Penicllin Mechanism of Action

Penicillin (sometimes abbreviated PCN or pen) is a group of Beta-lactam antibiotics used in the treatment of bacterial infections caused by susceptible, usually Gram-positive, organisms.

Developments from penicillin
The first major development was ampicillin, which offered a broader spectrum of activity than either of the original penicillins. Further development yielded beta-lactamase-resistant penicillins including flucloxacillin, dicloxacillin and methicillin. These were significant for their activity against beta-lactamase-producing bacteria species, but are ineffective against the methicillin-resistant Staphylococcus aureus strains that subsequently emerged.

The line of true penicillin was the antipseudomonal penicillins, such as ticarcillin and piperacillin, useful for their activity against Gram-negative bacteria. However, the usefulness of the beta-lactam ring was such that related antibiotics, including the mecillinams, the carbapenems and, most important, the cephalosporins, have this at the center of their structures.



Penicillin bind to protein called penicillin binding protein or PBP which are bacterial enzymes that catalyze cell wall peptidoglycon synthesis, this binding interferes with cell wall synthesis and leads to cell lysis and destruction of bacterium, Natural penicillin is effective against Gram-positive cocci and anaerobic pathogen and Spirochete.Penicillin G is also effective against Gram-Negative coccus. Semi synthetic penicillin which are chemically synthesized analogs are effective against gram-negative bacilli, Penicillin G is first commercially available penicillin .it is fully absorbed when administered orally. Due to acid inactivation in the stomach. But well absorbed if administered intra-muscularly. The drug is also excreted rapidly in the urine Penicillin V and some semi-synthetic such as ampicillin, which are resistant to acid and so administered orally, subsequently better absorbed have greater bio availability and extended duration of action.

Penicillin Binding Protein Animation

Penicillin-binding proteins (PBPs) are a group of proteins that are characterized by their affinity for and binding of penicillin. They are a normal constitutent of many bacteria;All beta-lactam antibiotics bind to PBP to have their effect of preventing cell wall construction by the bacterium.

There are a large number of PBPs, usually several in each organism, and they are found as both membrane-bound and cytoplasmic proteins. For example, Spratt (1977) reports that six different PBPs are routinely detected in all strains of E. coli ranging in molecular weight from 40000 to 91000. The different PBPs occur in different numbers per cell and have varied affinities for penicillin . The PBPs are usually broadly classified into high-molecular-weight (HMW) and low-molecular-weight (LMW) categories.







PBPs are all involved in the final stages of the synthesis of peptidoglycan, which is the major component of bacterial cell walls. Bacterial cell wall synthesis is essential to growth, cell division (thus reproduction) and maintaining the cellular structure in bacteria. Inhibition of PBPs leads to irregularities in cell wall structure such as elongation, lesions, loss of selective permeability, and eventual cell death and lysis.

PBPs have been shown to catalyse a number of reactions involved in the process of synthesising cross-linked peptidoglycan from lipid intermediates and mediating the removal of D-alanine from the precursor of peptidoglycan. Purified enzymes have been shown to catalyse the following reactions: D-alanine carboxypeptidase, peptidoglycan transpeptidase, and peptidoglycan endopeptidase. In all bacteria that have been studied, enzymes have been shown to catalyse more than one of the above reactions (Spratt, 1977). The enzyme has a penicillin-insensitive transglycosylase N-terminal domain (involved in formation of linear glycan strands) and a penicillin-sensitive transpeptidase C-terminal domain (involved in cross-linking of the peptide subunits) and the serine at the active site is conserved in all members of the PBP family



Antibiotics




PBPs bind β-lactam antibiotics because they are similar in chemical structure to the modular pieces that form the peptidoglycan (Disteche and Bouille et al, 1982). When they bind to penicillin, the β-lactam amide bond is ruptured to form a covalent bond with the serine residue at the PBPs active site. This is an irreversible reaction and inactivates the enzyme.

There has been a great deal of research into PBPs because of their role in antibiotics and resistance. Bacterial cell wall synthesis and the role of PBPs in its synthesis is a very good target for drugs of selective toxicity because the metabolic pathways and enzymes are unique to bacteria (Chambers, 1999). Resistance to antibiotics has come about through overproduction of PBPs and formation of PBPs that have low affinity for penicillins (among other mechanisms such as lactamase production). Research on PBPs has led to the discovery our new semi-synthetic β-lactams, wherein altering the side-chains on the original penicillin molecule has increased the affinity of PBPs for penicillin, and, thus, increased effectiveness in bacteria with developing resistance.

The β-lactam ring is an analogous structure to all β-lactam antibiotics. Shown here is the core structure of penicillin, the square in the middle with an oxygen double-bonded to it is the β-lactam ring

Peptidoglycan Animation

Peptidoglycan, also known as murein, is a polymer consisting of sugars and amino acids that forms a mesh-like layer outside the plasma membrane of Bacteria (cell-wall). The sugar component consists of alternating residues of β-(1,4) linked N-acetylglucosamine and N-acetylmuramic acid residues. Attached to the N-acetylmuramic acid is a peptide chain of three to five amino acids. The peptide chain can be cross-linked to the peptide chain of another strand forming the 3D mesh-like layer. Some Archaea have a similar layer of pseudopeptidoglycan. Peptidoglycan serves a structural role in the bacterial cell wall, giving structural strength, as well as counteracting the osmotic pressure of the cytoplasm. A common misconception is that peptidoglycan gives the cell its shape; however, whereas peptidoglycan helps maintain the structure of the cell, it is actually the MreB protein that facilitates cell shape. Peptidoglycan is also involved in binary fission during bacterial cell reproduction.








The peptidoglycan layer is substantially thicker in Gram-positive bacteria (20 to 80 nanometers) than in Gram-negative bacteria (7 to 8 nanometers), with the attachment of the S-layer. Peptidoglycan forms around 90% of the dry weight of Gram-positive bacteria but only 10% of Gram-negative strains. In Gram-positive strains, it is important in attachment roles and stereotyping purposes. For both Gram-positive and Gram-negative bacteria, particles of approximately 2 nm can pass through the peptidoglycan.


The peptidoglycan layer in the bacterial cell wall is a crystal lattice structure formed from linear chains of two alternating amino sugars, namely N-acetylglucosamine (GlcNAc or NAG) and N-acetylmuramic acid (MurNAc or NAM). The alternating sugars are connected by a β-(1,4)-glycosidic bond. Each MurNAc is attached to a short (4- to 5-residue) amino acid chain, containing D-alanine, D-glutamic acid, and meso-diaminopimelic acid in the case of Escherichia coli (a Gram negative) or L-alanine, D-glutamine, L-lysine, and D-alanine in the case of Staphylococcus aureus (a Gram positive). These amino acids, except the L-amino acids, do not occur in proteins and are thought to help protect against attacks by most peptidases.Cross-linking between amino acids in different linear amino sugar chains by an enzyme known as transpeptidase result in a 3-dimensional structure that is strong and rigid. The specific amino acid sequence and molecular structure vary with the bacterial species.