Review papers with raw data transparency

Key findings summarized (visual-first, evidence-cited)

• Reusch synthesizes biochemical and biophysical data showing short-chain PHB ("cPHB") widely present and often noncovalently complexed with inorganic polyphosphate (polyP), with cPHB/polyP complexes isolated from membranes and mitochondria and characterized by NMR and size-exclusion chromatography; proposes Ca2+-bridged cPHB/polyP channels that can be Ca2+-selective under physiological pH Reusch 2012 cPHB/polyP review
• Primary functional evidence comes from planar-lipid-bilayer reconstitution (channel activity dependent on polymer length, concentration, and endgroups) and synthetic mimic channels — robust in vitro but requiring high PHB:lipid ratios and specific oligomer lengths to observe channel activity Bilayer reconstitution evidence
• Reusch documents covalent cPHB attachment to proteins (PHBylation) in bacteria and eukaryotes and proposes that this modification changes protein hydrophobicity, folding, membrane targeting, and temperature sensitivity — biochemical detection via NMR and proteomics is cited but systematic proteome-wide mapping in living cells is lacking PHBylation observations
• Physiological connections to mitochondria (mitochondrial permeability transition, TRP channels) and human health (plasma cPHB in lipoproteins; correlations with diabetes/atherosclerosis) are proposed with some experimental correlates but without large-scale genetic or in vivo causal tests: plausible but speculative pathways require targeted perturbation studies Physiological & health claims

Critical appraisal — strengths

• Comprehensive synthesis connecting diverse experimental threads (bilayers, NMR, biochemical isolation, modeling) into coherent mechanistic proposals that are experimentally falsifiable (e.g., architecture-dependent channel formation, Ca2+ coordination geometry) Falsifiable structural models
• Provides mechanistic chemical reasoning (polymer electrolytes analogy, carbonyl coordination, polyP charge behavior) that maps onto observed pH- and cation-dependent selectivities in bilayer assays — plausible chemical logic supported by polymer electrolyte literature Polymer electrolyte rationale

Critical appraisal — limitations, blindspots, and biases

• Heavy reliance on in vitro reconstitution and modeling: channel activity is reproducible in bilayers but typically requires synthetic oligomers at high PHB:lipid ratios and defined oligomer lengths — these conditions may not reflect native membrane concentrations or organization in living cells Bilayer condition dependence
• Structural claims remain model-based: no high-resolution cryo-EM or X-ray structures of native cPHB/polyP complexes exist to validate spiral vs columnar models; polymer flexibility and low electron density complicate direct structural detection — fundamental unknown remains the in situ architecture Lack of high-res structure
• Limited genetic/in vivo causal tests: examples linking cPHB/polyP to specific protein channels (KcsA, TRPM8, Ca-ATPase) derive from biochemical detection and enzymatic perturbations (e.g., polyphosphatase) but lack orthogonal genetic removal/knock-in approaches that would establish necessity/sufficiency in living organisms — potential species differences and experimental artifacts are possible Need for in vivo genetics
• Detection challenges and potential artefacts: cPHB's low abundance, flexible backbone, lack of unusual atoms, and hydrophobic character make it invisible to many assays and potentially confounded with lipids/detergents in preparations; prior positive reports may be biased by detection methods or contamination unless orthogonally validated (mass spectrometry, modern proteomics) Analytical detection challenges

External corroboration and counterexamples (selected)

• Independent reports indicate bacteria synthesize PHB storage granules (long-chain PHA) and that some bacteria constitutively make PHB (LD12 SAR11): recent functional genomics demonstrates physiological PHB production and genetic pha loci in diverse microbes, supporting biological roles for PHB polymers, but these refer primarily to long-chain storage PHB (granules) rather than cPHB/polyP complexes highlighted by Reusch SAR11 PHB production (2025)
• Mitochondria/prohibitin literature: subsequent decades of prohibitin (PHB1/PHB2) research show mitochondrial and extra-mitochondrial roles; these independent PHB proteins (prohibitins) are distinct from polyhydroxybutyrate polymers but sometimes create naming confusion; careful distinction is required when reading "PHB" in different literatures (polyester PHB vs prohibitin proteins) Distinguish prohibitin vs PHB polymer

Where the conclusions could be overturned (falsification tests)

1. High-sensitivity mass-spectrometry/proteomics on intact native membranes that fail to detect cPHB covalent attachments or polyP co-complexes at scale would challenge claims of widespread PHBylation; conversely, robust proteome-wide identification of cPHB modification sites (with MS/MS evidence and controls) would strengthen the hypothesis (experimentally tractable via enrichment + MS/MS).
2. Genetic removal or targeted enzymatic depletion of polyP/cPHB in organisms where cPHB was reported (e.g., TRPM8-expressing cells, mitochondria) followed by electrophysiological and physiological readouts: loss-of-function would support functional necessity; lack of phenotype would weaken the model.
3. High-resolution structural determination (cryo-EM or solid-state NMR) of native cPHB/polyP complexes in membranes or of protein complexes proposed to bind cPHB/polyP (e.g., KcsA prepared under native-retaining conditions) would validate or falsify detailed spiral/columnar models.

Practical recommendations and next experiments

• Develop orthogonal detection pipelines: targeted chemical derivatization of ester carbonyls of cPHB + high-resolution LC-MS/MS peptide mapping to identify specific protein-conjugation sites (PHBylation) in vivo.
• Use genetic and enzymatic perturbations (e.g., inducible polyphosphatases, candidate cPHB synthetase knockouts) combined with patch-clamp/mitochondrial permeability assays and lipidomics to test causality in cells and tissues.
• Obtain cryo-EM of reconstituted complexes assembled from biomimetic proteins and synthetic cPHB/polyP under near-physiological lipid stoichiometries to test structural models; complement with MD simulations of Ca2+ coordination geometries.

Short confidence & bias statement

Assessment synthesizes the review and subsequent relevant primary work; confidence moderate (7/10) because the review draws legitimately on experimental data but the core functional claims require modern orthogonal in vivo validation and high-resolution structural support Reusch 2012 basis