A stepwise approach for reducing the size of a polypeptide hormone, atrial natriuretic peptide (ANP), from 28 residues to 15 while retaining high biopotency is described. Systematic structural and functional analysis identified a discontinuous functional epitope for receptor binding and activation, most of which was placed onto a smaller ring (Cys 6 to Cys 17 ) that was created by repositioning the ANP native disulfide bond (Cys 7 to Cys 23 ). High affinity was subsequently restored by optimizing the remaining noncritical residues by means of phage display. Residues that flanked the mini-ring structure were then deleted in stages, and affinity losses were rectified by additional phage-sorting experiments. Thus, structural and functional data on hormones, coupled with phage display methods, can be used to shrink the hormones to moieties more amenable to small-molecule design.
