Contents
Structure
Vancomycin is naturally produced by the soil bacterium Amycolatopsis orientalis [1]. It is a glycosylated nonribosomal peptide, which means that it is biosynthesized by the bacterium without the use of mRNA or a ribosome, through means of nonribosomal peptide synthesis. It is a heptapeptide consisting of of the following amino acids: Leucine1 (Leu1), β-hydroxytyrosine2 (β-OH-Tyr2), Asparagine3 (Asn3), 4-hydroxyphenylglycine4 (HPG4), HPG5, β-OH-Tyr6, 3,5-dihydroxyphenylglycine7 (DPG7). Of these seven only Asn3 and Leu1 are proteinogenic amino acids, the rest being non-proteinogenic or non-coded amino acids. The stereoisomeric configuration of the amino acid residues in the heptapeptide scaffold of vancomycin is D-D-L-L-D-D-L.[2]
Once the basic heptapeptide scaffold is assembled, further post translational modifications take place on the molecule. First, residues 2 and 4, 4 and 6, and 5 and 7, undergo oxidative crosslinking, become covalently bonded to each other, forming the highly rigid, dome-like structure of vancomycin. This conformation is what gives vancomycin its high affinity for forming hydrogen-bonds with its target - the N-acyl-D-Ala-D-Ala termini of the peptidoglycan precursors in bacteria. The molecule in this state is biologically active and is termed the aglycone backbone of vancomycin[3] . Finally, the Leucine is methylated to N-methyleucine, and two successive glycosylations on the phenolate of the HPG residue give the finished vancomycin molecule. These further modifications are not essential for the antibiotic functionality of vancomycin though they do allow for stronger interactions with the target. [2]
Mechanism of Action
Vancomycin kills and prevents the growth of gram-positive bacteria by inhibiting the cell-wall synthesis of these bacteria [3]. The cell walls of gram-positive bacteria are comprised of several layers of peptidoglycan, a mesh-like polymer made up of sugars and amino acids. It is this layer that provides the necessary mechanical support for bacteria to be able to withstand fluctuations in osmotic pressures in excess of 5-15 atm without lysing (rupturing)[4] .
A single peptidoglycan layer consists of many crosslinked glycan chains. A glycan chain is made up of repeating units of covalently bonded N-acetylglucosamine (NAG) and N-acetylmuramic acid (NAM) monomers joined together through transglycosylation. The newly elongated chains are mechanically weak till the pentapeptide chains found on every NAM molecule are crosslinked. This is done by a family of transpeptidases, which use the amide group of the Lys3 on one strand to attack the D-Ala4 on the other strand, liberating a D-Ala5 residue, and forming a Lys3-D-Ala4 interstrand isopeptide bond which acts as a strengthening covalent cross-link between the two strands.[4]
Vancomycin belongs to a class of antibiotics which interferes in both the polymerization and the cross linking of glycan strands. It does this by binding firmly to the substrate of the transpeptidation enzymes, the D-Ala4-D-Ala5 dipeptide, by means of five hydrogen bonds with its peptide backbone [3] [5]. The formation of this complex prevents both transglycosylation and transpeptidation via steric hindrance[5].
The final two steps of bacterial peptidoglycan biosynthesis constitute a good target for any antimicrobial agent, as both processes are extracellular and thus accessible to compounds that are unable to penetrate the cell membrane. Furthermore, the peptidoglycan layer is vital enough to survival that it is highly conserved across organisms, meaning that compounds such as vancomycin are effective against a variety of gram-positive bacteria. Lastly, targeting a process that involves multiple, related enzymes, is advantageous as a single, spontaneous mutation in one enzyme will not lead to resistance.[4]
References
- ↑ Grace, Y., Koteva, K.P., Thaker & Thaker, M.N. (2013). Glycopeptide antibiotic biosynthesis. The Journal of Antibiotics, 67, 31-41. DOI: 10.1038/ja.2013.117
- ↑ 2.0 2.1 Nolan, E. M., & Walsh, C. T. (2009). How Nature Morphs Peptide Scaffolds into Antibiotics. Chembiochem : A European Journal of Chemical Biolo1, 10(1), 34–53. http://doi.org/10.1002/cbic.200800438
- ↑ 3.0 3.1 3.2 Reynolds, P.E. (1989). Structure, Biochemistry and Mechanism of Action of Glycopeptide Antibiotics. European Journal of Clinical Microbiolo1 and Infectious Diseases, 8(11), 943-950. https://doi.org/10.1007/BF01967563
- ↑ 4.0 4.1 4.2 Kahne, D., Leimkuhler, C., Lu, W. & Walsh, C. (2005). Glycopeptide and Lipoglycopeptide Antibiotics. Chemical Reviews, 105, 425-448.
- ↑ 5.0 5.1 Bambeke, F. van, Laethem, Y. van, Courvalin, P. & Tulkens, P.M. (2004) Glycopeptide Antibiotics from Conventional Molecules to New Derivatives. Drugs, 64(9), 913-936.