Helical peptide models for protein glycation: proximity effects in catalysis of the Amadori rearrangement

Introduction: Non-enzymatic glycation of proteins has been implicated in va rious diabetic complications and age-related disorders. Proteins undergo gl ycation at the N-terminus or at the E-amino group of lysine residues. The o bservation that only a fraction of all lysine residues undergo glycation in dicates the role of the immediate chemical environment in the glycation rea ction. Here we have constructed helical peptide models, which juxtapose lys ine with potentially catalytic residues in order to probe their roles in th e individual steps of the glycation reaction. Results: The peptides investigated in this study are constrained to adopt h elical conformations allowing residues in the i and i+4 positions to come i nto spatial proximity, while residues i and i+2 are far apart. The placing of aspartic acid and histidine residues at interacting positions with lysin e modulates the steps involved in early peptide glycation (reversible Schif f base formation and its subsequent irreversible conversion to a ketoamine product, the Amadori rearrangement). Proximal positioning of aspartic acid or histidine with respect to the reactive lysine residue retards initial Sc hiff base formation. On the contrary, aspartic acid promotes catalysis of t he Amadori rearrangement. Presence of the strongly basic residue arginine p roximate to lysine favorably affects the pK(a) of both the lysine c-amino g roup and the singly glycated lysine, aiding in the formation of doubly glyc ated species. The Amadori product also formed carboxymethyl lysine, an adva nced glycation endproduct (AGE), in a time-dependent manner. Conclusions: Stereochemically defined peptide scaffolds are convenient tool s for studying near neighbor effects on the reactivity of functional amino acid sidechains. The present study utilizes stereochemically defined peptid e helices to effectively demonstrate that aspartic, acid is an efficient ca talytic residue in the Amadori arrangement. The results emphasize the struc tural determinants of Schiff base and Amadori product formation in the fina l accumulation of glycated peptides. (C) 2001 Elsevier Science Ltd. All rig hts reserved.