Probing Peptide Properties
Mass spec is also proving to be a highly sensitive tool for delineating the effects of charge distribution on peptide structure, which, in turn, affects the behavior of signaling peptides in regulating gene expression. Jianhua Ren, Ph.D., associate professor at the University of the Pacific, and her colleagues presented research on the acidities of short peptides and how this property affects their propensity to undergo conformational changes.
Dr. Ren’s group studies thioredoxins, ubiquitous proteins found throughout living organisms. These enzymes perform as antioxidants by cysteine thiol-disulfide exchange, catalyzing protein reduction. Cysteine residues are found in the active site of the thioredoxin molecule in a CXXC configuration, and they are essential to its antioxidant performance.
Dr. Ren and her colleagues are exploring the helix macrodipolar effects on the gas-phase acidity of peptides. The protein alpha helix has an intrinsic macrodipole moment running from the N-terminus to the C-terminus. In a solution with relatively low pH, the N-terminal cysteine will be ionized and the C-terminal cysteine will not. The ionized cysteine facilitates the thiol-disulfide exchange reaction.
By generating a family of related peptides, Dr. Ren and her team were able to investigate the effect of various amino acid substitutions on the gas-phase acidity of the peptides. “We found that N-terminal cystine peptides are more acidic than the corresponding C-terminal ones, and that longer peptides are more acidic than the corresponding shorter ones,” Dr. Ren explained. “Finally, our computational studies showed that the ionic N-terminal cystine peptides prefer helical conformations, while the ionic C-terminal cysteine analogues are mainly random coils.”
A number of pressing questions in rational drug development can be explored most expediently using mass spec and its many permutations. The discovery process requires refined molecular analysis that enables the modeling of compounds that precisely target the source of the disease process. This may prove to be the best route to achieving a strong pipeline of innovative drugs.