Analysis of protein-carbohydrate interaction at the lower size limit of the protein part (15-mer peptide) by NMR spectroscopy,electrospray ionization mass spectrometry,and molecular modeling

Structural analysis of minimally sized lectins will offer insights into fundamentals of intermolecular recognition and potential for biomedical applications. We thus moved significantly beyond the natural limit of lectin size to determine the structure of synthetic mini-lectins in solution, their carbohydrate selectivity and the impact of ligand binding on their conformational behavior. Using three disaccharide (Thomsen-Friedenreich antigen; Gal beta 1,3GalNAc alpha 1,R)-binding pentadecapeptides without internal disulfide bridges as role models, we successfully tested a combined strategy with different techniques of NMR spectroscopy, electrospray ionization mass spectrometry, and molecular modeling. In solution, the peptides invariably displayed flexibility with rather limited restrictions, shown by NMR experiments including nearly complete resonance assignments and molecular dynamics simulations. The occurrence of aromatic/nonpolar amino acids in the sequence did not lead to formation of a hydrophobic core known from microbial chitinase modules. Selectivity of disaccharide binding was independently observed by mass spectrometry and NMR analysis. Specific ligand interaction yielded characteristic NMR signal alterations but failed to reduce conformational flexibility significantly. We have thereby proven effectiveness of our approach to analyze even low-affinity interactions (not restricted to carbohydrates as ligands). It will be useful to evaluate the impact of rational manipulation of lead peptide sequences.