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Amino acid residues S70, K73, K234, E166, and N170 involved in the catalytic cycle of antibiotic hydrolysis are conservative and do not mutate, and residues S130 and A237 are rarely mutating. 6.2.2 Mutations Causing Phenotypes of TEM‐Type β‐Lactamases Based on differences in substrate specificity due to amino acid mutations, TEM‐type β‐lactamases are divided into four phenotypes: 2b, which are β‐lactamases hydrolyzing penicillins and first‐generation cephalosporins; 2be (ESBL), which are β‐lactamases hydrolyzing penicillins, first‐ to fourth‐generation cephalosporins, and monobactams; 2br (inhibitor‐resistant type [IRT]), which are β‐lactamases resistant to inhibitors of β‐lactam structure (clavulanic acid, sulbactam, tazobactam); and 2ber (complex mutation type [CMT]), which are inhibitor‐resistant ESBLs, mixed type.
Combinations of Key ESBL and IRT Mutations in CMT TEM‐Type β‐Lactamases (2ber)
β‐Lactamases with a confirmed CMT phenotype (2ber) involve 10 enzymes, which combine the key mutations of two types (ESBL and IRT) (Figure 6.4).
The analysis of changes in the catalytic properties of these mutants shows that ESBL and IRT mutations have a negative effect on each other: the efficiency of hydrolysis of various β‐lactams and resistance to inhibitors decrease in all CMT enzymes in comparison with the corresponding ESBL and IRT enzymes, and the magnitude of this effect depends on combinations of mutations (Figures 6.6 and 6.7).
However, a large number of their possible combinations with each other and with secondary mutations lead to a diversity of enzymes with different phenotypes (ESBL, IRT, and CMT).
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