Sandbox 123
From Proteopedia
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| - | == Introduction == | ||
| - | <scene name='36/365380/4dki_cartoon/5'>Transpeptidase (TP)</scene>, also known as penicillin-binding proteins (PBP), catalyze the cross-linking of peptidoglycan polymers during bacterial cell wall synthesis. | + | <scene name='36/365380/4dki_cartoon/5'>Transpeptidase (TP)</scene>, 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. | ||
| - | β-Lactam antibiotics stop the production of the cell wall by targeting bacterial PBPs. The cell wall, which is composed of peptidoglycan and surrounds the cell membrane, is crucial for maintaining the structural integrity of the bacterium. | ||
| - | The cell wall is composed | + | 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 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. | ||
<StructureSection load='4DKI' size=550 side=right scene='2H4M'/Com_view/1'> | <StructureSection load='4DKI' size=550 side=right scene='2H4M'/Com_view/1'> | ||
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| - | == The Mechanism of PBP2a == | ||
| - | PBP2a is composed of two domains: a <font color='orange'><b>non-penicillin binding domain </b><scene name='36/365380/4dki_cartoon/10'>(NPB) </scene></font> and a <font color='dodgerblue'><b><scene name='36/365380/4dki_cartoon/11'>TP</scene> 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/Ser403/15'>a serine residue at position 403 (ser403)</scene> which catalyzes the cross-linking of the peptidoglycan rows with pentaglycine cross-links. | + | 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/10'>(NPB) </scene></font> and a <font color='dodgerblue'><b><scene name='36/365380/4dki_cartoon/11'>TP</scene> 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/Ser403/15'>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 | |
| + | (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. | ||
| + | (c) The now terminal D-Ala residue forms a covalent bond to Ser403, while a crosslinking | ||
| + | pentaglycine chain enters the active site. | ||
| + | (d) 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. | |
| - | (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. | ||
| - | + | 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. | ||
| - | ( | + | Figure 2. Mechanism of action of β-lactams. (a) Structure of a β-lactam (penicillin) |
| + | showing the amide, carboxyl, and 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. | ||
| - | The entire process takes 4 milliseconds. | ||
| - | = | + | 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/4'>ceftobiprole</scene> and | ||
| + | ceftaroline – that have anti-MRSA activity have been developed. Ceftobiprole is able to | ||
| + | inhibit PBP2a because additional chemical groups at the R2 position of the cephalosporin | ||
| + | backbone are able to interact with additional amino acid residues in PBP2a; specifically | ||
| + | <scene name='36/365380/Ceftobiprole/3'>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. | ||
| - | + | Figure 5. Mechanism of action of ceftobiprole. (a) Structure of ceftobriprole.3 | |
| + | (b) The R2 group of ceftobiprole is bound by PBP2a, Tyr446, and Met641. This | ||
| + | increased binding allows the serine residue active site (Ser403) to hydrolyze the β- | ||
| + | lactam ring and become irreversibly inhibited. (c) Schematic of β-lactam covalently | ||
| + | bonded to active site blocking entrance to the substrate. | ||
Revision as of 18:14, 1 August 2013
, 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 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.
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