Transpeptidase, penicillin inhibition

Penicillin inhibits a D-alanyl-D-alanine transpeptidase that catalyzes the reaction... 

Inhibition of transpeptidase Some PBPs catalyze formation of the cross-linkages between peptidoglycan chains. Penicillins inhibit this transpeptidase-catalyzed reaction, thus hindering the formation of crosslinks essential for cell wall integrity. As a result of this blockade of cell wall synthesis, the Park peptide , UDP-acetylmuramyl-L-Ala-D-GIn-L-Lys-D-Ala-D-Ala, accumulates. 

Penicillin inhibits the cross-linking transpeptidase by the Trojan horse stratagem. The transpeptidase normally forms an acyl intermediate with the penultimate d-alanine residue of the d-Ala-d-Ala peptide (Figure 8.29). This covalent acyl-enzyme intermediate then reacts with the amino group of the terminal glycine in another peptide to form the cross-link. Penicillin is welcomed into the active site of the transpeptidase because it mimics the d-Ala-d-Ala moiety of the normal substrate (Figure 8.30). Bound penicillin then forms a covalent bond with a serine residue at the active site of the enzyme. This penicilloyl-enzyme does not react further. Hence, the transpeptidase is irreversibly inhibited and cell-wall synthesis cannot take place. 

Figure 7.10 Bacterial cell wall synthesis. 1) Alanine molecules are added to a carbohydrate tripeptide to form a "T" shaped cell wall precursor. This reaction is inhibited by D-cycloserine. 2) The precursor is transported across the plasma membrane by a carrier. Vancomycin inhibits the transport process. 3) The transporter is recycled to the inside of the cell to carry other precursors. Bacitracin inhibits this step. 4) The precursor is linked to the existing cell wall structure by transpeptidase. Penicillins, cephalosporins, imipenem and aztreonam inhibit the transpeptidase. Transpeptidase is one of several penicillin binding proteins and is not the only site of penicillin action.

By virtue of their fused /3-lactam-thiazolidine ring structure, the penicillins behave as acylating agents of a reactivity comparable to carboxylic acid anhydrides (see Section 5.11.2.1). This reactivity is responsible for many of the properties of the penicillins, e.g. difficult isolation due to hydrolytic instability (B-49MI51102), antibacterial activity due to irreversible transpeptidase inhibition (Section 5.11.5.1), and antigen formation via reaction with protein molecules. 

The antibiotic activity of certain (3-lactams depends largely on their interaction with two different groups of bacterial enzymes. (3-Lactams, like the penicillins and cephalosporins, inhibit the DD-peptidases/transpeptidases that are responsible for the final step of bacterial cell wall biosynthesis.63 Unfortunately, they are themselves destroyed by the [3-lactamases,64 which thereby provide much of the resistance to these antibiotics. Class A, C, and D [3-lactamases and DD-peptidases all have a conserved serine residue in the active site whose hydroxyl group is the primary nucleophile that attacks the substrate carbonyl. Catalysis in both cases involves a double-displacement reaction with the transient formation of an acyl-enzyme intermediate. The major distinction between [3-lactamases and their evolutionary parents the DD-peptidase residues is the lifetime of the acyl-enzyme it is short in (3-lactamases and long in the DD-peptidases.65-67... 

The answer is d. (Hardman, pp 1097—1098.) The antibiotic clavu-lanic acid is a potent inhibitor of p-lactamases. The mode of inhibition is irreversible. Although clavulanic acid does not effectively inhibit the transpeptidase, it maybe used in conjunction with a p-lactamase-sensitive penicillin to potentiate its activity... 

To be an effective antibacterial agent, a drng mnst inhibit an enzyme that is present in the bacteria bnt not in the host. One well-known example is a transpeptidase involved in cell wall synthesis in some bacteria. Inhibition prevents bacteria from synthesising their cell wall so that proliferation stops. A drng that inhibits this enzyme is the antibiotic, penicillin first nsed in 1941 (see Chapter 17). However, the first dnrg to inhibit bacterial growth was developed from a dye (Box 3.8). 

Penicillins disrupt cell wall synthesis by inhibiting transpeptidase. When bacteria are in their growth and replication phase, penicillins are bactericidal due to cell wall defects, the bacteria swell and burst. 

The antibiotic activity of penicillin is due to its ability to inhibit transpeptidases responsible for crosslink formation in construction of bacterial cell walls, leading to lysis of the weakened cells. 

Penicillin G Prevents bacterial cell wall synthesis by binding to and inhibiting cell wall transpeptidases Rapid bactericidal activity against susceptible bacteria Streptococcal infections, meningococcal infections, neurosyphilis IV administration rapid renal clearance (half-life 30 min, so requires frequent dosing (every 4 h) Toxicity Immediate hypersensitivity, rash, seizures... 

Several classes of (3-lactamases, often encoded in transmissible plasmids, have spread worldwide rapidly among bacteria, seriously decreasing the effectivenss of penicillins and other (3-lactam anti-biotics.t y Most (3-lactamases (classes A and C) contain an active site serine and are thought to have evolved from the dd transpeptidases, but the B typey has a catalytic Zn2+. The latter, as well as a recently discovered type A enzyme,2 hydrolyze imipenem, currently one of the antibiotics of last resort used to treat infections by penicillin-resistant bacteria. Some (3-lactam antibiotics are also powerful inhibitors of (3-lactamases.U/aa/bb These antibiotics may also have uses in inhibition of serine proteasesCC/dd such as elastase. Some antibiotic-resistant staphylococci produce an extra penicillin-binding protein that protects them from beta lactams.ee Because of antibiotic resistance the isolation of antibiotics from mixed populations of microbes from soil, swamps, and lakes continues. Renewed efforts are being... 

The monohactams, like pcncillins and cephalosporins, interfere with the synthesis of bacterial cell walls. /l-Lactani antibiotics bind to a series of penicillin-binding proteins (PBPs) on the cytoplasmic membrane and their antibacterial effect is believed to result from inhibition of a subset of these PBPs known as peplidoglycan transpeptidases. 

The penicillin antibiotics inhibit transpeptidase enzymes (penicillin-binding proteins (PBPs)) by acylation of the serinyl residue at their active site, which leads to cell wall lysis, since blocking PBPs circumvents proper murein membrane formation [3]. Several peptides and peptidomimetics containing the (3-lactam ring have been recently described as effective protease inhibitors and, consequently, as potential drugs for a wide range of diseases implicating proteases [5-8]. 

Ser residue in the active site of the enzyme acetylcholinesterase, irreversibly inhibiting the enzyme and preventing the transmission of nerve impulses (Fig. la). Iodoacetamide modifies Cys residues and hence may be used as a diagnostic tool in determining whether one or more Cys residues are required for enzyme activity (Fig. lb). The antibiotic penicillin irreversibly inhibits the glycopeptide transpeptidase enzyme that forms the cross-links in the bacterial cell wall by covalently attaching to a Ser residue in the active site of the enzyme (see Topic Al). 

As emphasized by the Strominger group [183,215,252] and depicted in Figure 8.5, the amide bonds in acyl-D-Ala-D-Ala and penicillins assume the same spatial position. This is considered to be very important in the inhibition of transpeptidase and DD-carboxypeptidase. 

---