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<StructureSection load= size=475 side='right' scene='36/365380/4dki_cartoon/19'>

Transpeptidase (TP), also known as penicillin-binding proteins (PBP), catalyze the cross-linking of peptidoglycan polymers during bacterial cell wall synthesis. Beta-lactam (β-lactam) antibiotics, which
include penicillins, cephalosporins and carbapenems, bind and irreversibly inhibit transpeptidases. The overuse and misuse of β-lactam antibiotics has led to strains of Staphylococcus aureus that are resistant to all β-lactams and are often only susceptible to “last resort antibiotics”, such as vancomycin. 


== Cell Wall Structure ==

The cell wall, which is composed of peptidoglycan, is crucial for maintaining the structural integrity of the bacterium.  Peptidoglycan consists of N-acetylmuramic Acid (NAM) and N-acetylglucosamine (NAG) polymers. Rows of peptidoglycan are cross-linked together with pentaglycine chains. The NAM residues have a five amino acid side chain that terminates with two D-Alanine (D-Ala) residues. 

[[Image:Cell Wall 7 30 2013.jpg|thumb|alt= Alt text| Figure 1. A.Bacterial Cell Wall  B.Peptidoglycan with D-Ala-D-Ala substrate |550px]] 

== Structure of a Resistant Transpeptidase ==

Methicillin resistant Staphylococcus aureus (MRSA) is resistant to all β-lactams because it acquires an alternative PBP, PBP2a, that is not bound or inhibited by any β-lactams. PBP2a is composed of two domains: a <font color='orange'><b>non-penicillin binding domain </b><scene name='36/365380/4dki_cartoon/20'>(NPB) </scene></font> and a <font color='dodgerblue'><b>transpeptidase <scene name='36/365380/4dki_cartoon/21'>(TP)</scene> binding domain </b></font>. The NBP domain of PBP2a is anchored in the cell membrane, while the TP domain “sits” in the periplasm with its active site facing the inner surface of the cell wall. The active site contains <scene name='36/365380/Ser403/19'>a serine residue at position 403 (ser403)</scene> which catalyzes the cross-linking of the peptidoglycan rows with pentaglycine cross-links.




== Catalytic Mechanism of PBP2a ==
[[Image:Schematic TP 3steps.jpg|thumb|alt= Alt text|Figure 2. Schematic diagram illustrating the mechanism of action of PBP2a |550px]]

(a)The D-Ala-D-Ala side-chain substrate of the peptidoglycan accesses the active site of the PBP2a.

(b)Ser403 nucleophilically attacks the peptide bond of the terminal D-Ala residues of the substrate. The terminal D-Ala residue then exits the active site. The now terminal D-Ala residue forms a covalent bond to Ser403, while a crosslinking pentaglycine chain enters the active site.

(c)A covalent bond forms between the pentaglycine chain and the  terminal D-Ala residue, regenerating the active site serine residue.

The entire process takes 4 milliseconds.

== How Do Antibiotics Work? == 

The β-lactam antibiotics inhibit bacterial growth by inhibiting PBPs and ultimately cell wall
synthesis. Specifically, β-lactams are molecular mimics of D-Ala-D-Ala, which is the normal
substrate of PBPs. Nucleophillic attack of the β-lactam results in the PBP being irreversibly
inhibited by the β-lactam. As a result, the synthesis of the cell wall is inhibited which leads
to cell lysis.

[[Image:Structures on penicillin and b lactam.jpg|thumb|alt= Alt text|Figure 3. Mechanism of action of β-lactams. A. Structure of a β-lactam (penicillin) showing the amide, carboxyl, and β-lactam ring groups β-lactam ring groups. B. Structure of the D-Ala-D-Ala substrate. C. Overlay of the D-Ala-D-Ala substrate in red with penicillin demonstrating molecular mimicry.|550 px]]



== PBP2a and Ceftobiprole ==

MRSA becomes resistant to β-lactams by acquiring an alternative PBP, PBP2a, that is
neither bound nor inhibited by β-lactams. Recently, two cephalosporins – <scene name='36/365380/Ceftobiprole/23'>ceftobiprole</scene> and
ceftaroline – that have anti-MRSA activity have been developed. Ceftobiprole is able to
inhibit PBP2a because additional chemical groups at the <scene name='36/365380/Ceftobiprole/12'>R2</scene> position of the cephalosporin backbone are able to interact with additional amino acid residues in PBP2a; specifically
<scene name='36/365380/Ceftobiprole/22'>Tyr446 and Met641</scene>. As a result of its tighter binding to PBP2a, ceftobiprole is able to more
efficiently react with the serine active site residue and therefore inhibit the activity of
PBP2a.




MRSA becomes resistant to β-lactams by acquiring an alternative PBP, PBP2a, that is
neither bound nor inhibited by β-lactams. Recently, two cephalosporins – 
<scene name='37/372724/Ceftobiprole/1'>ceftobiprole</scene> and
ceftaroline – that have anti-MRSA activity have been developed. Ceftobiprole is able to
inhibit PBP2a because additional chemical groups at the 
<scene name='37/372724/Ceftobiprole/7'>R2</scene>
position of the cephalosporin backbone are able to interact with additional amino acid
residues in PBP2a; specifically 
<scene name='37/372724/Tyr446_and_met641_label/2'>Tyr446 and Met641</scene>.
As a result of ceftobiprole <scene name='37/372724/R2_interaction/4⅝'>tighter binding</scene> to PBP2a as highlighted in green , <scene name='37/372724/Ceftobiprole_in_cpk/1'>the medicine</scene>, shown as colors of the atom types ([[CPK]]), is able to more efficiently react with the serine active site residue and therefore inhibit the activity of PBP2a.


<scene name='37/372724/Medicine_interaction/3'>R2</scene>
<scene name='37/372724/Medicine_interaction/2'>Tyr446 and Met641</scene>
<scene name='37/372724/Medicine_interaction/1'>ceftobiprole</scene>
<scene name='37/372724/R2_interaction/6'>tighter binding</scene>


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