Peptidoglycan transpeptidase

A comparison of the structures of penicillin and Dalanyl-Dalanine (cf. structures 41 and 42) shows that there is a great deal of similarity between the two molecules. Penicillin is essentially an acylated cyclic dipeptide of Dcysteine and Dvaline (84). As such, it contains a peptide bond, that of the /3-lactam ring, that can acylate the enzyme. Labeling studies of the peptidoglycan transpeptidase of Bacillus subtilis indicate that radioactive penicillin reacts with a sulfhydryl group of a cysteine residue of the enzyme (86). 

The fS-lactamases have mirrored the chemical strategy of proteases to cleave the fS-lactam bond. The main fS-lactamase families emulate Ser or metallo-protease mechanisms to perform ring-opening hydrolysis of the fS-lactam ring, destroying the antibiotic warhead in the process (Fig. 6). The Ser enzymes mimic the first step in peptidoglycan transpeptidase reaction by formation of an acyl-enzyme intermediate. However, unlike the transpeptidase, this intermediate is short lived and subsequent hydrolysis releases the inactive antibiotic (Fig. 6). 

Transpeptidation reactions are considered to be achieved by reversible acylation of peptidoglycan transpeptidase (Figure 8.6). According to the structural analogue model, the same acylation would take place in the... 

Finally, it may be worthwhile to point out that one of the most successful chemotherapeutic drugs provided by nature may be considered as a non-classical active-site-directed irreversible inhibitor This drug is penicillin (70). As a non-classical antimetabolite of D-alanyl-D-alanine, it first forms a reversible complex with the enzyme peptidoglycan transpeptidase, and then by a ringopening reaction of its p-lactam moiety, it forms a covalently linked penicil-loyl-enzyme complex97. However, the reaction involves the acylation of a sulfhydryl group in the active site of the enzyme in this respect, 70 resembles the classical-type endoalkylating antimetabolites, azaserine and DON (8 and 9, see Section 2.2.). Some of the more recently discovered antibiotics and natural products from plants with antitumor activity (e.g., camptothecin) are... 

The interactions of 8-lactams with PBPs indicate that these compounds are structural analogues of R-D-alanyl-D-alanine, the natural substrate of peptidoglycan transpeptidases and D-alanine carboxypeptidases, where R is the remainder of the pentapeptide. The mechanisms of the transpeptidase and carboxypeptidase reactions are thought to involve formation of an acyl-enzyme intermediate that can react with either a primary amine (e.g., an a-amino group) to form a peptide bond, or with water to form a carboxylic acid. In both reactions D-alanine is released before the acyl enzyme is formed. When a S-lactam antibiotic enters the binding site, the /1-lactam bond is hydrolyzed, and the resulting acyl group reacts with the active-site... 

Extensive investigations of the properties of PBPs of E. coli, strain K12, were carried out. The results are indicative of our present understanding of PBPs and will be outlined. It should be stated that it is now understood that penicillin-sensitive enzymes such as DD-Cbase, peptidoglycan transpeptidase, and endopeptidases identified earlier are almost certainly identical with the PBPs under discussion here. Multiple PBPs have been discovered in all bacterial membranes studied. It is also now apparent that the interactions of (3-lactam antibiotics with bacteria can result in one or more effects on the physiology and structure of the cell. Thus inhibition of cell division can be observed so can lysis, bulge formation, or even the development of ovoid cell forms stable to osmosis. 

Peptidoglycan transpeptidase (PBP) PonA1/PonA2 (Rv0050/Rv3682)... 

It has been suggested, although the evidence is inconclusive, that penicillin provides transition state inhibition of peptidoglycan transpeptidase (Section 13.1) before this antibiotic acylates the enzyme (Lee, 1971). Several 1,6-dihydro-8-azapurines have shown high therapeutic indices in experimental cancer of mice (Albert, 1980). These candidate drugs were intended as transition-state inhibitors of purine biosynthesis. In general, it would appear that the subject of transition-state inhibitors may have a rosy future but not, so far, many achievements in therapy. 

P-lactam ring binds to the enzyme peptidoglycan transpeptidase, which the bacterium needs to build its cell walls, so the bacteria grow bigger but cannot divide, and eventually burst. 

Simple, animal cells do not have cell walls, so we do not have peptidoglycan transpeptidase and therefore penicillin is not toxic to humans. 

The three enzymes sensitive to penicillins, i.e., peptidoglycan transpeptidase, D-alanine earboxypeptldase, and endopeptidase, were not inhibited at bactericidal concentrations of FL 1060 , but the formation of peptido-glycans covalently bound to the cell wall or to lipoproteins was impaired. 

The main site of action of the penicillins is apparently a membrane-bound enzyme like the peptidoglycan transpeptidase isolated from E, coli. 

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