Benzylpenicillin penicillin hydrolysis

X 10" for benzylpenicillin. The ratio of the third-order rate constant, k K, for the copper(II) ion plus hydroxide ion catalysed hydrolysis of cephaloridine to the second-order rate constant for hydroxide-ion catalysed hydrolysis of the same substrate is 1.6 x 10 M. The corresponding ratio for benzylpenicillin is 1.2 x 10 °M. The transition state for cephaloridine hydrolysis is therefore stabilised by copper(II) ions ca 100-fold less than that for penicillin hydrolysis, but both transition states are greatly stabilised by the metal ion. Again, ad hoc explanations for this difference may be found in the lower basicity of the ring nitrogen in the tetrahedral intermediate formed from cephaloridine and/or a less favourable geometry. 

Benzylpenicillin isocyanate has a /S-Iactam ring that could be hydrolyzed by the enzyme. When benzylpenicillin isocyanate binds to the catalytic site and hydrolysis and inactivation proceed simultaneously, these reactions should take place through the same reversible complex (E-BPI) as represented above. Although the rate was over 100 times less than that with benzylpenicillin, the hydrolysis of benzylpenicillin isocyanate did occur with concomitant inactivation of the enzyme. The apparent Michaelis constant, k-iks)/ki, was coincident with K (400 fJiM) obtained in the inactivation reaction described. Other penicillin isocyanates may be hydrolyzed in a similar manner by yS-lactamase. 

Fig. 5.20. Modes of coordination of transition metal ions with /3-lactam antibiotics. Complex A In penicillins, the metal ion coordinates with the carboxylate group and the /3-lactam N-atom. This complex stabilizes the tetrahedral intermediate and facilitates the attack of HO-ions from the bulk solution. Complex B In benzylpenicillin Cu11 binds to the deprotonated N-atom of the amide side chain. The hydrolysis involves an intramolecular attack by a Cu-coordinated HO- species on the carbonyl group. Complex C In cephalosporins, coordination of the metal ion is by the carbonyl O-atom and the carboxylate group. Because the transition state is less stabilized than in A, the acceleration factor of metal ions for the hydrolysis of cephalosporins is lower than for penicillins. Complex D /3-Lactams with a basic side chain bind the metal ion to the carbonyl and the amino group in their side chain. This binding mode does not stabilize the tetrahedral transition complex and, therefore, does not affect the rate of... 

Following the realization that the presence of phenylacetic acid in the fermentation led to a simplification of the mixture of penicillins produced by the fungus due to preferential uptake of this acid and its incorporation into benzylpenicillin (4), a wide variety of other acids were added to the growing culture. Inclusion of the appropriate acids in the culture medium thus afforded, respectively, phenoxymethylpenicillin (5, penicillin V), phenethicillin (6), > propicillin (7), and phenbencillin (8). These modifications served to increase stability of the lactam bond towards hydrolysis and thus conferred some degree of oral activity. 

Theoretical studies were reported of the thiolysis by 2-mercaptoethanol and 2-mer-captoethylamine of simple models of the bicyclic ring systems of penicillin and cephalosporin88 and of the alkaline hydrolysis of sanfetrinem (93)89 The major determinant in allergies induced by penicillins is the penicilloyl group bound to the amino group of the Lys residues present in the carrier protein. Now a study is reported of the polyelectrolyte polyethylenimine (94) as a model of the carrier protein and its catalysis of the aminolysis of benzylpenicillin.90... 

Before interest developed in the structure of penicillin, the imidazo[5,1-b ]thiazole ring system was unknown, and even today, almost all of the examples which have been described are those prepared in connection with penicillin studies. Very mild acidic hydrolysis converts benzylpenicillin methyl ester (293) into benzylpenicillic acid (294) by a mechanism suggested... 

During the same period, Sheehan was working toward a total synthesis of penicillins. In 1958, he announced the synthesis of 6-amino-penicillanic acid (6-APA) and its utility for the preparation of new penicillins by acylation (67, 68). (Almost 10 years earlier, this substance had been postulated to be an intermediate in the biosynthesis of penicillins (69, 70). Prior Japanese literature also contained clear suggestions that it had been formed by enzymatic hydrolysis of benzylpenicillin (71) and in fermentations carried out in the absence of side chain precursors... 

Rapson HOC and Bird AE, Icmization cxmstants of some penicillins and of their alkaline and penicillinase hydrolysis products, /. Pharm. Pharmacol, Suppl.15,222-231T, (1963). Gted in Perrin Si pl. No. 7780 Ref. R6. Also cited in Kirschbaum J and Penicillin G, Potassium (Potassium Benzylpenicillin), APDS, 15,427-507 (1986). NB The study used pH measurements with a g ss electrode and liquid junction potentials. 

It is important to understand the mechanism of the cleavage of the (3-lactam ring catalyzed by 3-lactamase involving a zinc atom as an active site, because the zinc-containing enzyme is a factor in bacterial resistance to the 0-lactam antibiotics such as penicillin, cephalosporin, and so on. The first functional model for (3-lactamase was reported by Kimura and coworkers. " They demonstrated that the zinc complex involving 1,4,7,10-tetra-azacyclododecane ([12]aneN4) forms the [Zn—OH]" species under neutral conditions, and it accelerated the hydrolysis of benzylpenicillin in aqueous solution. In contrast, the native P-lactamase has two zinc atoms in the active site. From the three-dimensional structure infor-... 

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