hemH (locus PP_0744; UniProt Q88PV4) of Pseudomonas putida KT2440 encodes protoporphyrin ferrochelatase (heme synthase / protoheme ferro-lyase; EC 4.98.1.1), the terminal enzyme of the tetrapyrrole/heme‑biosynthetic pathway. Its primary function is to catalyze the insertion of one ferrous iron (Fe²⁺) ion into protoporphyrin IX, releasing two protons and producing protoheme IX (heme b) — the essential cofactor of cytochromes, catalases, and peroxidases. Because P. putida is a Gram‑negative (diderm) bacterium, its ferrochelatase belongs to the classical protoporphyrin‑dependent branch of heme biosynthesis and acts on protoporphyrin IX (not coproporphyrin III). The enzyme works in the cytoplasm / at the cytoplasmic face of the inner membrane, where it obtains Fe²⁺ and the porphyrin precursor and hands off heme to apo‑hemoproteins.
| Attribute | Value |
|---|---|
| Gene symbol | hemH |
| Ordered locus | PP_0744 |
| UniProt | Q88PV4 |
| Organism | Pseudomonas putida KT2440 (ATCC 47054 / DSM 6125), a Gram‑negative γ‑proteobacterium |
| Protein | Ferrochelatase / heme synthase / protoheme ferro‑lyase |
| EC | 4.98.1.1 (reclassified from the historical 4.99.1.1) |
| Length / mass | 338 aa / 37.8 kDa |
| Family / domains | Ferrochelatase family; PF00762, Ferrochelatase_N (IPR033659), Ferrochelatase_C (IPR033644) |
Verification outcome — identity confirmed and unambiguous. The gene symbol hemH is the canonical bacterial designation for ferrochelatase and is fully consistent with the protein description, family, and Pfam/InterPro domains. Orthology is quantitatively confirmed: Q88PV4 shares 43.9 % sequence identity with E. coli HemH (P23871) with full conservation of the catalytic core (see §4). The catalytic activity, pathway assignment (protoheme from protoporphyrin‑IX, step 1/1), and the presence of the diagnostic ferrochelatase sequence motifs (see §4) all agree. There is no ambiguity of the kind that affects poorly characterized symbols; hemH → ferrochelatase is a one‑to‑one, well‑established assignment across bacteria. Direct genetic proof exists for orthologues: the cloned hemH gene is necessary and sufficient to rescue ferrochelatase‑deficient mutants, and its purified ~39‑kDa product carries ferrochelatase activity (Frustaci & O'Brian, 1993; PMID 8368826).
Ferrochelatase catalyzes the final committed step of heme biosynthesis:
Protoporphyrin IX + Fe²⁺ → protoheme IX (heme b) + 2 H⁺
(UniProt/Rhea RHEA:22584, written in the reverse sense as heme b + 2 H⁺ = protoporphyrin IX + Fe²⁺; EC 4.98.1.1)
"Ferrochelatase catalyzes the insertion of ferrous iron into a porphyrin macrocycle to produce the essential cofactor, heme" (Medlock et al., 2021; PMID 34402499). UniProt assigns the pathway as "protoheme biosynthesis; protoheme from protoporphyrin‑IX: step 1/1" — i.e., this enzyme performs the single, terminal metallation step. The product, heme b, is the prosthetic group of respiratory cytochromes (b/o/d oxidases, cytochrome bc₁), catalases and peroxidases, and other hemoproteins that are indispensable for the aerobic, respiratory lifestyle of P. putida.
Substrate specificity. P. putida is a diderm, so it uses the protoporphyrin‑dependent pathway and its ferrochelatase is a protoporphyrin ferrochelatase (PpfC‑type) whose porphyrin substrate is protoporphyrin IX (2 propionate + 2 vinyl substituents). This contrasts with the coproporphyrin‑dependent pathway "used almost exclusively by monoderm bacteria" (Gabler et al., 2024; PMID 38390750), discovered in 2015 (Gabler et al., 2022; PMID 34719106), in which coproporphyrin ferrochelatases (CpfC) act on the four‑propionate substrate coproporphyrin III. Correctly placing HemH in the protoporphyrin‑dependent branch is essential for defining its true substrate.
Metal‑ion selectivity. In vitro, ferrochelatases can insert several divalent metals (Fe²⁺, Co²⁺, Ni²⁺, Zn²⁺, Cu²⁺), but Fe²⁺ is the physiological and kinetically preferred substrate and, uniquely, is not subject to severe substrate inhibition (Hunter et al., 2008; PMID 18593702). Metals such as Mn²⁺, Pb²⁺, Cd²⁺, Hg²⁺ act as inhibitors; high‑resolution crystallography shows that this "inhibition" is not a failure to insert the metal but is set at the product‑release step via a conserved π‑helix conformational switch (Medlock et al., 2009; PMID 19703464). Thus physiological Fe²⁺ specificity is achieved largely through iron delivery and controlled product release rather than by simple exclusion of competing metals.
Sequence analysis of Q88PV4 localizes the invariant ferrochelatase catalytic apparatus onto the P. putida protein:
LGSP motif (residues 11–14) — part of the conserved porphyrin/metal‑binding pocket signature of the family.SFHGLP motif — the conserved active‑site histidine (positionally homologous to B. subtilis His183 and human His263); annotated by UniProt as a binding site.The catalytic cycle, established from structural, kinetic, HDX‑MS and QM/MM studies of homologues, comprises:
Possible [2Fe‑2S] cluster (bioinformatic + structural inference, not annotated). Q88PV4 has 8 cysteines; four of them (C213, C214, C225, C230) fall within an internal ~18‑residue segment (AADCC…VSSVCYRGQC). This pattern matches the "new class" of bacterial protoporphyrin ferrochelatases that ligate a [2Fe‑2S] cluster via four cysteines in an internal ~20‑residue segment (Shepherd et al., 2006; PMID 16548850), distinct from the C‑terminal cluster of animal enzymes; such clusters were also confirmed in Caulobacter and Mycobacterium ferrochelatases (Dailey & Dailey, 2002; PMID 11948160).
Structural support (Iteration 2). The AlphaFold model AF‑Q88PV4‑F1 (overall mean pLDDT 96.5) shows these internal cysteines converging into a compact pocket — SG–SG distances C214–C225 = 3.4 Å, C213–C225 = 4.1 Å, C213–C214 = 3.9 Å (with C230 ~8.6 Å away), all at high local confidence (pLDDT 85–98). Because AlphaFold does not place metals, the convergence of three thiols within ~3.5–4 Å is the geometric signature of a metal/cofactor‑coordination site rather than random surface cysteines.
Comparative‑genomics support (Iteration 3). Global alignment of Q88PV4 against E. coli HemH (P23871) — an enzyme experimentally known to lack a [2Fe‑2S] cluster — gives 43.9 % identity (clear orthology) with a fully conserved catalytic core (P. putida His189 ↔ E. coli His194; Glu294 ↔ Glu275; N‑terminal Leu11 ↔ Leu13). Crucially, E. coli HemH has only 4 cysteines total, and three of P. putida's four internal cysteines are absent in E. coli at the aligned positions: C213 ↔ E. coli Arg210, C225 ↔ Thr214, C230 ↔ Glu217 (only C214 ↔ E. coli Cys211). Thus P. putida carries exactly the extra cysteines expected of the cluster‑containing subclass, precisely where the cluster‑less enzyme does not.
Importantly, HAMAP rule MF_00323 does not annotate a cluster for Q88PV4, so this remains a testable hypothesis (best resolved by EPR/UV‑vis of the purified protein and Cys→Ser mutagenesis), not an established fact; if present, the cluster is proposed to modulate iron affinity/activity rather than be strictly required for catalysis.
UniProt annotates the subcellular location of Q88PV4 as cytoplasm. Consistent with this, bacterial ferrochelatases are commonly peripheral inner‑membrane proteins acting at the cytoplasmic face; for example, the Caulobacter crescentus enzyme "is a homodimeric, membrane‑associated protein" (Dailey & Dailey, 2002; PMID 11948160). This localization places HemH where it can draw Fe²⁺ and protoporphyrin IX from the cytoplasm and deliver newly made heme b to membrane‑embedded apo‑hemoproteins (e.g., respiratory oxidases of the inner membrane).
HemH acts at the very end of the conserved tetrapyrrole pathway:
glutamate → 5‑aminolevulinate (Hem A/L pathway) → porphobilinogen (HemB) → hydroxymethylbilane (HemC) → uroporphyrinogen III (HemD) → coproporphyrinogen III (HemE) → protoporphyrinogen IX (HemF/HemN) → protoporphyrin IX (HemG/HemY) → heme b (HemH / ferrochelatase). In diderms such as P. putida, HemH is the sole terminal metallation step supplying heme for the respiratory chain and for oxidative‑stress defense enzymes (catalase/peroxidases). Its product heme b is also the precursor for heme o/heme d and c-type cytochrome maturation. Because heme is essential and Fe²⁺ is potentially toxic, this step sits at the intersection of iron homeostasis and respiratory metabolism, which is why the pathway (and ferrochelatase specifically) is an attractive antimicrobial target in pathogens (e.g., N‑methyl mesoporphyrin inhibition; Wu et al., 2009; PMID 19597542).
Supported
- H1 — HemH/Q88PV4 is a protoporphyrin ferrochelatase catalyzing Fe²⁺ insertion into protoporphyrin IX to form heme b (EC 4.98.1.1). Strongly supported (UniProt/Rhea, HAMAP MF_00323, conserved motifs, homologue genetics PMID 8368826).
- H2 — Substrate is protoporphyrin IX because P. putida is a diderm using the protoporphyrin‑dependent pathway. Supported (PMID 34719106, 38390750).
- H3 — Catalysis proceeds via porphyrin distortion + His/Glu‑mediated proton abstraction/metal insertion; His189 and Glu294 are the catalytic residues. Supported (PMID 18423489, 34402499).
- H4 — Physiological Fe²⁺ specificity is governed by iron delivery / product release. Supported (PMID 19703464, 18593702).
- H5 — Enzyme functions in the cytoplasm / cytoplasmic face of the inner membrane. Supported (UniProt; PMID 11948160).
Open / Not yet resolved
- H6 — P. putida HemH binds an internal‑ligand [2Fe‑2S] cluster (four internal Cys). Plausible but unproven, now with three converging lines of inference: (i) internal 4‑Cys motif matching the Shepherd/Dailey cluster subclass (PMID 16548850, 11948160); (ii) AlphaFold geometry placing three thiols within 3.4–4.1 Å; (iii) comparative alignment showing three of these cysteines are absent in the cluster‑less E. coli enzyme. Still requires experimental spectroscopy/mutagenesis; not annotated by UniProt/HAMAP.
Refuted / Excluded
- The enzyme is not a coproporphyrin ferrochelatase (CpfC) and does not use coproporphyrin III as its physiological substrate, because that pathway is restricted to monoderm bacteria (PMID 38390750).