At the molecular level, proteins are constantly being modified, either through endogenous processes, or from the influence of outside stressors. These modifications necessarily cause a structural change in the protein, which can act as a signal or recognition site and are repaired by specific enzymes. However, as we age, these modifications accumulate, often causing diseases typically attributed to advancing age. Alzheimer's Disease and several prion-related neurodegenerative diseases such as Creutzfeld-Jakob disease and fatal familial insomnia have been linked to conformational changes in modified proteins, specifically a conversion from an """"""""inactive"""""""" alpha-helical conformation to the abnormal, disease- related beta-sheet conformation. Thus it has been hypothesized that modifications that stabilize alpha-helical conformations should prevent these diseases, whereas damage that destabilizes helical structures should facilitate the onset of these diseases. A common form of damage modification to proteins is the deamidation of asparaginyl residues. This type of modification, if not efficiently repaired, can lead to the introduction of a D-aspartyl residue or an isoaspartyl peptide linkage. If the asparginyl residue is located in an alpha-helix, both of these modifications can decrease the stability of the helix, thus favoring the formation of the beta-sheet conformation. High-resolution nuclear magnetic resonance (NMR) spectroscopy can be used to determine the detailed structures of asparagine-, D-aspartyl- and isoaspartyl-containing peptides in order to understand the effects of age-related protein damage on protein structure. The effects of increased negative charge density (from incorporation of D-Asp) and the introduction of an additional methylene group to the peptide backbone (from isoAsp) will strongly alter alpha-helical conformations, causing a destabilization of peptide helical structures. The relative helical stability of the normal and the modified peptides can be compared by monitoring individual alpha-proton chemical shifts of model peptides in a structure-inducing solvent like trifiuoroethanol as a function of temperature by NMR. These results will yield insights into the important role age-related protein structural changes and helical stability may play in dictating the onset of specific neurodegenerative diseases.
| Lam, Sik Lok; Hsu, Victor L (2003) NMR identification of left-handed polyproline type II helices. Biopolymers 69:270-81 |
