DESIGN, SYNTHESIS AND SOLUTION STRUCTURE OF A HELIX-LOOP-HELIX DIMER - A TEMPLATE FOR THE RATIONAL DESIGN OF CATALYTICALLY ACTIVE POLYPEPTIDES

A polypeptide with 42 amino acids, SA-42, has been designed to fold in to a 'hairpin' helix-loop-helix motif. Its intended use is to serve as a template for the rational design of catalytically active polypeptid es. SA-42 is made up of two amphiphilic helices, that are designed to interact through the hydrophobic effect and are connected by a short l oop. Evidence is presented that it does adopt the intended motif and t hat it dimerises in an antiparallel mode at concentrations between 2 m u mol dm(-3) and 5.6 mmol dm(-3). The structural evidence was obtained from NMR and CD spectroscopy and the state of aggregation was determi ned with equilibrium sedimentation ultracentrifugation and CD spectros copy. Special attention was paid to the design of SA-42 with respect t o the expected NMR spectrum. The mean residue ellipticity at 222 nm of SA-42, -25 000 deg cm(2) dmol(-1), compares well with that of other p ublished helix-loop-helices, Two polypeptides that were developed from SA-42 by the replacement of three and five residues, respectively, we re recently shown to have catalytical activity.