Folded conformations of antigenic peptides from riboflavin carrier protein in aqueous hexafluoroacetone

Carrier protein influences immunodominance of a known epitope: implication in peptide vaccine design

We investigated how the processing of a given antigen by antigen presenting cells (APC) is dictated by the conformation of the antigen and how this governs the immunodominance hierarchy. To address the question, a known immunodominant sequence of bacteriophage lambda repressor N-terminal sequence 12-26 [λR(12-26)] was engineered at the N and C termini of a heterologous leishmanial protein, Kinetoplastid membrane protein-11 (KMP-11); the resulting proteins were defined as N-KMP-11 and C-KMP-11 respectively. The presence of λR(12-26) in N-KMP-11 and C-KMP-11 was established by western blot analysis with antibody to λR(12-26) peptide. N-KMP-11 but not C-KMP-11 could stimulate the anti λR(12-26) T-cell clonal population very efficiently in the presence of APCs. Priming of BALB/c mice with N-KMP-11 or C-KMP-11 generated similar levels of anti-KMP-11 IgG, but anti-λR(12-26) specific IgG was observed only upon priming with N-KMP-11. Interestingly, uptake of both N-KMP-11 and C-KMP-11 by APCs was similar but catabolism of N-KMP-11 but not C-KMP-11 was biphasic and fast at the initial time point. Kratky plots of small angle X-ray scattering showed that while N-KMP-11 adopts flexible Gaussian type of topology, C-KMP-11 prefers Globular nature. To show that KMP-11 is not unique as a carrier protein, an epitope (SPITBTNLBTMBK) of Plasmodium yoelii (PY) apical membrane protein 1[AMA-1 (136-148)], is placed at the C and N terminals of a dominant T-cell epitope of ovalbumin protein OVA(323-339) and the resulting peptides are defined as PY-OVA and OVA-PY respectively. Interestingly, only OVA-PY could stimulate anti-OVA T-cells and produce IgG response upon priming of BALB/c mice with it. Thus for rational design of peptide vaccine it is important to place the dominant epitope appropriately in the context of the carrier protein.

Separation and identification of the regioisomers of verdoheme by reversed-phase ion-pair high-performance liquid chromatography, and characterization of their complexes with heme oxygenase

We report an HPLC method for separating the four regioisomers of verdoheme formed in the coupled oxidation of hemin with oxygen and ascorbate in aqueous pyridine. The reversed-phase ion-pair system uses hexafluoroacetone and pyridine as ion-pair agents. The regiochemistry of the separated isomers was established both by HPLC of the corresponding biliverdin IX derivatives and by 1H NMR of each isomer. Optical spectra of the pyridine verdohemochrome isomers were similar to each other, but showed differences in the absorption maxima in the red region, which appear at 680, 663, 648 and 660 nm for the alpha, beta, gamma, and delta-isomers, respectively. Each of the four isomers was incorporated anaerobically into heme oxygenase-1, yielding the corresponding verdoheme-enzyme complex. The ferrous forms had absorption maxima at 690, 667, 655, and 663 nm, and their CO-bound forms had maxima at 638, 624, 616, and 626 nm for alpha, beta, gamma, and delta-isomer, respectively. Addition of ferricyanide to the alpha-verdoheme-heme oxygenase complex brought about a ferric low-spin heme-like signal, which is identical with the ferric alpha-verdoheme complexed with the heme oxygenase that was observed in the heme oxygenase reaction.

Helix-enhancing propensity of fluoro and alkyl alcohols: influence of pH, temperature and cosolvent concentration on the helical conformation of peptides

We have analyzed the effects of trifluoroethanol (TFE) and three other alcohols(1-propanol, 2-propanol and hexafluoro-2-propanol) on S-peptide (residues 1-20) of ribonuclease A, an analog of S-peptide (QHM-->AAA, Sa-peptide) and TC-peptide (residues 295-316) of thermolysin to assess the helix-enhancing propensity of fluoro and alkyl alcohols under different environmental conditions of cosolvent concentration, pH and temperature by circular dichroism (CD). The dependence of cosolvent concentration on helix-induction showed a plateauing effect in all cases. 1-Propanol and 2-propanol were as effective as TFE in all the three peptides. Hexafluoro-2-propanol (HFIP) was a better helix enhancer in all cases however, the relative effectiveness varied with the peptide sequence. The alcohol transitions were analyzed assuming a two-state transition. The free energy decreased linearly in the cosolvent concentration range of 0-5 m for all the three peptides. The m-value (constant of proportionality) varied between peptides but was similar for any given peptide for TFE, 1-propanol or 2-propanol. The m-values of HFIP for all three peptides was much higher compared to other cosolvents. The isothermal cosolvent helix-induction curves for the three peptides exhibited similar features of shape and character for 1-propanol, 2-propanol and TFE. The additivity of cosolvent-induced helix formation was observed for different blends of alkyl and/or fluoro cosolvents. The pH-dependence of helix formation was observed in both TFE and 1-propanol solutions for S-peptide and TC-peptide, respectively, while in Sa-peptide, which was designed to perturb the pH-effect, helix formation was unaffected. The overall results provide some insight into the mechanism of cosolvent-mediated helix-enhancement in protein segments and are likely to facilitate optimization of conditions for cosolvent usage in chemistry and biology.