Functional Annotation Report: PaaH (PP_3282, UniProt Q88HS1)

3-Hydroxyadipyl-CoA dehydrogenase of Pseudomonas putida KT2440

1. Summary (Answer to the Research Question)

PaaH (gene paaH; ordered locus PP_3282; UniProt Q88HS1) of Pseudomonas putida KT2440 is a soluble, cytoplasmic, NAD⁺-dependent 3-hydroxyacyl-CoA dehydrogenase (EC 1.1.1.35) that acts in the lower, β-oxidation-like segment of the aerobic phenylacetate (phenylacetyl-CoA) degradation pathway. Its primary catalytic function is the stereospecific oxidation of (S)-3-hydroxyadipyl-CoA to 3-oxoadipyl-CoA (β-ketoadipyl-CoA), coupled to reduction of NAD⁺ to NADH. This is the penultimate redox reaction of the pathway; the resulting 3-oxoadipyl-CoA is thiolytically cleaved to succinyl-CoA + acetyl-CoA, which enter the TCA cycle. The identification rests on database/EC annotation, membership in the 3-hydroxyacyl-CoA dehydrogenase (PaaH) family with a confirmed N-terminal Rossmann NAD-binding fold, and functional characterization of the surrounding paa pathway in P. putida and E. coli.

Gene-identity verification: The gene symbol paaH, the organism (P. putida KT2440), the annotated function (3-hydroxyadipyl-CoA dehydrogenase, EC 1.1.1.35), and the InterPro family/domains (3-OHacyl-CoA_DH_PaaH, IPR011967; NAD-binding and C-terminal domains) are mutually consistent with the literature on the bacterial phenylacetate catabolon. No ambiguity or misassignment was detected.

2. Biochemical Function

2.1 Reaction catalyzed

(S)-3-hydroxyadipyl-CoA + NAD⁺ ⇌ 3-oxoadipyl-CoA + NADH + H⁺ (EC 1.1.1.35)

EC 1.1.1.35 is the general "3-hydroxyacyl-CoA dehydrogenase" that oxidizes the C3 hydroxyl of an (S)/L-3-hydroxyacyl-CoA thioester to the corresponding 3-oxo (β-keto) acyl-CoA, generating NADH. The reaction is the third of the four classic β-oxidation reactions (hydration → dehydrogenation → thiolysis) and is freely reversible in vitro [Barycki et al., 2000, PMID 10840044]. In PaaH the acyl substrate is the C6 dicarboxylic thioester 3-hydroxyadipyl-CoA, a ring-cleavage product of phenylacetyl-CoA catabolism rather than a straight-chain fatty acyl-CoA.

2.2 Substrate specificity

The substrate is defined by the pathway context and by the enzyme's subfamily assignment:
- The InterPro PaaH subfamily signature (IPR011967) distinguishes phenylacetate-pathway 3-hydroxyacyl-CoA dehydrogenases from general fatty-acid β-oxidation HADs, indicating specificity for the short-chain dicarboxylic 3-hydroxyacyl-CoA intermediate (3-hydroxyadipyl-CoA) generated after aromatic-ring opening [Ferrández et al., 1998, PMID 9748275; Olivera et al., 1998, PMID 9600981].
- Downstream chemistry constrains the substrate: because the terminal products of the pathway are succinyl-CoA + acetyl-CoA [Nogales et al., 2007, PMID 17259607], the immediate PaaH product must be 3-oxoadipyl-CoA (β-ketoadipyl-CoA), and therefore its substrate is 3-hydroxyadipyl-CoA.
- Family members are stereospecific for the (S)/L-3-hydroxy configuration [Barycki et al., 2000, PMID 10840044].

2.3 Cofactor

NAD⁺ is the electron acceptor. The N-terminal Rossmann dinucleotide-binding domain carries the cofactor; the homologous PaaH-type enzyme RePaaH1 shows "the cofactor-binding site is located on the Rossmann fold" [Kim et al., 2014, PMID 24792376].

3. Structural / Bioinformatic Evidence

Direct sequence analysis of Q88HS1 (this study) confirms the annotation:

• Length: 505 residues — consistent with the PaaH subfamily (e.g., E. coli PaaH ≈ 475 aa), which is larger than the ~300-aa mammalian mitochondrial HAD owing to extra subfamily-specific segments (InterPro IPR041040, 3HCDH_RFF).
• N-terminal Rossmann NAD-binding fingerprint: residues 12–22 read VAVIGAGAMGAG, containing the canonical G-x-G-x-x-G glycine-rich motif (GAGAMG, position 14) that binds the ADP moiety of NAD(H). This is diagnostic of the dinucleotide-binding fold.
• Two-domain architecture (by homology): N-terminal Rossmann NAD-binding domain + C-terminal α-helical dimerization domain, as crystallographically established for the closely related PaaH-family enzyme RePaaH1 [Kim et al., 2014, PMID 24792376] and for L-3-hydroxyacyl-CoA dehydrogenase [Barycki et al., 2000, PMID 10840044].
• Catalytic machinery (by family conservation): a His–Glu catalytic dyad in the interdomain cleft abstracts a proton from the C3 hydroxyl while NAD⁺ accepts the hydride [Barycki et al., 2000, PMID 10840044; Mugo et al., 2015, PMID 25446130].
• InterPro domains (from the target record): 3-OHacyl-CoA_DH_CS (IPR006180), 3-OHacyl-CoA_DH_NAD-bd (IPR006176), 3-OHacyl-CoA_DH_PaaH (IPR011967), 3HC_DH_C (IPR006108), 3HCDH_RFF (IPR041040) — all congruent with an NAD-dependent 3-hydroxyacyl-CoA dehydrogenase of the phenylacetate pathway.

This constitutes convergent bioinformatic/structural inference (sequence motif + family + homologous crystal structures) supporting the annotated function.

4. Pathway Context

PaaH is one enzyme of the phenylacetyl-CoA catabolon, an aerobic "hybrid" pathway that funnels many aromatic compounds into central metabolism [Ferrández et al., 1998, PMID 9748275; Luu et al., 2013, PMID 23377939]. The pathway, defined genetically in P. putida U/KT2440 and E. coli, comprises [Olivera et al., 1998, PMID 9600981; Ferrández et al., 1998, PMID 9748275]:

1. Activation — phenylacetate → phenylacetyl-CoA (PaaK, phenylacetate–CoA ligase).
2. Ring 1,2-epoxidation/hydroxylation — a multicomponent oxygenase (PaaABC(D)E) acting on the CoA-activated ring.
3. Ring opening — a hydratase/aldehyde-dehydrogenase (PaaZ) that cleaves the ring to a linear dicarboxylic-CoA (a 3-oxo-unsaturated adipyl/suberyl-CoA species).
4. β-oxidation-like processing — enoyl-CoA hydratase/isomerase, then PaaH (3-hydroxyacyl-CoA dehydrogenase), then a β-ketoadipyl-CoA thiolase.
5. Termination — thiolytic cleavage of 3-oxoadipyl-CoA to succinyl-CoA + acetyl-CoA [Nogales et al., 2007, PMID 17259607].

PaaH's specific position: it performs step 4's dehydrogenation, converting 3-hydroxyadipyl-CoA → 3-oxoadipyl-CoA (β-ketoadipyl-CoA). The product is the direct substrate of the terminal thiolase (in P. putida shown to yield succinyl-CoA + acetyl-CoA), so PaaH sits immediately upstream of the pathway's convergence with central carbon metabolism and with the β-ketoadipate node. This pathway allows P. putida to use phenylacetate as sole carbon and energy source (and it is chemotactically attractive) [Luu et al., 2013, PMID 23377939]. The catabolon integrates upper routes for phenylethylamine/phenylethanol and styrene, all of which converge on phenylacetyl-CoA [Olivera et al., 1998, PMID 9600981; Arias et al., 2008, PMID 18177365; Alonso et al., 2003, PMID 14597173].

Note on nomenclature: In the P. putida KT2440 gene naming (used for PP_3282), paaH denotes the 3-hydroxyacyl-CoA dehydrogenase. Homologous paa clusters in other organisms use different letters for the same enzymatic steps (e.g., in some P. putida studies the terminal thiolase is designated by a different letter), so care is needed when mapping gene letters across strains. The enzymatic identity of PP_3282 as the 3-hydroxyadipyl-CoA dehydrogenase (EC 1.1.1.35) is unambiguous.

5. Subcellular Localization

PaaH is a soluble cytoplasmic (cytosolic) enzyme. Evidence:
- Its substrate and product are CoA thioesters, which are cytosolic metabolites of the intracellular β-oxidation-like module [Olivera et al., 1998, PMID 9600981].
- The 505-aa sequence has no N-terminal signal peptide and no transmembrane segment, and the 3-hydroxyacyl-CoA dehydrogenase family is uniformly soluble/globular [Barycki et al., 2000, PMID 10840044; Kim et al., 2014, PMID 24792376].
- The enzyme is predicted to function as a homodimer (via the C-terminal domain), like characterized family members.

6. Supported and Refuted Hypotheses

7. Limitations and Future Directions

• No direct enzymology on Q88HS1 itself: substrate specificity and kinetic parameters are inferred from pathway context and close homologs, not measured for the KT2440 protein. Direct assay with (S)-3-hydroxyadipyl-CoA vs. straight-chain and other dicarboxylic 3-hydroxyacyl-CoAs would establish specificity and any promiscuity.
• No experimental structure of Q88HS1: the two-domain fold, dimerization, and His–Glu dyad positions are inferred from RePaaH1 and human HAD. A crystal/AlphaFold structure would localize the catalytic residues and confirm cofactor geometry.
• Nomenclature mapping: cross-strain paa gene-letter differences warrant care; confirming PP_3282 operon context and coexpression with the other paa genes in KT2440 would solidify the assignment.
• Regulation: the pathway is induced by phenylacetyl-CoA via a PaaX-type repressor in E. coli; verifying PaaH's transcriptional control in KT2440 would complete the physiological picture.

Key References

• Olivera et al. 1998, PNAS — P. putida U phenylacetyl-CoA catabolon; β-oxidation-like system (PMID 9600981)
• Ferrández et al. 1998, J Biol Chem — E. coli paa cluster; β-oxidation-like enzymes (PMID 9748275)
• Nogales et al. 2007, Microbiology — terminal step; succinyl-CoA + acetyl-CoA products (PMID 17259607)
• Luu et al. 2013, Appl Environ Microbiol — PAA as funneling pathway / growth substrate (PMID 23377939)
• Kim et al. 2014, Biochim Biophys Acta — PaaH1 (Ralstonia) crystal structure; Rossmann + dimerization domains (PMID 24792376)
• Barycki et al. 2000, Biochemistry — L-3-hydroxyacyl-CoA dehydrogenase mechanism, His-Glu dyad (PMID 10840044)
• Arias et al. 2008 (PMID 18177365); Alonso et al. 2003 (PMID 14597173) — upstream catabolon routes converging on phenylacetyl-CoA

Artifacts

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• OpenScientist final report