Aminoglycoside antibiotics modifying enzymes

Figure 9 Aminoglycoside antibiotic modifying enzymes. The aminoglycoside antibiotics such as kanamycin B (shown) bind to the 16 S rRNA of the bacterial ribosome impairing cognate codon-anticodon discrimination (A). Resistance occurs via acetylation (AAC), phosphorylation (APH), or adenylylation (ANT) of the antibiotic (B). A wide variety of enzymes are known with different regiospecificities of chemical modification, and the sites of some clinically important enzymes are shown in panel C.

Beside AAC enzymes two different enzyme classes, nucleotidyltransferases (ANT enzymes), and phosphotransferases (APH enzymes) modify the hydroxyl groups of aminocyclitol-aminoglycoside antibiotics. 

New concepts in the strategy of the synthesis of drugs rarely appear, such as from the observation that microorganisms often get resistance from enqrmes that inactivate the drug through phosphorylation. To avoid the problem, the aminoglycoside antibiotic kanamycin A was modified in a way that it was re-obtained whenever it was modified by the microorganism resistance enzymes (Haddad 1999). 

Production of enzymes to degrade the antibiotic, e.g. novel (3-lactamases, extended-spectrum p-lactamases, or aminoglycoside-modifying enzymes. 

Chloramphenicol (9) is liable to breakdown by chloramphenicol acetyl-transferases [185]. Fluoro derivatives (57, 58) resist enzymatic attack but little has been heard of these, apparently because of their toxicity [319], Aminoglycoside antibiotics (AGACs) may be chemically modified by AMEs. Some derivatives (e.g. amikacin, 43) are more recalcitrant than others, e.g. kanamycin (42) (see Figure 4.2). Other enzyme-resistant AGACs of low toxicity are needed. 

To date, six enzymes have been isolated and identified from strains of Ps. aeruginosa which modify aminoglycoside antibiotics. These are listed in Table 7.15. 

Although Table 20.7 lists only benzylpenicillin and ampicillin as being inactivated by p-lactamase (from B. cereus), other P-lactams may also be hydrolysed by P-lactamases. Other antibioticinactivating enzymes are also known (Chapter 13) and have been considered as possible inactivating agents, e.g. chloramphenicol acetyltransferase (inactivates chloramphenicol) and enzymes that modify aminoglycoside antibiotics. 

This RNA-paromomycin structure also explains a number of features that help show why modification of the aminoglycoside antibiotic by the resistance enzymes prevents the modified drug from binding to the ribosomal site. These features are as follows. 

A variety of enzymatic mechanisms for antibiotic resistance are known. Hydrolysis of the lactam rings of /3-lactams, cephalosporins, and carbapenams destroys their ability to inhibit transpeptidases that cross-link peptidoglycan in bacterial cell walls. Modification of aminoglycoside antibiotics by acetylation, phosphorylation, or adenylation interferes with their ability to bind to the 16S subunit of the ribosome. Streptogramin activity can be destroyed by acetylation or by an elimination reaction that opens the lactone ring. The enzymes responsible for these detoxification reactions evolved in response to naturally occurring antibiotics, but are easily adapted to modify semisynthetic and completely synthetic antibiotics. For example, only a few point mutations are needed to enhance the ability of TEM /3-lactamases to hydrolyze third-generation cephalosporins such as cefotaxime and ceftazidime. ... 

In order to analyze the effect that conformational restriction has on the antibiotic enzymatic inactivation, three different enzymes were chosen as model systems Staphylococcus aureus ANT(4 ), Mycobacterium tuberculosis AAC(2 ) and Enterococcus faecalis APH(3 ). These proteins are representative of the three main families of enzymes that modify aminoglycosides adenyltrans-ferases, acyltransferases and phosphotransferases. In addition, there is high resolution X-ray structural information available for the three enzymes in complex with several antibiotics. 

Drug inactivation. Bacterial resistance to aminoglycosides and /3-lactam antibiotics usually results from production of enzymes that modify or destroy the antibiotic, respectively. A variation in this mechanism—the failure of bacteria to activate a prodrug—commonly underlies resistance of Mycobacterium tuberculosis to isoniazid. 

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