Molecular Mechanics Of Cyclic Antibiotics Biology Essay



The development of new antibacterial curative agents capable of holding microbic opposition is a main chase in clinical medical specialty. [ 1 ] In an attempt to turn to this menace of microbic antibiotic opposition, research workers have pursued several schemes, one being the development of peptide based antibiotics. [ 2 ] These antibiotic peptides exhibit a fast and deadly manner of action that is rather different from the manner of action of other man-made antibiotics, doing peptide antibiotics attractive curative marks. [ 2 ]

Presently, more than 500 antimicrobic peptides have been isolated from a broad scope of beings. Peptides are classified based on their construction of which there are four major categories: I?-sheet, I±-helical, loop and extended peptides [ 3 ] , with the first two categories being the most common in nature. [ 4 ] The I?-sheet peptides represent a extremely diverse group of molecules at the degree of primary construction. Despite such differences, these peptides portion common characteristics, including amphipathic composing, with distinguishable hydrophobic and hydrophilic surfaces. [ 5 ] Both the I±-helical and I?-sheet peptides are amphiphilic in nature. [ 6 ] I?-sheet peptides are chiefly cyclic, and can be subdivided into two subgroups: those incorporating disulfide bonds, such as tachyplesin and those that do non, such as gramacidine S [ 7, 8 ] and the tyrocidines.

Cyclic I?-sheet peptides

Peptides incorporating disulfide Bridgess

Cyclic I?-sheet peptides can be constrained by disulfide cross-links or anchor cylization. Tachyplesins, polyphemusins, bactenecins, and protegrins are illustrations of disulfide-constrained antibiotic peptides.

Peptides missing disulfide Bridgess

An apprehension of the cardinal characteristics of the secondary and third constructions of the antimicrobic peptides and their effects on bactericidal and haemolytic activity can help the rational design of improved parallels for clinical usage. [ 9 ] Studies on a wide scope of peptides reveal two of import demands for antimicrobic activity, ( 1 ) a cationic charge and ( 2 ) an induced amphipathic conformation. [ 4 ]

Michael R. Yeaman ; Nannette Y. Yount. Mechanisms of antimicrobic peptide action and opposition. Pharmacological reappraisals. 2003 ; 55:27 – 55.

Cationicity is doubtless of import for the initial electrostatic attractive force of antimicrobic peptides to negatively charged phospholipids membranes of bacteriums and other micro beings, and common electroaffinity likely confers selective antimicrobic aiming comparative to host tissues. The fact that bacterial membranes are rich in the acidic phospholipids PG, PS, and CL confers their overall negative charge. Furthermore, LPS and teichoic or teichuronic acids of Gram-negative and Gram-positive bacteriums, impart extra negative charge to the surfaces of these several beings.

Michael R. Yeaman ; Nannette Y. Yount. Mechanisms of antimicrobic peptide action and opposition. Pharmacological reappraisals. 2003 ; 55:27 – 55.

The amphipathicity is characterized by a variable figure of I?-strands, with comparatively few or no coiling spheres, organized to make both polar and non-polar surfaces. These I?-strands are often antiparallel, and are stablized by a series of disulfide bonds, with every bit many as eight cysteins in some peptides [ eg works defensis ( sitaram and nagaraj 1999 ) and muscel mytilins ( dimarcq 1998 ) ] , or by cyclization of the peptide anchor ( e.g protegrins, gramicidin, or I?-defensins ) . The conformational rigidness observed in many I?-sheet antimicrobic peptides in aqueous solution may besides advance multimerization, restricting exposure of hydrophobic aspects to hydrophilic environments A figure of I?-sheet peptides have been shown to be as dimmers in aqueous solution

The proposed mechanism by which antimicrobic peptides perturb mark membranes involve amphipathicity and hydrophobic minute. For illustration, interpolation of the hydrophobic peptide face into the lipid bilayer, and association of the charged arginine side ironss with polar lipid caput groups, relies upon 3-dimensional separation of hydrophobic and charge. Once associated with the membrane, the amphipathic nature of I?-sheet peptides likely enables their formation of transmembrane channels.Several theoretical accounts have been proposed to explicate the exact mechanism by which these peptides may organize and track the channel ; nevertheless the precise conformation adopted by such peptides in the hydrophobic membrane environment remains to be determined.

Three attacks are presently being used to develop antibiotics. The first involve alteration of bing peptides ( and presumptively besides isolation of fresh peptides from nature and alteration of these ) . For illustration, the streptogramins are a household of cyclic peptides discovered in the 1950s, which are quite powerful but instead indissoluble. Recent work has resulted in two water-soluble, semi man-made streptogramins, dalfopristin and quinupristin. A 2nd instead exciting attack involves the modular nature of synthesis of the antibiotics. Schneider et al discovered that one can set together a fresh combination of peptide synthesis faculties and arrive at a fresh construction. Therefore there is great possible for obtaining important chemical diverseness in the anchor amino acids or their alterations, and a combinative attack to bring forthing diverseness ( i.e. commixture and duplicate faculties ) is possible. The 3rd attack is to utilize these constructions as templets for chemical synthesis and diverseness. Gramicidin S and tyrocidine A are illustrations of this attack. Discrepancies of gramicidin S with altered ring size, charge amino acid sequences, hydrophobicity, etc have been constructed and shown to hold greater selectivity for bacteriums than for mammalian cells.

Proposed mechanism of action

An overview of the interaction of peptides with Gram-negative bacteriums is as shown in the fig below. The initial association of peptides with the bacterial membrane occurs through electrostatic interactions between the cationic peptide and the LPS in the outer membrane taking to membrane disturbance. It has besides been shown that cationic peptides have a higher affinity for LPS in the outer cusp of the outer membrane of Gram-negative bacteriums than do native divalent cations such as Mg2+ and Ca2+ . [ 10 ] Passage across the outer membrane is proposed to happen by ego promoted consumption. Harmonizing to this hypothesis, unfolded cationic peptides are proposed to tie in with the negatively charged surface of the outer membrane and either nuetralize the charge over a spot of the outer membrane, making clefts through which the peptide can traverse the outer membrane ( A ) , or really adhere to the divalent cation adhering sites on LPS and interrupt the membrane ( B ) . Once the peptide has transited the outer membrane, it will adhere to the negatively charged surface of the cytol Intelligence Community membrane, created by the caput groups of phosphatidylglycerol and cardiolipin, and the amphipathic peptide will infix into the membrane interface ( the part where the phospholipid headgroups meet the fatty acyl ironss of the phospholipid membrane ) ( C ) . It is non known at which point in this procedure the peptide really folds into its amphipathic construction ( i.e during theodolite across the outer membrane or during interpolation into the cytoplasmatic membrane ) . Many peptide molecules will infix into membrane inteface and are proposed to so either sum into micelle-like composite which spans the membrane ( D ) or impudent floating-point operation under the influence of big transmembrane electrical possible gradient ( about 140mV ) ( E ) . The micelle-like sums ( D ) are proposed to hold H2O associated with them, and this provides channels for the motion of ions across the membrane and perchance escape of larger water-soluble molecules. These sums would be variable in size and life clip and will disassociate into monomers that possibly disposed at either side of the membrane. The net consequence of ( D ) and ( E ) is that some monomers will be translocated into the cytol and can disassociate from the membrane and bind to cellur polyanions such as Deoxyribonucleic acid and RNA ( F ) [ 10


Understanding the relationship between construction and map of these peptides has been a ambitious undertaking. This survey is limited to the probe of the constructions, ego assembly, specifically the dimerization and collection of two cyclic peptides: tyrocidine A and tyrocidine C in aqueous solution every bit good as in decane. We besides investigate the binding of Ca ( II ) ions to tyrocidine C.

1. Michael A. Marques, Diane M. Citron, Clay C. Wang. Development of Tyrocidine A analogues with improved antibacterial activity. Bioorganic & A ; Medicinal Chemistry 2007 ; 15:6667- 6677

2. Hans G. Holman. Peptide antibiotics and their function in innate unsusceptibility 1995 ; 13:61-92

3. Robert E. W. Hancock, R. Lehrer. Cationic peptides: a new beginning of antibiotics.

4. Jon-Paul S.Powers, Robert E. W. Hancock. The relationship between peptide construction and antibacterial activity. Peptides 2003 ; 24:1681- 1691

5. Tushar K. Chakraborty, Dipankar Koley, Rapolu Ravi, Viswanatha Krishnakumari, Ramakrishnan Nagaraj, Ajit C. Kunwar. Synthesis, conformational analysis and biological surveies of cyclic cationic antimicrobic peptides incorporating sugar amino acids. J.Org. Chem 2008 ; 73:8731 – 8744

6. Andreu David, Luis Rivas. Animal antimicrobic peptides: an overview. Biopolymers 1998 ; 47:415 – 433