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N Sitaram

Publications and source records attributed to N Sitaram.

At least 19 recordsLinked to original sources
PubMed2006

Antimicrobial peptides with unusual amino acid compositions and unusual structures.

Antimicrobial peptides, which constitute an important component of innate immunity in animal and plant kingdom, are ubiquitously distributed in nature. However, they differ widely in their sizes, sequences and structures. On the basis of their structure they have been broadly classified into three classes: a) linear peptides with propensity for amphiphilic alpha-helical structure, b) peptides with beta or alphabeta structure stabilized by different number of disulfide bridges and c) peptides with over-representation of certain amino acids or unusual structures. Although considerable amount of work has been done on peptides of all the three classes, recent reviews have emphasized on peptides belonging to the first two classes. The present review focuses on the peptides belonging to the third group. The antimicrobial peptides discussed in this article include aromatic amino acid-rich peptides, (Pro-Arg)-rich peptides, unusual defensins and defensin-like molecules, unusual antimicrobial peptides from amphibians, bacteriocins with unusual structure and anionic antimicrobial peptides.

Amino Acid Sequence↗
PubMed2002

Host-defense antimicrobial peptides: importance of structure for activity.

Antimicrobial peptides are important components of innate immunity in species across the evolutionary scale. Unlike therapeutically used antibiotics, this class of peptides exert their activity by permeabilizing bacterial membranes. Despite the seemingly common mechanism of action, there is considerable variation in their primary structures, length and number of positive charges. Host-defense antimicrobial peptides have been the subject of extensive biophysical studies with a view at delineate structural requirements for activity. In this article, the structures of host defence antibacterial peptides and the structural requirements for activity are reviewed.

Amino Acid Sequence↗
PubMed2002

The therapeutic potential of host-defense antimicrobial peptides.

Species across the evolutionary scale from insects to mammals deploy peptides to counter microbial invaders. Although, there are considerable differences in the primary and secondary structures of these peptides, they exert their antimicrobial activity by common mechanisms ie. by membrane permeabilization. Extensive structure-function studies have been carried out on this class of antimicrobial peptides and 'designer' peptides have been generated which possess specific antimicrobial activity. In this article the physico-chemical properties and biological activities of host-defense peptides and 'designer' peptides would be reviewed with a view to explore whether they could be used as effective therapeutic agents.

Amino Acid Sequence↗
PubMed2001

Biological activities of retro and diastereo analogs of a 13-residue peptide with antimicrobial and hemolytic activities.

The biological activities of synthetic retro and diastereo analogs of PKLLKTFLSKWIG (SPFK), a 13-residue peptide with antimicrobial and hemolytic activities, have been investigated. Retro peptides with C-terminal acid and amide exhibited antibacterial activities comparable with those of SPFK. Their hemolytic activities were, however, only marginally lower. The diastereo analog with C-terminal acid was not antibacterial and was weakly hemolytic. Amidation of this analog could restore antibacterial activity. Both retro analogs were unordered in aqueous medium but had a propensity for a helical structure in trifluoroethanol. However, diastereo analogs were unordered in both aqueous medium and trifluoroethanol. Thus, reversing the sequence in a short amphiphilic peptide may not always result in the selective loss of biological activity such as hemolytic activity. Also, introduction of enantiomeric amino acids in a short peptide to generate a diastereomer may result in loss of structure as well as antimicrobial and hemolytic activities, unless compensated by an increase in positive charges.

Amino Acid Sequence↗
PubMed2000

Tigerinins: novel antimicrobial peptides from the Indian frog Rana tigerina.

Four broad-spectrum, 11 and 12 residue, novel antimicrobial peptides have been isolated from the adrenaline-stimulated skin secretions of the Indian frog Rana tigerina. Sequences of these peptides have been determined by automated Edman degradation, by mass spectral analysis and confirmed by chemical synthesis. These peptides, which we have named as tigerinins, are characterized by an intramolecular disulfide bridge between two cysteine residues forming a nonapeptide ring. This feature is not found in other amphibian peptides. Conformational analysis indicate that the peptides tend to form beta-turn structures. The peptides are cationic and exert their activity by permeabilizing bacterial membranes. Tigerinins represent the smallest, nonhelical, cationic antimicrobial peptides from amphibians.

Amino Acid Sequence↗
PubMed2000

Antibacterial and hemolytic activities of single tryptophan analogs of indolicidin.

The structure and biological activities of analogs of the bovine neutrophil antibacterial and hemolytic peptide indolicidin, ILPWKWPWWPWRR-amide, where one tryptophan at 4th, 8th, or 11th position has been retained and the others replaced by leucine, have been investigated. All the single tryptophan analogs exhibit antibacterial activity. However, unlike indolicidin, they do not lyse erythrocytes. Structure analysis by circular dichroism spectroscopy indicates that the analogs are unordered in aqueous medium and adopt beta-turn structures in trifluoroethanol and micelles. The tryptophan residues in indolicidin appear to be essential for hemolytic activity but not antibacterial activity. The nonspecific biological activities of indolicidin and specific antibacterial activity of single tryptophan analogs suggest that in short peptides, a motif composed of hydrophobic amino acids with the exception of tryptophan, interspaced with proline residues and cationic amino acids at the N or C termini would favor selective antibacterial activity.

Amino Acid Substitution↗
PubMed1999

Interaction of antimicrobial peptides with biological and model membranes: structural and charge requirements for activity.

Species right across the evolutionary scale from insects to mammals use peptides as part of their host-defense system to counter microbial infection. The primary structures of a large number of these host-defense peptides have been determined. While there is no primary structure homology, the peptides are characterized by a preponderance of cationic and hydrophobic amino acids. The secondary structures of many of the host-defense peptides have been determined by a variety of techniques. The acyclic peptides tend to adopt helical conformation, especially in media of low dielectric constant, whereas peptides with more than one disulfide bridge adopt beta-structures. Detailed investigations have indicated that a majority of these host-defense peptides exert their action by permeabilizing microbial membranes. In this review, we discuss structural and charge requirements for the interaction of endogenous antimicrobial peptides and short peptides that have been derived from them, with membranes.

Amino Acid Sequence↗
PubMed1999

Biological activities of C-terminal 15-residue synthetic fragment of melittin: design of an analog with improved antibacterial activity.

Melittin, the 26-residue predominant toxic peptide from bee venom, exhibits potent antibacterial activity in addition to its hemolytic activity. The synthetic peptide of 15 residues corresponding to its C-terminal end (MCF), which encompasses its most amphiphilic segment, is now being shown to possess antibacterial activity about 5-7 times less compared to that of melittin. MCF, however, is 300 times less hemolytic. An analog of MCF, MCFA, in which two cationic residues have been transpositioned to the N-terminal region from the C-terminal region, exhibits antibacterial activity comparable to that of melittin, but is only marginally more hemolytic than MCF. The biophysical properties of the peptides, like folding and aggregation, correlate well with their biological properties.

Amino Acid Sequence↗
PubMed1999

Consequences of introducing a disulfide bond into an antibacterial and hemolytic peptide.

The effect of introducing a disulfide bridge between the N- and C-terminal ends on the structure and biological activities of the 13-residue linear peptide PKLLKTFLSKWIG(SPFK), which has both antibacterial and hemolytic activity, have been investigated. The terminal amino acids P and G in SPFK were replaced by cysteines to form a disulfide bridge. The linear peptides C(Acm)KLLKTFLSKWIC(Acm) and C(Acm) KLLKTFLSKWIC(Acm)-amide, where Acm is acetamidomethyl group, showed antibacterial activity but did not possess hemolytic activity unlike SPFK. Introduction of an S-S bridge resulted in enhanced hemolytic activity compared with SPFK. The hemolytic activity was particularly pronounced in the cyclic peptide CKLLKTFLSKWIC-amide. Circular dichroism studies indicate that the cyclic peptides tend to adopt distorted helical structures. The cyclic peptides also have a greater affinity for lipid vesicles, which could be the reason for the effective perturbation of the erythrocyte membrane.

Amino Acid Sequence↗
PubMed1999

Addition and omission analogs of the 13-residue antibacterial and hemolytic peptide PKLLKTFLSKWIG: structural preferences, model membrane binding and biological activities.

The consequences of selective addition or deletion of polar amino acids in a 13-residue antibacterial peptide PKLLKTFLSKWIG on structure, membrane binding and biological activities have been investigated. The variants generated are (a) S and T residues replaced by K, (b) S and T residues deleted individually and together, (c) introduction of two additional K and (d) deletion of L and L with T. In the aqueous environment all the peptides were unordered. In trifluoroethanol, the spectra of peptides belonging to groups (a-c) suggest distorted helical conformation. Peptides in group (d) appear to adopt beta-sheet conformation. The peptides bind to zwitterionic and negatively charged lipid vesicles, although to different extents. With the exception of peptides in group (d), all the other peptides exhibited comparable antibacterial activity against Escherichia coli and Staphylococcus aureus. However, the changes made in the peptides in groups (a-c) resulted in reduction of hemolytic activity compared to the parent peptide. Extent of binding to lipid vesicles composed of phosphatidylcholine and cholesterol appears to correlate with hemolytic activity. It appears that polar and charged residues play a major role in modulating the biological activities of the 13-residue peptide PKLLKTFLSKWIG. The 11-residue peptide-like PKLLKFLKWIG has selective antibacterial activity. Thus, by judicious engineering it should be possible to generate short peptides with selective antibacterial activity.

Animals↗
PubMed1998

Mechanism of antimicrobial action of indolicidin.

Indolicidin, a 13-residue antimicrobial peptide isolated from cytoplasmic granules of bovine neutrophils, exhibits activity against Gram-positive and Gram-negative bacteria as well as fungi. Although indolicidin is bactericidal and permeabilizes the bacterial membranes, it does not lyse the bacterial cells. We examined the effect of bactericidal concentrations of indolicidin on the morphology of Escherichia coli cells and found that it induces filamentation. Further investigations showed that indolicidin inhibits DNA synthesis in E. coli cells at concentrations at which RNA and protein synthesis are either partially affected or not affected at all. Since inhibition of DNA synthesis is also known to induce filamentation in E. coli, it appears to contribute to the antimicrobial activity of indolicidin.

Anti-Bacterial Agents↗
PubMed1998

Structural features of helical aggregates of antibacterial peptides via simulated annealing and molecular modeling.

A 27-residue stretch of amino acids encompassing two putative 13-residue amphiphilic helical segments is an important determinant of activity in the 47-residue antibacterial peptide bovine seminalplasmin. Synthetic peptides corresponding to the 27-residue stretch (P27) SLSRYAKLANRLANPKLLETFLSKWIG as well as the 13-residue segments PKLLETFLSKWIG (SPF),exhibit antimicrobial activity. An analog of SPF where E has been replaced by K(SPFK) showed improved antimicrobial properties as compared to SPF. The peptides have the ability to bind and permeabilize membranes. We have modeled helical bundles of P27 and the two 13-residue peptides SPF and SPFK using simulated annealing via molecular dynamics. Octameric but not hexameric aggregates of P27 can form channels which would allow the passage of ions. In the case of 13-residue peptides, aggregates formed by 6 monomers can conceivably form ion conducting channels. Since the ability to form channels which would allow the passage of ions across the membranes is an important determinant of the biological activities of these peptides, knowledge of the pore forming structures should help in the design of analogs with improved activities.

Amino Acid Sequence↗
PubMed1997

Identification of the region that plays an important role in determining antibacterial activity of bovine seminalplasmin.

Seminalplasmin (SPLN) is a 47-residue protein isolated from bovine seminal plasma having potent antimicrobial activity against a broad spectrum of microorganisms. SPLN, also known as caltrin, acts as a calcium transport regulator in bovine sperms. Analysis of the sequence of SPLN reveals a 27-residue stretch with the sequence SLSRYAKLANRLANPKLLETFLSKWIG more hydrophobic than the rest of the protein. It is demonstrated that a synthetic peptide corresponding to this 27-residue segment has antimicrobial activity comparable to that of SPLN. It does not exhibit hemolytic activity at concentrations where antibacterial activity is observed. Since P27 can be conveniently obtained in large amounts by chemical synthesis, it could serve not only as a starting compound to obtain peptides with improved antibacterial activity but also to understand the role of SPLN in reproductive physiology.

Amino Acid Sequence↗
PubMed1996

Requirements for antibacterial and hemolytic activities in the bovine neutrophil derived 13-residue peptide indolicidin.

The antimicrobial and hemolytic activities of the 13-residue peptide indolicidin (ILPWKWPWWPWRR-NH2), present in bovine neutrophils, and its analogs have been determined with a view to gaining insight into the structural roles of tryptophan and proline. Peptides where proline was replaced by alanine and tryptophan by phenylalanine showed antibacterial activities comparable to that of indolicidin. The peptides do not exhibit a strong propensity to occur in either helical or beta-sheet conformation. The peptides also do not appear to exert their activity by permeabilizing the bacterial plasma membrane unlike other endogenous antibacterial peptides. The presence of tryptophan appears to be essential for hemolytic activity as the phenylalanine analog does not exhibit any hemolytic activity.

Animals↗
PubMed1995

Seminal plasmin.

The importance of seminal plasma in fertilization was appreciated as early as 1677 and would thus hardly seem a source for the search of antibacterial agents. The observation that seminal plasma had the ability to inhibit the growth of microorganisms in 1940 led to a systematic search for molecules possessing antimicrobial activity in addition to factors that might have a role in reproductive physiology. Extensive investigations led to the discovery in bovine seminal fluid of a 47-residue peptide, possessing potent antimicrobial activity as well as calcium transport modulatory properties in bovine sperm. We describe in this article the two, apparently unrelated, biological activities of this peptide.

Amino Acid Sequence↗
PubMed1995

Structural and charge requirements for antimicrobial and hemolytic activity in the peptide PKLLETFLSKWIG, corresponding to the hydrophobic region of the antimicrobial protein bovine seminalplasmin.

Several analogs of the 13-residue antimicrobial and hemolytic peptide PKLLETFLSKWIG (SPF), which is the most hydrophobic region of the 47-residue antimicrobial protein seminalplasmin [Sitaram, N. & Nagaraj, R. (1990) J. Biol. Chem. 265, 10438-10442] have been synthesized. The antimicrobial and hemolytic properties of the peptides were investigated with a view to gain a insight into the structural and charge requirements for these activities of SPF. Peptides in which E was replaced by K exhibited considerably improved antimicrobial activity with no concomitant increase in hemolytic activity. A peptide in which the aromatic amino acids were replaced by leucine exhibited antimicrobial activity like those of the peptides which had aromatic amino acids. Interchange in the positions of E and K and total replacement of K by E resulted in complete loss of activity. The peptides having antimicrobial activity like those of the peptides which had aromatic amino acids. Interchange in the positions of E and K and total replacement of K by E resulted in complete loss activity. The peptides having antimicrobial activities showed appreciable helical content in a hydrophobic environment, whereas inactive peptides did not. Thus, by suitable 'engineering' the biological activity of a short 13-residue peptide can be altered by yield peptides specifically having only antimicrobial activity with increased potency.

Amino Acid Sequence↗
PubMed1993

Identification of a second membrane-active 13-residue peptide segment in the antimicrobial protein, bovine seminalplasmin.

Seminalplasmin (SPLN) is a 47-residue protein from bovine seminalplasma having broad-spectrum antibacterial activity. The protein has no hemolytic activity. SPLN interacts with lipid vesicles and its antibacterial activity appears to stem from its ability to permeabilize the bacterial plasma membrane. Analysis of SPLN's primary structure, with respect to its relative hydrophobicity and hydrophilicity, revealed a segment, PKLLETFLSKWIG, more hydrophobic than the rest of the protein. A synthetic peptide corresponding to this region had not only antibacterial activity but also hemolytic properties. Analysis of the SPLN sequence based on hydrophobic moment plots has revealed a second segment, SLSRYAKLANRLA, which could be membrane active. A synthetic peptide corresponding to this region shows only antibacterial activity with no hemolytic activity.

Amino Acid Sequence↗