| Category | celM (P55742) summary | Evidence scope | Key references with publication date and URL |
|---|---|---|---|
| Identifier | **Gene/protein:** celM; **UniProt accession:** P55742; **Organism:** *Acetivibrio thermocellus* (= *Hungateiclostridium thermocellum*, formerly *Clostridium thermocellum*); protein is in the **M42 peptidase/TET aminopeptidase family**, not a bona fide cellulase in the tested biochemical assays (pqac-00000000, pqac-00000010) | **Direct CelM experiment** for reannotation; family assignment supported by sequence/structural comparison | Dutoit et al., **2012-11**, PLoS ONE, https://doi.org/10.1371/journal.pone.0050639 (pqac-00000000, pqac-00000010) |
| Function | Primary experimentally supported function is **aminopeptidase (leucine aminopeptidase-like) activity**, releasing N-terminal amino acids from short peptide-like substrates; **no significant cellulase activity** detected on cellobiose, carboxymethyl cellulose, or filter paper cellulose under assay conditions (pqac-00000000, pqac-00000010) | **Direct CelM experiment** | Dutoit et al., **2012-11**, https://doi.org/10.1371/journal.pone.0050639 (pqac-00000000, pqac-00000010) |
| Enzyme class | **Putative aminopeptidase, EC 3.4.11.-; M42 metallopeptidase / TET aminopeptidase.** Family-level work describes M42/TET enzymes as **dinuclear metallo-aminopeptidases** that form self-compartmentalized tetrahedral complexes and function in intracellular peptide turnover (pqac-00000006, pqac-00000011) | **Family-level inference** with CelM-specific sequence/functional consistency | Appolaire et al., **2016-03**, https://doi.org/10.1016/j.biochi.2015.11.001 (pqac-00000006, pqac-00000011); Bhat, **2024-04-24**, https://doi.org/10.1007/s12551-024-01192-8 (pqac-00000008) |
| Substrates tested & preference | CelM hydrolyzed **L-Leu-pNA best**; also active on **L-Ile-pNA, L-Met-pNA, L-Val-pNA, L-Ala-pNA**; no detectable activity on **D-Leu-pNA, L-Gly-pNA, L-Phe-pNA, L-His-pNA, L-Pro-pNA, L-Glu-pNA**; no aminoacylase activity toward **N-acetyl-L-Leu-pNA**. Reported specific activities included **L-Leu-pNA 291.7 ± 12.6**, **L-Ile-pNA 53.1 ± 13.1**, **L-Met-pNA 14.6 ± 1.5**, **L-Val-pNA 8.4 ± 0.016**, **L-Ala-pNA 2.1 ± 0.1** mmol min⁻¹ mmol⁻¹ enzyme. Catalytic efficiencies: **kcat/Km** for CelM were **114.3 s⁻¹ M⁻¹ (Leu-pNA)**, **39.3 s⁻¹ M⁻¹ (Ile-pNA)**, **24.8 s⁻¹ M⁻¹ (Met-pNA)** (pqac-00000003, pqac-00000004, pqac-00000010) | **Direct CelM experiment** | Dutoit et al., **2012-11**, https://doi.org/10.1371/journal.pone.0050639 (pqac-00000003, pqac-00000004, pqac-00000010) |
| Metal dependence | Assay optimization identified **1 mM Co²⁺** as optimal; at 100 µM, CelM activity increased from **3.8 ± 1.3** (no added metal) to **319.6 ± 20.8 mmol min⁻¹ mmol⁻¹** with **Co²⁺**; other divalent ions gave much lower activity (**Ni²⁺ 8.5 ± 0.6; Zn²⁺ 1.0 ± 0.1; Mn²⁺ 4.5 ± 0.4; Fe²⁺ 4.5 ± 0.6; Mg²⁺ 3.9 ± 0.3; Cu²⁺ 4.3 ± 0.4; Ca²⁺ 2.9 ± 0.2**). Family-level studies indicate M42 enzymes are **dinuclear metal enzymes** whose catalytic state and oligomerization depend on metal loading (pqac-00000005, pqac-00000009, pqac-00000011) | **Direct CelM experiment** plus **family-level mechanistic support** | Dutoit et al., **2012-11**, https://doi.org/10.1371/journal.pone.0050639 (pqac-00000005, pqac-00000009, pqac-00000012); Appolaire et al., **2016-03**, https://doi.org/10.1016/j.biochi.2015.11.001 (pqac-00000011); Bhat, **2024-04-24**, https://doi.org/10.1007/s12551-024-01192-8 (pqac-00000008) |
| Inhibitors | CelM was strongly inhibited by **EDTA** (>98% inhibition under reported conditions) and by **bestatin**, with apparent **Ki ≈ 292 ± 66 nM**, supporting assignment as a **metalloaminopeptidase** (pqac-00000002, pqac-00000010) | **Direct CelM experiment** | Dutoit et al., **2012-11**, https://doi.org/10.1371/journal.pone.0050639 (pqac-00000002, pqac-00000010) |
| Oligomerization | Purified recombinant CelM resolved into two species by gel filtration: approximately **407 ± 34 kDa** and **75 ± 6 kDa**, interpreted as **dodecamer** and **dimer**, respectively, from a **36.2 kDa monomer**. The **dodecameric** form was catalytically active (**291.7 mmol min⁻¹ mmol⁻¹** on Leu-pNA), whereas the **dimer** was only weakly active (**6.4 mmol min⁻¹ mmol⁻¹**) (pqac-00000002, pqac-00000010). M42/TET literature defines these complexes as self-compartmentalized **12-subunit tetrahedral particles**, with activity toward polypeptides coupled to assembly (pqac-00000006, pqac-00000011) | **Direct CelM experiment** plus **family-level structural inference** | Dutoit et al., **2012-11**, https://doi.org/10.1371/journal.pone.0050639 (pqac-00000002, pqac-00000010); Appolaire et al., **2016-03**, https://doi.org/10.1016/j.biochi.2015.11.001 (pqac-00000006, pqac-00000011); Gauto et al., **2022-04**, https://doi.org/10.1038/s41467-022-29423-0 (pqac-00000006) |
| Temperature/pH optima | CelM leucine aminopeptidase activity was maximal at about **60–65°C** (figure text notes around **65°C**), with sharp decline above ~75°C; **optimal pH ~6.7–7.1** (neutral range). Activation energy from Arrhenius analysis was **60.47 ± 2.31 kJ mol⁻¹** (pqac-00000002, pqac-00000005, pqac-00000009, pqac-00000013) | **Direct CelM experiment** | Dutoit et al., **2012-11**, https://doi.org/10.1371/journal.pone.0050639 (pqac-00000002, pqac-00000005, pqac-00000009, pqac-00000013) |
| Evidence type | **Strong direct evidence:** recombinant CelM biochemical assays, substrate profiling, inhibitor tests, metal dependence, gel filtration, pH/temperature optimization. **Moderate inference:** cytosolic localization and role in intracellular peptide turnover are inferred from the broader M42/TET literature, which places TET peptidases among **ATP-independent cytosolic prokaryotic peptidases** involved in peptide degradation (pqac-00000006, pqac-00000007, pqac-00000011) | Mixed: **direct CelM** + **family-level inference** | Dutoit et al., **2012-11**, https://doi.org/10.1371/journal.pone.0050639 (pqac-00000000, pqac-00000002); Appolaire et al., **2016-03**, https://doi.org/10.1016/j.biochi.2015.11.001 (pqac-00000006, pqac-00000011); Basbous et al., **2018-09**, https://doi.org/10.1128/jb.00059-18 (pqac-00000007) |
| Key references with publication date and URL | **CelM-defining study:** Dutoit et al., *Functional Characterization of Two M42 Aminopeptidases Erroneously Annotated as Cellulases*, **2012-11**, PLoS ONE, https://doi.org/10.1371/journal.pone.0050639. **Family review:** Appolaire et al., *TET peptidases: A family of tetrahedral complexes conserved in prokaryotes*, **2016-03**, Biochimie, https://doi.org/10.1016/j.biochi.2015.11.001. **Family specificity context:** Basbous et al., *Characterization of a Glycyl-Specific TET Aminopeptidase Complex from Pyrococcus horikoshii*, **2018-09**, J Bacteriol, https://doi.org/10.1128/jb.00059-18. **Dynamic mechanism context:** Gauto et al., *Functional control of a 0.5 MDa TET aminopeptidase by a flexible loop revealed by MAS NMR*, **2022-04**, Nat Commun, https://doi.org/10.1038/s41467-022-29423-0. **Recent metal-cofactor review:** Bhat, *Drug targeting of aminopeptidases: importance of deploying a right metal cofactor*, **2024-04-24**, Biophysical Reviews, https://doi.org/10.1007/s12551-024-01192-8 (pqac-00000000, pqac-00000006, pqac-00000007, pqac-00000008) | Reference row | Dutoit 2012; Appolaire 2016; Basbous 2018; Gauto 2022; Bhat 2024 (pqac-00000000, pqac-00000006, pqac-00000007, pqac-00000008) |


*Table: This table compiles the key experimentally supported and inferred properties of celM (UniProt P55742) from Acetivibrio thermocellus/Hungateiclostridium thermocellum. It distinguishes direct CelM evidence from broader M42/TET family inference and highlights the most relevant references, dates, and URLs.*