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Core finding. The glmS holoribozyme (with GlcN6P) shows very large normal thio effects at the scissile phosphate — k_O/k_S of 200 (R_P thio), 15 (S_P thio), and ~9,000 (dithio) in 10 mM Mg²⁺ — versus only ~3-fold effects previously reported under conditions where a conformational step, not chemistry, was rate-limiting (k_obs ~100-fold slower). The optimized construct here cleaves at 80 min⁻¹, monophasic and complete, so thio effects report on the chemical step Large holoribozyme thio effects.

No divalent metal in the active site of the holoribozyme. Thio effects were essentially identical in 10 mM Mg²⁺ versus 3 M K⁺/100 mM EDTA (R_P: 340, S_P: 270, dithio: 410), and Cd²⁺ (up to 2 mM) gave no rescue (k_O/k_S ~150–170 and ~15–18 with or without Cd²⁺). Catalysis is therefore carried entirely by nucleobases and the cofactor No metal-ion rescue in holoribozyme.

The striking aporibozyme result. Without cofactor, the R_P thio substrate cleaves ~30-fold faster than oxo (large inverse thio effect), and Cd²⁺ rescues R_P (rescue 30) and dithio (≥12) substrates with non-saturating binding (Hill coefficients 0.5 and 0.9). Raising Mg²⁺ from 1 to 50 mM diminished the inverse thio effect ~3-fold (50→17). QM/MM simulations explain this: sulfur substitution or Mg²⁺ binding at pro-R_P disrupts an inhibitory A-1(O2′)–pro-R_P hydrogen bond that sequesters the nucleophile; GlcN6P(O1) plays the same role in the holoribozyme Aporibozyme inverse thio effect and rescue.

Synthesis and implications. Four cofactor roles are supported: (i) general acid (previously established Klein & Ferré-DCochrane glmS cofactor structure); (ii) active-site alignment; (iii) nucleophile activation by disrupting the inhibitory 2′-OH interaction; (iv) charge stabilization of the nonbridging oxygens. The conceptual advance — an organic cofactor assuming roles normally played by an inorganic metal ion — is mechanistically coherent with the broader view that glmS relies on nucleobase/cofactor chemistry rather than inner-sphere metal catalysis Chemical Reviews on nucleic acid catalysis.

Confidence and caveats. Kinetic evidence (triplicate trials, ≥3–6 measurements per thio-effect point) is strong; the mechanistic assignments for roles (ii)–(iv) rest partly on simulations that the authors themselves flag as trajectory-dependent (hydrogen-bond patterns varied across MD runs) and distance-based rather than free-energy-based for holo thio effects, and 1-ps QM/MM MD is too short for robust sampling. Free-energy simulations also acknowledge possible bias in the pro-R_P Mg²⁺ placement. SAXS confirms apo and holo enzymes are similarly folded, controlling against allostery SAXS folding control. Falsification would require showing GlcN6P-independent cleavage rates matching the holoribozyme, or loss of the stereospecific aporibozyme rescue pattern.

 BGPT Paper Review

Study Novelty

First demonstration that a small-molecule cofactor performs four distinct catalytic roles in a ribozyme, including nucleophile activation and charge stabilization, and that a metal ion can substitute for the cofactor.

Scientific Quality

Rigorous combination of fast-kinetics, stereospecific thio substitution, metal rescue, SAXS controls, and QM/MM; simulations are acknowledged as trajectory-dependent and short (1 ps), and hydrogen-bonding analyses are distance-based only.

Study Generality

Insights on cofactor-versus-metal catalytic equivalence extend to RNA-world and in vitro-selected cofactor ribozymes, though demonstrated only in one ribozyme system.

Study Usefulness

Defines a mechanistic paradigm for cofactor-assisted RNA catalysis relevant to ribozyme engineering, antibiotic strategies targeting glmS, and understanding riboswitch chemistry.

Study Reproducibility

Detailed protocols, multiple independent trials (3–6 per rate constant), HPLC-verified stereoisomers, deposited SAXS data, and no competing interests support replication.

Explanatory Depth

Mechanistic account integrates kinetics, stereochemistry, and simulation to a hydrogen-bond-level explanation, though free-energy barriers are not quantified for all roles.