazolone-5-propanoate to N-formimidoyl-L-glutamate. Terminal processing then differs among organisms. A two-step route uses HutF and HutG to release ammonium and formate; a one-step route uses a formimidoylglutamase to release formamide directly. The folate-coupled route instead transfers the formimino group to tetrahydrofolate and feeds it into one-carbon metabolism.

This document replaces a one-part human-only wrapper. The common chemistry is now exposed as three ordered reaction parts and the terminal route is modeled explicitly as three implementation variants. The Pseudomonas putida KT2440 genes provide concrete exemplars for the two-step formate route, Bacillus subtilis HutG provides an exemplar for the one-step formamide route, and completed human reviews ground the folate-coupled FTCD route. These exemplars do not restrict the taxonomic scope of the route variants. Histidine transport, Hut transcriptional regulation, ammonia reassimilation, and downstream glutamate metabolism and folate one-carbon metabolism beyond the FTCD-intrinsic cyclodeamination are outside the boundary. The OpenScientist synthesis is retained as research provenance: it generally expects one terminal route but leaves coexistence unresolved. ONE_OR_MORE is a conservative relaxation that does not assert either exclusivity or parallel route occurrence.