Adding backbone to protein folding: why proteins are polypeptides.

It is argued that the chemical nature of the polypeptide backbone is the central determinant of the three-dimensional structures of proteins. The requirement that buried polar groups form intramolecular hydrogen bonds limits the fold of the backbone to the well known units of secondary structure while the amino acid sequence chooses among the set of conformations available to the backbone. 'Sidechain-only' models, based for example on hydrophobicity patterns, fail to account for the properties of the backbone and thus will have difficulty capturing essential features of a folding pathway. This is evident from the incorrect predictions they make for the conformations of the limiting cases of all-hydrophobic or all-polar sequences.