
Penicillin-binding proteins (PBPs) are a group of
protein
Proteins are large biomolecules and macromolecules that comprise one or more long chains of amino acid residue (biochemistry), residues. Proteins perform a vast array of functions within organisms, including Enzyme catalysis, catalysing metab ...
s that are characterized by their affinity for and binding of
penicillin
Penicillins (P, PCN or PEN) are a group of beta-lactam antibiotic, β-lactam antibiotics originally obtained from ''Penicillium'' Mold (fungus), moulds, principally ''Penicillium chrysogenum, P. chrysogenum'' and ''Penicillium rubens, P. ru ...
. They are a normal constituent of many
bacteria
Bacteria (; : bacterium) are ubiquitous, mostly free-living organisms often consisting of one Cell (biology), biological cell. They constitute a large domain (biology), domain of Prokaryote, prokaryotic microorganisms. Typically a few micr ...
; the name just reflects the way by which the protein was discovered. All
β-lactam antibiotics (except for
tabtoxinine-β-lactam, which inhibits
glutamine synthetase
Glutamine synthetase (GS) () is an enzyme that catalyzes the condensation of glutamate and ammonia to form glutamine:
Glutamate + ATP + NH3 → Glutamine + ADP + phosphate
Glutamine synthetase uses ammonia produced by nitrate reduction ...
) bind to PBPs, which are essential for bacterial
cell wall
A cell wall is a structural layer that surrounds some Cell type, cell types, found immediately outside the cell membrane. It can be tough, flexible, and sometimes rigid. Primarily, it provides the cell with structural support, shape, protection, ...
synthesis. PBPs are members of a subgroup of transpeptidase enzymes called
DD-transpeptidases.
Diversity
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
''Escherichia coli'' ( )Wells, J. C. (2000) Longman Pronunciation Dictionary. Harlow ngland Pearson Education Ltd. is a gram-negative, facultative anaerobic, rod-shaped, coliform bacterium of the genus ''Escherichia'' that is commonly foun ...
'' ranging in molecular weight from 40,000 to 91,000.
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.
High Molecular Mass (HMM) PBP’s are essential for cell viability and they are divided between two classes. Class A enzymes catalyze both the polymerization of a peptidoglycan from disaccharide peptides (glycosyltransferase) and the cross-linking of muramyl peptides (transpeptidase). On the other hand, class B enzymes possess transpeptidase activity (only cross linking). Low Molecular-Mass (LMM) PBP’s are dispensable for normal cell growth and control how tightly the peptidoglycan chains are linked together.
Proteins that have evolved from PBPs occur in many higher organisms and include the mammalian
LACTB protein.
Function
PBPs are all involved in the final stages of the synthesis of
peptidoglycan
Peptidoglycan or murein is a unique large macromolecule, a polysaccharide, consisting of sugars and amino acids that forms a mesh-like layer (sacculus) that surrounds the bacterial cytoplasmic membrane. The sugar component consists of alternating ...
, 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 defects in cell wall structure and irregularities in cell shape, for example
filamentation
Filamentation is the anomalous growth of certain bacteria, such as ''Escherichia coli'', in which cells continue to elongate but do not divide (no septa formation). The cells that result from elongation without division have multiple chromosomal ...
, pseudomulticellular forms, lesions leading to
spheroplast
A spheroplast (or sphaeroplast in British usage) is a microbial cell from which the cell wall has been almost completely removed, as by the action of penicillin or lysozyme. According to some definitions, the term is used to describe Gram-negative ...
formation, and eventual cell death and
lysis
Lysis ( ; from Greek 'loosening') is the breaking down of the membrane of a cell, often by viral, enzymic, or osmotic (that is, "lytic" ) mechanisms that compromise its integrity. A fluid containing the contents of lysed cells is called a ...
.
PBPs have been shown to catalyze a number of reactions involved in the process of synthesizing cross-linked peptidoglycan from lipid intermediates and mediating the removal of
D-
alanine
Alanine (symbol Ala or A), or α-alanine, is an α-amino acid that is used in the biosynthesis of proteins. It contains an amine group and a carboxylic acid group, both attached to the central carbon atom which also carries a methyl group sid ...
from the precursor of peptidoglycan. Purified
enzyme
An enzyme () is a protein that acts as a biological catalyst by accelerating chemical reactions. The molecules upon which enzymes may act are called substrate (chemistry), substrates, and the enzyme converts the substrates into different mol ...
s have been shown to catalyze the following reactions:
D-alanine carboxypeptidase, peptidoglycan transpeptidase, and peptidoglycan endopeptidase. In all bacteria that have been studied, enzymes have been shown to catalyze more than one of the above reactions.
The enzyme has a penicillin-insensitive transglycosylase
N-terminal
The N-terminus (also known as the amino-terminus, NH2-terminus, N-terminal end or amine-terminus) is the start of a protein or polypeptide, referring to the free amine group (-NH2) located at the end of a polypeptide. Within a peptide, the amin ...
domain (involved in formation of linear glycan strands) and a penicillin-sensitive transpeptidase
C-terminal
The C-terminus (also known as the carboxyl-terminus, carboxy-terminus, C-terminal tail, carboxy tail, C-terminal end, or COOH-terminus) is the end of an amino acid chain (protein or polypeptide), terminated by a free carboxyl group (-COOH). When t ...
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.
Some low-molecular-weight PBPs associate with the
MreB cytoskeleton and follow its rotation around the cell, inserting petipdoglycan in an oriented manner during cell growth. In contrast, high-molecular-weight PBPs are independent from MreB and maintain cell wall integrity by detecting and repairing defects in the peptidoglycan.
Antibiotics
PBPs bind to
β-lactam antibiotics because they are similar in chemical structure to the modular pieces that form the peptidoglycan.
When they bind to penicillin, the β-lactam amide bond is ruptured to form a covalent bond with the catalytic 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.
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). These experiments change the structure of PBP by adding different amino acids into the protein, allowing for new discovery of how the drug interacts with the protein. Research on PBPs has led to the discovery of 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.
Presence of the protein
penicillin binding protein 2A (PBP2A) is responsible for the
antibiotic resistance
Antimicrobial resistance (AMR or AR) occurs when microbes evolve mechanisms that protect them from antimicrobials, which are drugs used to treat infections. This resistance affects all classes of microbes, including bacteria (antibiotic resis ...
seen in
methicillin-resistant ''Staphylococcus aureus'' (MRSA).
The β-lactam ring is a structure common to all β-lactam antibiotics.
Structure of Penicillin Binding Protein 3
Penicillin binding protein 3 is important for bacteria wall synthesis and is a main target in β-lactam antibiotics. It is a two-domain protein containing a C-terminal transpeptidase linked to an extended N-terminal domain. This protein is similar to other class B PBP’s since it contains an α-helical subdomain or “head” domain towards the N-terminus. The N-terminal domain’s function is still not known but it is thought it serves to position the transpeptidase domain away from the inner membrane as part of a multienzyme complex involved in cell wall biosynthesis.
Active Site of Penicillin Binding Protein 3 and a β-lactam (carbenicillin)

The active site is located in a long cleft running parallel with the 3 strand across the lower part of the transpeptidase domain. When
carbenicillin
Carbenicillin is a bactericidal antibiotic belonging to the carboxypenicillin subgroup of the penicillins. It was discovered by scientists at Beecham and marketed as Pyopen. It has Gram-negative coverage which includes ''Pseudomonas aeruginosa' ...
binds to penicillin binding protein 3, it forms an acyl-enzyme complex which means the β-lactam is chemically attached to PBP3. The β-lactams are covalently bound to S294 which inactivates the enzyme. Also, the N-terminal end of PBP3 is more flexible, however, the C-terminal part, which contains the enzyme’s active site, is very stable and does not change much. The binding of carbenicillin to the active site increases the enzymes thermostability with conformational changes. The first carboxylate group in carbenicillin forms hydrogen bonding interactions with S485, T487, and N351. These hydrogen bonding interactions help stabilize the binding between carbenicillin and PBP 3.
Other images
File:Penicillin Core.svg, Penicillin core.
File:Filamentation 1.jpg, Filamentation
Filamentation is the anomalous growth of certain bacteria, such as ''Escherichia coli'', in which cells continue to elongate but do not divide (no septa formation). The cells that result from elongation without division have multiple chromosomal ...
(top right of electron micrograph) occurs in some bacteria when PBP3 is inhibited.
See also
PASTA domain
References
{{reflist
Bacterial proteins