Benzylpenicillin penicillin 3-lactamases

Penicillinacylase is used industrially to catalyze the hydrolytic removal of the side chain in naturally occurring penicillins such as benzylpenicillin (4.49) and phenoxymethylpenicillin (4.50). The nucleus 6-aminopenicillic acid (4.48) is then used as the starting material for the preparation of semisynthetic penicillins. Appropriate acylation of the 6-NH2 group leads to )3-lactamase-stable and broad-spectrum penicillins. 

Y. Song, R. Kluger, Benzylpenicillin Methyl Phosphate. A Penicillin Prodrug That Inactivates RTEM /L Lactamase , Bioorg. Med. Chem. Lett. 1994, 4, 1225- 1228. 

Penicillin V is a narrow-spectrum penicillin and has similar antibacterial activity to benzylpenicillin. It is active against many streptococcal infections, but it is inactivated by penicillinases. Flucloxacillin is a penicillinase-resistant antibiotic and is effective against infections caused by penicillin-resistant staphylococci. In comparison to penicillin V, attachment of carbocyclic/heterocyclic ring directly to the C6 carbonyl group confers resistance to beta-lactamases due to steric hindrance around the amide group. 

The major penicillin produced by P. chrysogenum in submerged culture was benzylpenicillin (PenG) (Figure 4), which was rather unstable under acid conditions and also deactivated by P-lactamases. However, the discovery of 6-aminopenicillanic acid (6-APA) also produced by P. chrysogenum, led to the preparation of new semisynthetic derivatives with improved stability e.g. methicillin, ampicillin and amoxycillin (Figure... 

The activity of these semis)mthetic penicillins extends beyond the Gram-positive and Gram-negative cocci which are susceptible to benzylpenicillin, and includes many Gram-negative bacilli. They do not resist P-lactamases and their usefulness has reduced markedly in recent years because of the increased prevalence of organisms that produce these enzymes. 

As a general rule these agents are rather less active than benzylpenicillin against Gram-positive cocci, but more active than the P-lactamase-resistant penicillins (above). They have useful activity against Enterococcus faecalis and many strains of Haemophilus influenzae. Enterobacteriaceae are variably sensitive and laboratory testing for sensitivity is... 

All penicillins (Fignre 74) are composed of a thiazolidine ring attached to a beta-lactam, which in turn carries a free amide gronp (0=CNH) on which a substitution and an attachment (R) are made. In the case of benzylpenicillin, the R is a benzyl gronp. Penicillin may be metabolized by amidase to 6-aminopenicillanic acid, which has antibacterial activity, or by penicillinase (bacterial beta-lactamase), to penicilloic acid, which is devoid of antibacterial activity bnt is antigenic in natnre and acts as a sensitizing structure. The main sonrce of bacterial resistance to penicillin is in fact the prodnction of penicillinase by the microorganisms. 

The penioillins usually are discussed under various groups based on spectrum of activity and sensitivity or resistanoe toward (3-lactamase. One of the earliest and still most commonly used penicillin is benzylpenicillin. 

Lactamases are bacterial enzymes, some of which play a crucial role in the resistance of pathogens to /3-lactam antibiotics. They are grouped into four classes (A, B, C, and Classes A, C, and D are serine enzymes using serine as a nucleophile, which are excluded in this text. Class B /3-lactamases include mononuclear zinc(II)-/3-lactamases and dinuclear zinc(II)-/3-lactamases. " A Zn -containing /3-lactamase II hydrolyzes the /3-lactam ring of a variety of penicillins (e.g., benzylpenicillin) and cephalosporins. At pH 7 and 30 ""C, the half-life of benzylpenicillin bound to /3-lactamase II is ca. 0.5 ms. The first structure of the... 

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-... 

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. 

Benzylpenicillin is hydrolysed, to the corresponding penicilloic acid (i2.7)y by alkali, cupric ions, or p-lactamase (usually called penicillinase), and to penillic acid, and penicillamine (11.21), In natural biosynthesis it is formed by the condensation of 6-penicillamine, D-valine, and phenyl-acetic acid. The presence of the highly strained four-membered lactam ring in penicillin makes it a powerful, but specific, acylating agent. This ring readily opens between C-7 and the nitrogen atom. 

Cephalosporin derivatives are expensive, but useful for organisms that are resistant to penicillins, and they may be given (but cautiously ) to patients who have become sensitized to the penicillins. They are more active than benzylpenicillin against Gram-negative bacteria, and specially effective in the respiratory and urinary tracts where they tend to concentrate. Although cephalosporins are not sensitive to penicillinase, resistant bacteria have been found to harbour a parallel (l-lactamase called cephalosporase. 

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