hisI encodes phosphoribosyl-AMP cyclohydrolase (EC 3.5.4.19), which hydrolyzes the adenine ring of phosphoribosyl-AMP in the third reaction of L-histidine biosynthesis. The 130-residue Pseudomonas putida KT2440 enzyme is a standalone cyclohydrolase rather than a HisIE fusion and binds magnesium and zinc ions. The preceding phosphoribosyl-ATP diphosphatase reaction is carried out by the adjacent standalone HisE protein, Q88D14.
| GO Term | Evidence | Action | Reason |
|---|---|---|---|
| GO:0000105 L-histidine biosynthetic process | IEA GO_REF:0000120 | ACCEPT | Summary: HisI performs the third reaction of the PRPP-to-histidine pathway. Reason: The reviewed UniProt entry and UniPathway assignment place the exact cyclohydrolase reaction at step 3 of 9 in L-histidine biosynthesis. |
| GO:0000287 magnesium ion binding | IEA GO_REF:0000104 | KEEP AS NON CORE | Summary: Magnesium binding is supported for the standalone cyclohydrolase. Reason: UniRule records one Mg2+ ion per subunit. This is relevant catalytic context but is less informative than the specific cyclohydrolase activity. |
| GO:0004635 phosphoribosyl-AMP cyclohydrolase activity | IEA GO_REF:0000120 | ACCEPT | Summary: This is the defining HisI activity in KT2440. Reason: GO:0004635 matches EC 3.5.4.19, RHEA:20049, the reviewed product name, and the HisI/PRA-CH family assignments. The support cited is the curated HAMAP MF_01021 FUNCTION statement and the PRA-CH family/domain classifications (rule-based, not a KT2440 assay). Supporting Evidence: file:PSEPK/hisI/hisI-uniprot.txt Catalyzes the hydrolysis of the adenine ring of phosphoribosyl-AMP. file:PSEPK/hisI/hisI-uniprot.txt HAMAP; MF_01021; HisI; 1. file:PSEPK/hisI/hisI-uniprot.txt Pfam; PF01502; PRA-CH; 1. |
| GO:0004636 phosphoribosyl-ATP diphosphatase activity | IEA GO_REF:0000117 | REMOVE | Summary: The diphosphatase activity belongs to the separate HisE protein in KT2440. Reason: This ARBA assignment appears to have propagated across a family that also contains bifunctional HisIE proteins. Q88D15 is a 130-residue standalone phosphoribosyl-AMP cyclohydrolase, whereas adjacent Q88D14 is the reviewed standalone phosphoribosyl-ATP diphosphatase with EC 3.6.1.31 and RHEA:22828. Supporting Evidence: file:PSEPK/hisE/hisE-uniprot.txt Full=Phosphoribosyl-ATP pyrophosphatase file:PSEPK/hisI/hisI-deep-research-openscientist.md *hisI* is a **standalone monofunctional gene** (PP_5014, 130 aa) sitting adjacent to a separate monofunctional *hisE* gene |
| GO:0005737 cytoplasm | IEA GO_REF:0000120 | KEEP AS NON CORE | Summary: HisI is a soluble cytoplasmic biosynthetic enzyme. Reason: The location is supported by UniProt but is contextual rather than the core biochemical function. |
| GO:0008270 zinc ion binding | IEA GO_REF:0000104 | KEEP AS NON CORE | Summary: Zinc binding is supported for the standalone cyclohydrolase. Reason: UniRule records one Zn2+ ion per subunit. This is supporting cofactor context rather than a pathway-defining activity. |
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