Mimesis of the biotin mediated carboxyl transfer reactions

The lithio derivative of N-methylethyleneurea (or thiourea), a model for the isourea form of biotin, is capable of causing the rearrangement of 1-methyl-4-methylene-3,1-benzoxazin-2-one to 4-hydroxy-1-methyl-2-quinolinone. In a number of respects this reaction mimicks the carboxylation of biotin on nitrogen and the subsequent carboxyl transfer to a carbon atom bonded to a carbonyl group.     

Identification of disulfide bonds in carboxy-terminal propeptides of human type I procollagen

Intra-chain and inter-chain disulfide bonds within the carboxy-terminal propeptides of human type I procollagen were studied using cyanogen bromide cleavage of the propeptides and electrophoresis on SDS-polyacrylamide/glycerol gels. The results could provide a better understanding of the assembly of pro alpha-chains into procollagen.     

Transition-state structure for a conformation change of ribonuclease

The rate constants k₁₂ⁿ for isomerization of the E₁H isomer (pK_a 8 in H₂O) of ribonuclease-A to the E₂H isomer (pK_a = 6.1 in H₂O), determined from proton-uptake measurements by the temperature-jump technique, in mixtures of protium and deuterium oxides (atom fraction of deuterium n), are described by the equation k₁₂ⁿ = (733 ± 16)(1 − n + [0.46 ± 0.04]n)(1 − n + 0.69n)²sec⁻¹ at 25°C. On the basis of the absolute magnitude of the rate constant, the magnitude of the solvent isotope effect and the proton inventory, it appears that the rate-determining step is proton transfer to a water molecule from the imidazolium form of a histidine residue, with a product-like activated complex resembling a hydronium ion. The subsequent motion of the protein structure to generate the new isomer (conformation change) must then occur in a time approaching a vibrational period. Alternative but less likely mechanisms include rate-limiting protein reorganization concerted with proton transfer to water, rate-limiting diffusion of hydronium ion away from the enzyme, or “solvation catalysis” of protein reorganization.     

The structure of sodium beta-fluoropyruvate: a gem-diol.

Sodium β-fluoropyruvate is shown by X-ray diffraction and infrared spectroscopy to exist as a gem-diol when crystallized from water in the monoclinic space group $\text{P2}{}_1\text{a}$. The unit cell dimensions are a = 8.693(1), b = 11.556(1), c = 5.252(1) Å, β = 94.85(8) °. Molecular dimensions of the gem-diol are presented. The carbon-carbon bond to the carboxyl group is long [1.551(2) Å], indicative of a bond that can break easily. The hydration of the carbonyl group is attributed to the high electronegativity of the fluorine atom. While reports exist in the literature of a crystalline form of β-fluoropyruvic acid that contains a carbonyl, rather than a gem-diol group, ¹³C nmr studies presented here demonstrate that, in aqueous solution, the gem-diol is the predominant (≥95%) form. The significance of these results in the study of the mechanisms of enzymes utilizing pyruvate as a substrate is discussed. If the carbonyl oxygen activity is important for the action of an enzyme, e.g., in the formation of a Schiff base, it is possible that fluoropyruvate may not be a substitute for pyruvate. In other cases the fluoropyruvate can behave as a substrate analog, although its greater bulk may slow down or prevent its entry into the active site.