RESUMO
The bulky trityl steric buttress is used to effect an intramolecular, uncatalysed ene reaction that operates at room temperature, whilst smaller buttresses require heat.
RESUMO
The kinetics of the inactivation of beta-lactamase I from Bacillus cereus 569 by preparations of 6 alpha-bromopenicillanic acid showed unexpected features. These can be quantitatively accounted for on the basis of the inactivator being the epimer, 6 beta-bromopenicillanic acid. At pH 9.2, the rate-determining step in the inactivation is the formation of the inactivator. When pure 6 beta-bromopenicillanic acid is used to inactivate beta-lactamase I, simple second-order kinetics are observed. The inactivated enzyme has a new absorption peak at 326 nm. The rate constant for inactivation has the same value as the rate constant for appearance of absorption at 326 nm; the rate-determining step may thus be fission of the beta-lactam ring of 6 beta-bromopenicillanic acid. Inactivation is slower in the presence of substrate, and the observed kinetics can be quantitatively accounted for on a simple competitive model. The results strongly suggest that inactivation is a consequence of reaction at the active site.
Assuntos
Ácido Penicilânico/farmacologia , Inibidores de beta-Lactamases , Bacillus cereus/enzimologia , Cinética , Matemática , Ligação Proteica , Espectrofotometria UltravioletaRESUMO
There are two extracellular beta-lactamases produced by Bacillus cereus 569. One of these enzymes, beta-lactamase I, is inactivated by 6-beta-bromopenicillanic acid: the site of reaction is serine-44. This is a conserved amino acid residue in the other beta-lactamases whose structures have been determined, and it becomes a good candidate for an active-site group in these enzymes. The inactivation may involve a rearrangement leading to a dihydrothiazine. The other extracellular enzyme produced by B. cereus, beta-lactamase II, is exceptional in requiring metal ions for activity. The Zn II and Co II enzymes (the former is more active) have been studied by nuclear magnetic resonance, and by absorption spectroscopy. The groups that bind the metal ion required for activity are three histidine residues and the enzyme's sole thiol group.
Assuntos
Bacillus cereus/enzimologia , beta-Lactamases/metabolismo , Sítios de Ligação , Cefalosporinase/metabolismo , Cinética , Metais/metabolismo , Inibidores de beta-LactamasesRESUMO
The inactivation of beta-lactamase I by preparations of 6alpha-bromopenicillanic acid showed unexpected kinetic features that indicated that the active species was the 6beta-epimer. Samples containing 6beta-bromopenicillanic acid have been synthesized and shown to inactivate the enzyme in a rapid stoicheiometric reaction.
