Saturday, September 3, 2011

Classification of β-lactamases

The classification of β-lactamases has been problematic; the first proposal was to divide β-lactamases into the penicillinases that hydrolyzed penicillin and cephalosporinases that attacked cephalosporins (Datta and Kontomi-Chalou, 1965).

On the basis of their structures and catalytic mechanisms, β-lactamases have been divided into four major groups (Ambler, 1980).

Class A β-lactamases such as the extensively studied TEM and SHV enzymes. They are serine proteases mainly plasmid mediated and are widely spread among Gram-negative bacteria. Generally, class A β-lactamases are considered ''penicillinases'' because they display higher activity with penicillins than with cephalosporins. They may be produced constitutively or may require induction.

Class B β-lactamases are metalloenzymes that require a Zn2+ ion for activity.

Class C β-lactamases are the so-called cephalosporinases that are chromosomally encoded in many Gram-negative bacteria. They are either constitutive or inducible.
Class D β-lactamases exhibit original substrate profiles by hydrolyzing oxacillin (oxacillinases). Following the developments in molecular biology, sequence homology studies were able to resolve difficulties with previous classification schemes (Bush et al., 1995). Some β-lactamases, such as those produced by Staphylococcus aureus, have been stable over several decades. These enzymes have a narrow spectrum of activity aimed at penicillin molecules (Ambler, 1980).

However, a series of enzymatic variants appeared that had a broadened spectrum of activity to include penicillins, the extended-spectrum cephalosporins e.g., ceftazidime (CAZ); cefotaxime (CTX); and ceftriaxone (CRO), and aztreonam such enzymes are called extended spectrum β-lactamases (Heritage et al., 1999).

There are two basic mechanisms responsible for resistance of S. aureus to β-lactam antimicrobial agents. One mechanism is the production of β-lactamases that destroy these drugs. β-lactamases were responsible for the development of widespread resistance to penicillin and led to the development of β-lactamase resistant antistaphylococcal penicillins such as methicilin. The other mechanism is an alteration in membrane bound enzymes called penicillin binding proteins (PBPs) (Mulligan et al., 1993).

Action of β-lactamases:
β-lactamases interact with β-lactam antibiotics and cleave an amide bond by an acyl-enzyme (Bishop and Weiner, 1992), a mechanism which issimilar to that of the transpeptidase penicillin-binding proteins (Nicholas and Strominger, 1988). The number of different enzymes is increasing and now it exceeds 170. So the specificity of the β-lactamase for a β-lactam antibiotic is an important determinant in the efficiency with which the enzyme hydrolyzes the antibiotic. A point mutation in one or more amino acids in a structurally critical area of the enzyme can change the specificity of the molecule (Blazquez et al., 1995 & Bush et al., 1995).

Accordingly, researchers have presented evidence to support the concept that the physiologic role of β-lactamases is to restructure the peptidoglycan during bacterial cell growth (Tuomanen et al., 1991 & Bishop and Weiner, 1992). They found that synthesis of β-lactamases was induced both by the presence of β-lactam antibiotics and cell wall precursors in the extracellular environment, recalling the structural similarity of penicillin and the d-alanine-d-alanine dipeptide terminus of peptidoglycan chains.With regard to β-lactams, P. aeruginosa contains an inducible, chromosomally encoded cephalosporinase. P.aeruginosa also contain a chromosomally encoded kanamycin phosphotransferase (Bush et al., 1995).

Chloramphanicol can be inactivated by acetylation, the penicillins and cephalosporins by hydrolysis of the β-lactam ring, and the aminoglycoside antibiotics by aminoglycoside-modifying enzymes which specifically adenylated, phosphorylate, or acetylate certain members of this class.

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