Cantu C, Palzkill T
Program in Structural and Computational Biology and Molecular Biophysics, Baylor College of Medicine, Houston, Texas 77030, USA.
J Biol Chem. 1998 Oct 9;273(41):26603-9. doi: 10.1074/jbc.273.41.26603.
beta-Lactamases inactivate beta-lactam antibiotics by catalyzing the hydrolysis of the amide bond in the beta-lactam ring. The plasmid-encoded class A TEM-1 beta-lactamase is a commonly encountered beta-lactamase. It is able to inactivate penicillins and cephalosporins but not extended-spectrum antibiotics. However, TEM-1-derived natural variants containing the G238S amino acid substitution display increased hydrolysis of extended-spectrum antibiotics. Two models have been proposed to explain the role of the G238S substitution in hydrolysis of extended-spectrum antibiotics. The first proposes a direct hydrogen bond of the Ser238 side chain to the oxime group of extended-spectrum antibiotics. The second proposes that steric conflict with surrounding residues, due to increased side chain volume, leads to a more accessible active site pocket. To assess the validity of each model, TEM-1 mutants with amino acids substitutions of Ala, Ser, Cys, Thr, Asn, and Val have been constructed. Kinetic analysis of these enzymes with penicillins and cephalosporins suggests that a hydrogen bond is necessary but not sufficient to achieve the hydrolytic activity of the G238S enzyme for the extended-spectrum antibiotics cefotaxime and ceftazidime. In addition, it appears that the new hydrogen bond interaction is to a site on the enzyme rather than directly to the extended-spectrum antibiotic. The data indicate that, for the G238S substitution, a combination of an optimal side chain volume and hydrogen bonding potential results in the most versatile and advantageous antibiotic hydrolytic spectrum for bacterial resistance to extended-spectrum antibiotics.
β-内酰胺酶通过催化β-内酰胺环中酰胺键的水解使β-内酰胺类抗生素失活。质粒编码的A类TEM-1β-内酰胺酶是一种常见的β-内酰胺酶。它能够使青霉素和头孢菌素失活，但不能使广谱抗生素失活。然而，含有G238S氨基酸取代的TEM-1衍生天然变体对广谱抗生素的水解作用增强。已提出两种模型来解释G238S取代在广谱抗生素水解中的作用。第一种模型提出Ser238侧链与广谱抗生素的肟基形成直接氢键。第二种模型提出，由于侧链体积增加，与周围残基的空间冲突导致活性位点口袋更容易接近。为了评估每种模型的有效性，构建了具有Ala、Ser、Cys、Thr、Asn和Val氨基酸取代的TEM-1突变体。对这些酶与青霉素和头孢菌素的动力学分析表明，氢键对于实现G238S酶对广谱抗生素头孢噻肟和头孢他啶的水解活性是必要的，但不是充分的。此外，新的氢键相互作用似乎是与酶上的一个位点，而不是直接与广谱抗生素相互作用。数据表明，对于G238S取代，最佳侧链体积和氢键潜力的组合导致了细菌对广谱抗生素耐药性的最通用和最有利的抗生素水解谱。
