hemBB (locus PP_3322; UniProt Q88HN1) encodes delta-aminolevulinic acid dehydratase (ALAD), also known as porphobilinogen synthase (PBGS), EC 4.2.1.24. Its primary function is to catalyze the asymmetric (Knorr-type) condensation of two molecules of 5-aminolevulinic acid (ALA) into one molecule of the monopyrrole porphobilinogen (PBG), releasing two water molecules. This is the second, committed step of the tetrapyrrole biosynthesis pathway that in P. putida leads principally to heme (and siroheme). The enzyme works in the cytoplasm as a soluble homo-oligomer built on a (β/α)₈ TIM-barrel fold. Bioinformatic analysis of the active site indicates it belongs to the Zn-independent, Mg²⁺-dependent PBGS subclass, like its well-characterized ortholog in Pseudomonas aeruginosa.
Identity verification (required): The UniProt annotation (ALAD, EC 4.2.1.24, ALAD/PF00490 family, Aldolase_TIM domain, P. putida KT2440) is fully consistent with the enzyme identity established here. The gene symbol hemBB is the annotation used in the original KT2440 genome submission (EMBL AAN68929.1); it denotes a single hemB/PBGS gene and is not ambiguous with respect to protein function. Direct experimental literature exists for the very close Pseudomonas ortholog; findings for the KT2440 protein itself are supported by combining this ortholog evidence with sequence/structure inference.
Reaction: 2 × 5-aminolevulinic acid → porphobilinogen + 2 H₂O (EC 4.2.1.24).
During the initial steps of heme biosynthesis, two molecules of 5-aminolevulinic acid are asymmetrically condensed to porphobilinogen (Heinemann et al., 2010, PMID 19822707). The two substrate molecules bind at distinct subsites — the A-side (which contributes the acetate/aminomethyl-bearing part) and the P-side (which contributes the propionate side) — and are joined through an aldol condensation followed by Schiff-base–mediated C–N bond formation to build the single asymmetrically substituted pyrrole ring of PBG.
Substrate specificity: The enzyme is specific for 5-aminolevulinic acid (ALA). Specificity and catalysis depend on active-site lysines that form Schiff bases with the two ALA molecules. In Q88HN1 the invariant catalytic lysine motif "VMVKPGM" (Lys262) is conserved (aligning with the A-side catalytic lysine, human Lys252), confirming an intact, catalytically competent active site (sequence analysis, this work; mechanism per Jaffe, 2016, PMID 27783504). The ALA analog antibiotic alaremycin is a direct competitive-type inhibitor of the Pseudomonas PBGS active site (Ki ≈ 1.33 mM), underscoring the strict ALA specificity of the pocket (Heinemann et al., 2010, PMID 19822707).
PBGS enzymes evolved to use an unusual variety of metal ions both for catalytic function and to control protein multimerization (Jaffe, 2016, PMID 27783504). Two subclasses are recognized:
Q88HN1 clearly belongs to the Mg²⁺-dependent subclass. The 333-aa sequence has only 5 cysteines (positions 113, 229, 313, 322, 325), none in the catalytic metal loop, where the aligned region reads GDVALDPYTDHGHDG — aspartates in place of the zinc-binding cysteines. This is the diagnostic Asp-for-Cys signature of the magnesium-utilizing PBGS exemplified by the P. aeruginosa ortholog, which is Zn-independent and Mg²⁺/K⁺-responsive (sequence analysis, this work; framework from Jaffe, 2016, PMID 27783504). Functional consequence: the P. putida enzyme is expected to be insensitive to lead inhibition at a catalytic-Zn site and structurally distinct from the human host enzyme.
ALAD/PBGS catalyzes step 2 of tetrapyrrole biosynthesis:
Thus hemBB supplies the universal pyrrole building block for all cellular tetrapyrroles. In the non-photosynthetic aerobe P. putida, the dominant end-product is heme for cytochromes and other hemoproteins that support aerobic respiration and redox metabolism, making this enzyme essential for viability. Because PBGS is essential in bacteria and structurally divergent from the human enzyme (Mg vs Zn; bacterial oligomer interfaces), it is a validated antibacterial drug target (alaremycin; Heinemann et al., 2010, PMID 19822707).
| Claim | Evidence type | Source |
|---|---|---|
| Enzyme = ALAD/PBGS, EC 4.2.1.24, catalyzes 2 ALA → PBG | Database annotation + direct biochemistry on ortholog | UniProt Q88HN1; PMID 19822707; PMID 27783504 |
| Second/committed step of tetrapyrrole (heme) biosynthesis | Authoritative review | PMID 27783504 |
| Mg²⁺-dependent, Zn-independent subclass | Sequence/active-site analysis (Asp-for-Cys) + evolutionary framework | This work; PMID 27783504 |
| Conserved Schiff-base lysine (KPGM, Lys262) | Sequence motif analysis | This work; mechanism PMID 19822707 |
| (β/α)₈ TIM-barrel, homo-octamer, morpheein | Domain signatures + reviews | InterPro/Pfam; PMID 31952692; PMID 23409765 |
| Cytoplasmic localization | Inference (soluble enzyme, no targeting signals) | This work; general PBGS biology |
| Antibacterial target (active-site inhibitor alaremycin) | Co-crystal structure + inhibition kinetics on ortholog | PMID 19822707 |
Supported:
- H1: hemBB is a functional ALAD/PBGS catalyzing 2 ALA → PBG. (Supported: annotation, conserved catalytic motifs, ortholog biochemistry.)
- H2: The enzyme is Mg²⁺-dependent (not the mammalian Zn-type). (Supported: absence of the catalytic triple-Cys motif; Asp substitutions.)
- H3: It functions cytoplasmically as an oligomeric TIM-barrel enzyme. (Supported: domain signatures and PBGS structural biology.)
Refuted / ruled out:
- The protein is not a zinc metalloenzyme of the human/E. coli type (no catalytic Cys ligands) and is therefore not expected to be a lead-toxicity Zn-site target.
- The gene symbol "hemBB" does not indicate a functionally distinct protein or a mis-assigned ortholog; it is the KT2440 genome annotation name for the single hemB/PBGS gene.
Prepared over Iterations 1–2. Citations refer to PubMed IDs (PMID). Sequence-based analyses were performed on the UniProt Q88HN1 sequence.