Aminoglycoside antibiotics, enzymic inactivation

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. 

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). 

Structures of several important aminoglycoside antibiotics. Ring II is 2-deoxystreptamine. The resemblance between kanamycin and amikacin and between gentamicin, netilmicin, and tobramycin can be seen. The circled numerals on the kanamycin molecule indicate points of attack of plasmid-mediated bacterial transferase enzymes that can inactivate this drug. , , and , acetyltransferase , phosphotransferase , adenylyltransferase. Amikacin is resistant to modification at , , , and . 

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. 

However, by far the most important strategy involves the production of enzymes that destroy or deactivate the antibiotics. Bacterial enzymes that inactivate chloramphenicol (e.g., chloramphenicol acetyl transferase) and the aminoglycosides (usually enzymes that acetylate or phosphorylate the aminoglycosides) are quite common, but it is the penicillinases or, as they are now called, the beta-lactamases, that have had the most impact on antibacterial chemotherapy. The beta-lactam antibiotics, including the penicillins, cephalosporins and the newer variants called penems, carbapen-ems, cephamycins and monobactams, all possess the four-membered ring... 

Studies on biochemical mechanisms of resistance to aminoglycoside antibiotics have revealed that they are enzymically inactivated in several... 

Escherichia coli K12 ML1629, . coU K12 ML1410 R81, and E. coli K12 J5 Rll-2 showed similar resistant patterns to aminoglycosidic antibiotics, and the S-100 fraction contained kanamycin-neomycin phosphate transferase I. This enzyme solution also inactivated lividomycins A and B (6 and 7) in the presence of adenosine 5 -triphosphate. A crude powder of the inactivated lividomydn A extracted by column chroma-... 

Most strains of Pseudomonas aeruginosa are resistant to various antibiotics, and the natural resistance, at least to aminoglycosidic antibiotics, has been confirmed as being related to formation of enzymes that inactivate these antibiotics. The presence of such an enzyme in Pseudomonas aeruginosa was first reported by Umezawa and coworkers. ... 

Benveniste R, Davies J. Aminoglycoside antibiotic inactivating enzymes in actinomycetes similar to those present in clinical isolates of arttibiotic resistant bacteria. Proc Natl Acad... 

On the other hand, the phosphorylative inactivation of kanamycin by the RF-mediated resistant E j coli strain does differ from that involved in the resistance of laborata y acquired strains of . coli 12. The RF-mediated enzyme of E. coli (ml 1629) responsible for the inactivation is absent in sensitive ,. coli 12 strains, strongly suggesting a direct relationship between this inactivating process and the observed resistance. Adenylation as a means for the inactivation of streptomycin by RF-resistant Ej coli has been reported from two additional laboratories Whether adenylation, rather than phosphorylation, is actually the first step of inactivation of aminoglycoside antibiotics other than streptomycin remains to be established. 

Since tobramycin is a broad-spectrum antibiotic its application may be followed by bacterial colonization of the patient with resistant organisms (27 ). So far, the clinical use of tobramycin has only occasionally been reported to be followed by the development of bacterial resistance. On theoretical grounds and on the basis of a comparison with gentamicin it can be presumed that widespread or indiscriminate use of tobramycin will be associated with the risk of the development of bacterial resistance due to resistance factor coding for a number of bacterial enzymes inactivating aminoglycosides and probably other antibiotics as well. 

The three-dimensional structure of the aminoglycosides plays an essential role in their interaction with both RNA and the enzymes involved in the antibiotic inactivation and thus determines their biological function. A proper understanding of the different factors that govern aminoglycoside-RNA/protein interactions requires a detailed knowledge of the three-dimensional structure, and conformational properties of these oligosaccharides in both the... 

The different 3D-shapes adopted by aminoglycosides in the RNA- and enzyme-bound states suggest a possible structure-based chemical strategy to obtain antibiotics with better activity against resistant bacteria. Assiun-ing that, in these cases, some degree of conformational distortion of the substrates is required for enzymatic activity, it should be possible to design a conformationally locked oHgosaccharide that still retains antibiotic activity, but that is not susceptible to enzymatic inactivation (Fig. 8) [41]. 

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