Role of amino acid hydrophobicity, aromaticity, and molecular volume on IAPP(20-29) amyloid self-assembly

Aromatic amino acids strongly promote cross-beta amyloid formation; whether the amyloidogenicity of aromatic residues is due to high hydrophobicity and beta-sheet propensity or formation of stabilizing pp interactions has been debated. To clarify the role of aromatic residues on amyloid formation, the islet amyloid polypeptide 2029 fragment [IAPP(2029)], which contains a single aromatic residue (Phe 23), was adopted as a model. The side chain of residue 23 does not self-associate in cross-beta fibrils of IAPP(2029) (Nielsen et al., Angew Chem Int Ed 2009;48:21182121), allowing investigation of the amyloidogenicity of aromatic amino acids in a context where direct pp interactions do not occur. We prepared variants of IAPP(2029) in which Tyr, Leu, Phe, pentafluorophenylalanine (F5-Phe), Trp, cyclohexylalanine (Cha), a-naphthylalanine (1-Nap), or beta-naphthylalanine (2-Nap) (in order of increasing peptide hydrophobicity) were incorporated at position 23 (SNNXGAILSS-NH2), and the kinetic and thermodynamic effects of these mutations on cross-beta self-assembly were assessed. The Tyr, Leu, and Trp 23 variants failed to readily self-assemble at concentrations up to 1.5 mM, while the Cha 23 mutant fibrillized with attenuated kinetics and similar thermodynamic stability relative to the wild-type Phe 23 peptide. Conversely, the F5-Phe, 1-Nap, and 2-Nap 23 variants self-assembled at enhanced rates, forming fibrils with greater thermodynamic stability than the wild-type peptide. These results indicate that the high amyloidogenicity of aromatic amino acids is a function of hydrophobicity, beta-sheet propensity, and planar geometry and not the ability to form stabilizing or directing pp bonds. Proteins 2012;. (c) 2011 Wiley Periodicals, Inc.