| Analysis aspect | P. putida fcs (Q88HK0) / aromatic CoA ligase interpretation | Characterized 4CL / hydroxycinnamate-CoA ligases | Medium-chain fatty acid-CoA ligases (ACSM/MCS) | Implication for GO:0031956 |
|---|---|---|---|---|
| Superfamily membership | fcs belongs to the ANL (acyl-CoA synthetase/NRPS adenylation/luciferase) superfamily and therefore is expected to share the canonical AMP-forming fold and domain alternation chemistry with other acyl-CoA synthetases (pqac-00000025, pqac-00000026) | 4CL enzymes are established ANL-family adenylating enzymes using the same two-step adenylation/thioesterification mechanism (pqac-00000023, pqac-00000025) | Medium-chain FA-CoA ligases are also ANL-family members with the same broad fold and chemistry (pqac-00000008, pqac-00000013) | Shared superfamily does **not** justify direct transfer of the ACSM-specific MF term |
| Conserved catalytic core | Expected to retain the ANL catalytic core; no evidence of pseudization or catalytic loss, and pathway genetics show activity in vivo (pqac-00000025, pqac-00000026, pqac-00000030) | Conserved A1-A10 motifs, P-loop, catalytic His, catalytic Thr, and invariant A10 Lys are essential for adenylation in 4CL enzymes (pqac-00000025, pqac-00000029) | ACSM enzymes also retain the same superfamily-level catalytic motifs for adenylation/thioesterification (pqac-00000008, pqac-00000026) | Catalytic motif conservation supports “active ANL enzyme,” but not “medium-chain fatty acid substrate” |
| Invariant Lys / catalytic motifs | By ANL-family placement, fcs is expected to carry the invariant A10 Lys and associated catalytic motifs required for adenylate formation; functional complementation indicates these motifs are intact enough for catalysis in vivo (pqac-00000025, pqac-00000026, pqac-00000030) | Nt4CL2 uses invariant Lys526 (A10), His237, Thr336, and P-loop residues for catalysis; mutation abolishes or severely reduces activity (pqac-00000025, pqac-00000029) | ACSM/MCS enzymes use the same catalytic logic but with different substrate-recognition determinants around the pocket (pqac-00000008, pqac-00000013) | Failure mode is **not** pseudo-enzyme/lost activity |
| Aromatic substrate-recognition features | fcs function and physiology fit an aromatic hydroxycinnamate-binding pocket rather than an aliphatic chain tunnel: growth depends on fcs specifically on ferulate, caffeate, and p-coumarate (pqac-00000001, pqac-00000030) | 4CL pockets include residues enabling aromatic binding and π-stacking, e.g. Tyr239 equivalent, with cavity features formed by residues such as Met306, Gly308, Val341, Met344; optimized for flat hydroxycinnamic acids (pqac-00000022, pqac-00000028) | ACSM/MCS enzymes instead use hydrophobic aliphatic-chain tunnels rather than aromatic planar-substrate cavities (pqac-00000006, pqac-00000013) | Strongly inconsistent with medium-chain fatty acid-CoA ligase activity |
| Pocket geometry | Best explained by a broader aromatic-acid pocket in the 4CL-like branch of ANL enzymes (pqac-00000006, pqac-00000022) | Broader cavity accommodates coumarate/caffeate/ferulate-class substrates; aromatic face and steric gate shape hydroxycinnamate selectivity (pqac-00000022, pqac-00000016) | Narrower hydrophobic tunnel acts as a chain-length selector or “molecular ruler” for aliphatic medium-chain substrates (pqac-00000006, pqac-00000013) | Indicates wrong neighboring subfamily assignment rather than coarse family-level correctness |
| SDR position 210 | fcs should not be interpreted with the ACSM-specific medium-chain chain-length signature at this position (pqac-00000013) | Not the defining medium-chain signature in 4CL enzymes; aromatic-ligase classification depends on a different SDR pattern (pqac-00000009, pqac-00000012) | Position 210 is informative for fatty-acyl chain length, with His associated with medium-chain and Asn with long-chain CoA ligases (pqac-00000013) | Presence of ANL-family homology alone cannot override subfamily-specific SDR logic |
| SDR position 234 | fcs is more consistent with the aromatic-ligase SDR regime that includes conserved His at this position in 4CL-like enzymes (pqac-00000008) | Conserved His234 is characteristic in 4CL/LCS/MCS comparisons and contributes to attraction/positioning of larger carboxylic acid substrates in deep pockets (pqac-00000008) | ACSM subfamily is distinguished by a different overall 15-residue profile, not by simple transfer from 4CL-like profiles (pqac-00000009, pqac-00000015) | Supports need for subfamily-specific annotation |
| SDR position 324 | fcs should be interpreted in the context of aromatic-substrate accommodation rather than the small-substrate/chain-length logic used for aliphatic ACSs (pqac-00000008, pqac-00000012) | In ANL comparisons, pocket-opening effects around this position help define capacity for larger aromatic substrates (pqac-00000008, pqac-00000022) | Trp at equivalent position constricts pocket in small-substrate enzymes, while Gly can permit larger substrates; this illustrates how a few SDRs shift specificity across ANL neighbors (pqac-00000008) | Demonstrates how TreeGrafter can misplace proteins within a shared fold superfamily |
| SDR position 301 | fcs should follow the aromatic-CoA ligase active-site profile rather than ACSM-specific chain-binding constraints (pqac-00000011, pqac-00000015) | 4CL-like enzymes show subfamily-specific residue usage across the 15-SDR profile that predicts aromatic-acid activation (pqac-00000009, pqac-00000015) | Larger or different residues at position 301 contribute to acyl-chain pocket properties in fatty-acid ligases (pqac-00000011, pqac-00000013) | Another indicator that correct annotation requires SDR-level classification |
| Closest characterized functional neighborhood | fcs clusters functionally with hydroxycinnamate-activating enzymes such as CouL/fcs homologs, not ACSM enzymes (pqac-00000017, pqac-00000019) | CouL and related 4CL/fcs-type enzymes activate p-coumarate, ferulate, caffeate, and dihydroferulate; they do not behave as fatty-acid ligases (pqac-00000017, pqac-00000018, pqac-00000019) | ACSM enzymes specialize in aliphatic medium-chain fatty acids and are classified separately by SDR/pocket architecture (pqac-00000008, pqac-00000013) | Nearest characterized subfamily carries a sibling activity, not the propagated one |
| Genomic/pathway context | In KT2440, fcs is embedded in the ferulic acid/phenylpropenoid catabolic module with ech and vdh, exactly where a hydroxycinnamoyl-CoA ligase is expected (pqac-00000002, pqac-00000004, pqac-00000005) | 4CL/fcs-type enzymes function in hydroxycinnamate conversion to CoA esters in aromatic metabolism (pqac-00000002, pqac-00000003) | ACSM genes are associated with fatty-acid activation/metabolism, not ferulate-to-vanillin pathways (pqac-00000003, pqac-00000004) | Strong refutation of GO:0031956 by orthogonal pathway evidence |
| Experimental activity status | Catalytically active in vivo: deleting fcs abolishes growth on ferulate, caffeate, and p-coumarate; complementation restores growth (pqac-00000001, pqac-00000030) | Active hydroxycinnamoyl-CoA ligases show direct or inferred conversion of hydroxycinnamates to CoA esters (pqac-00000017, pqac-00000018) | No evidence that fcs supports utilization of medium-chain fatty acids (pqac-00000001, pqac-00000030) | Activity is intact, but the substrate class is wrong |
| Placement verdict | **fcs belongs to the 4CL / hydroxycinnamate-CoA ligase branch of the ANL superfamily** based on pathway context, mutant phenotype, characterized homologs, and aromatic-pocket structural logic (pqac-00000001, pqac-00000002, pqac-00000017, pqac-00000022) | Correct functional neighborhood: aromatic hydroxycinnamate CoA ligases / feruloyl-CoA synthetases (pqac-00000017, pqac-00000022) | Incorrect neighboring branch: medium-chain fatty acid-CoA ligases (pqac-00000008, pqac-00000013) | **Refutes GO:0031956; this is a within-superfamily mis-placement** |
| Curation consequence | Assign aromatic hydroxycinnamate CoA-ligase function, e.g. 4-coumarate-CoA ligase / feruloyl-CoA synthetase-type activity, rather than medium-chain fatty acid-CoA ligase (pqac-00000001, pqac-00000002, pqac-00000017) | More specific and biologically coherent with known hydroxycinnamate catabolism (pqac-00000002, pqac-00000017) | GO:0031956 should not be retained for fcs (pqac-00000001, pqac-00000030) | Replace with sibling-term in aromatic CoA-ligase branch |


*Table: This table compares P. putida fcs with aromatic 4CL-type ligases and medium-chain fatty acid-CoA ligases across catalytic motifs, substrate-pocket architecture, SDRs, and pathway evidence. It is useful for localizing the TreeGrafter error as a within-superfamily misplacement rather than loss of activity.*