TEM-1 beta-lactamase is the most prevalent plasmid-mediated beta-lactamase in gram-negative bacteria. To identify amino acid substitutions that alter the activity of TEM-1 towards extended-spectrum ...cephalosporins, regions around the active-site pocket were probed by random-replacement mutagenesis.
We extracted maximum information for structure–function analysis of the PSE-4 class A β-lactamase by random replacement mutagenesis of three contiguous codons in the H4 α-helix at amino acid ...positions Ala125, Thr126, Met127, Thr128 and Thr129. These positions were predicted to interact with suicide mechanism-based inhibitors when examining the PSE-4 three-dimensional model. Structure–function studies on positions 125–129 indicated that in PSE-4 these amino acids have a role distinct from those in TEM-1, in tolerating substitutions at Ala125 and being invariant at Met127. The importance of Met127 was suspected to be implicated in a structural role in maintaining the integrity of the H4 α-helix structure together, thus maintaining the important Ser130–Asp131–Asn132 motif positioned towards the active site. At the structural level, the H4 region was analyzed using energy minimization of the H4 regions of the PSE-4 YAM mutant and compared with wild-type PSE-4. The Tyr 125 of the mutant YAM formed an edge to face π–π interaction with Phe 124 which also interacts with the Trp 210 with the same interactions. Antibiotic susceptibilities showed that amino acid changes in the the H4 α-helix region of PSE-4 are particularly sensitive to mechanism based-inhibitors. However, kinetic analysis of PSE-4 showed that the two suicide inhibitors belonging to the penicillanic acid sulfone class, sulbactam and tazobactam, were less affected by changes in the H4 α-helix region than clavulanic acid, an inhibitor of the oxypenam class. The analysis of H4 α-helix in PSE-4 suggests its importance in interactions with the three clinically useful inhibitors and in general to all class A enzymes.
To determine which amino acids in TEM-1 β-lactamase are important for its structure and function, random libraries were previously constructed which systematically randomized the 263 codons of the ...mature enzyme. A comprehensive screening of these libraries identified several TEM-1 β-lactamase core positions, including F66 and L76, which are strictly required for wild-type levels of hydrolytic activity. An examination of positions 66 and 76 in the class A β-lactamase gene family shows that a phenylalanine at position 66 is strongly conserved while position 76 varies considerably among other β-lactamases. It is possible that position 76 varies in the gene family because β-lactamase mutants with non-conservative substitutions at position 76 retain partial function. In contrast, position 66 may remain unchanged in the gene family because non-conservative substitutions at this location are detrimental for enzyme structure and function. By determining the β-lactam resistance levels of the 38 possible mutants at positions 66 and 76 in the TEM-1 enzyme, it was confirmed that position 76 is indeed more tolerant of non-conservative substitutions. An analysis of the Protein Data Bank files for three class A β-lactamases indicates that volume constraints at position 66 are at least partly responsible for the low tolerance of substitutions at this position.
Four fragments of Saccharomyces cerevisiae chromosome III DNA which carry ARS elements have been sequenced. Each fragment contains multiple copies of sequences that have at least 10 out of 11 bases ...of homology to a previously reported llbp core consensus sequence (10, 11). A survey of these new ARSsequences and previously reported sequences revealed the presence of an additional 11bp conserved element located on the 3′ side of the T-rich strand of the core consensus. Subcloning analysis as well as deletion and transposon insertion mutagenesis of ARS fragments support a role for 3′ conserved sequence in promoting ARS activity.
This chapter discusses structure and function studies of β‐lactamase inhibitory protein (BLIP) with a focus on the interactions between TEM‐1 β‐lactamase and BLIP. The first proteinaceous inhibitor ...of β‐lactamases, BLIP, was isolated from Streptomyces clavuligerus by a researcher's group in 1990. A BLIP nonproducer mutant and a BLIP/clavulanic acid nonproducer double mutant were constructed with the aim of elucidating BLIP's physiological function. Two hypotheses have been proposed for the function of BLIP. First, BLIP may be produced in response to the production of β‐lactamases by other organisms in the surrounding environment in order to inhibit these β‐lactamases and prevent the hydrolysis of antibiotics produced by S. clavuligerus. Alternatively, BLIP may play a role in cell wall growth or morphogenesis. Protein‐protein interactions play a significant role in most cellular processes. The importance of such interactions in biology has made protein‐protein recognition an area of considerable interest. The two domains join with each other to form an 8‐strand antiparallel β‐sheet. The study of a series of homologous interfaces provides an opportunity to study specificity as well as overall affinity determinants.
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