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Serendipitous Discovery: Two Common Enzymes Form Ordered Filaments

This Observation paper reports the first cryo-EM structures of filaments formed by alkaline phosphatase (ALP) from P. aeruginosa and proteinase K (ProtK), both discovered unintentionally in unrelated samples. The ALP filament is scaffold-dependent, anchored by unassigned electropositive-contacting densities (likely eDNA or acidic polysaccharides), while the ProtK nanotube appears concentration-driven with minimal two-hydrogen-bond interfaces Sonani et al. 2026 Microbiology Spectrum.

Structural Comparison

Critical Assessment

Strengths: Atomic-resolution structures from crude samples; honest self-critique (authors acknowledge ProtK nanotube may be an artifact of concentration and monitored stock solution, finding no tubes there). Data deposited for independent verification. Automated model building (ModelAngelo) demonstrates a generalizable discovery workflow Jamali et al. 2024 Nature - ModelAngelo.

Limitations and blindspots: The chemical identity of the ALP scaffold densities is unresolved—eDNA, RNA, exopolysaccharides, teichoic acid, and polyphosphate are all listed as candidates without discriminating evidence. No physiological relevance is demonstrated for ProtK nanotubes in the native fungus Tritirachium album. No concentration-dependence curves or catalytic activity comparisons (polymeric vs. monomeric) are presented. Sample sizes, particle counts, and map validation metrics (FSC, Q-scores) are not reported in the main text. The authors' interpretation that ALP polymerization is "evolved" rests solely on absence of homologous KKK/R219/R248 residues in E. coli ALP (74% identity)—a suggestive but not conclusive evolutionary argument.

What would change the conclusions: A higher-resolution map showing different protofilament count or symmetry would weaken the ProtK structural claim. Identification of the scaffold density as a specific molecule (or demonstration that ALP polymerizes without any scaffold) would test the scaffold-dependence model. Concentration-dependent assembly/disassembly curves for ProtK would distinguish artifact from biology.

Proteinase K Crystal Structure (3PRK Reference)

The paper shows the ProtK nanotube monomer superposes on this crystal structure with 0.8 Å RMSD, confirming no packing-induced rearrangement.

Author Reviews

 BGPT Paper Review

Study Novelty

First report of filament formation by two widely-used enzymes (ALP and ProtK) discovered serendipitously. The finding that cryo-EM can identify atomic structures of uncharacterized assemblies without purification adds methodological novelty. However, the conceptual framework (enzyme filamentation as regulation) is well-established.

Scientific Quality

High-resolution structures with deposited coordinates are a strength. Honest self-critique about ProtK artifact possibility. However, missing map validation metrics, no functional characterization, no concentration-dependence data, and unassigned scaffold densities limit completeness. As an Observation paper it meets its scope but does not test hypotheses.

Study Generality

Findings are specific to two enzymes in particular contexts. The broader point—that cryo-EM can discover uncharacterized oligomeric states in crude samples—is generalizable but not deeply explored.

Study Usefulness

Useful for the enzyme polymerization community and as a methodological demonstration. Limited immediate practical utility given unresolved scaffold identity and unclear physiological relevance. Structural data may inform future studies of biocatalytic nanomaterials.

Study Reproducibility

Structures deposited (PDB/EMDB), methods described in supplemental material, commercial ProtK source identified. However, exact conditions triggering filament formation are incompletely characterized, and particle counts/processing details are not fully reported in the main text.

Explanatory Depth

Structural mechanisms are well described (contact interfaces, electrostatics, symmetry). However, the chemical identity of the ALP scaffold and the physiological/biophysical driving forces for ProtK assembly remain unknown, limiting mechanistic depth.