We have analyzed the amino acid sequences situated around the putative proteolytic cleavage sites in twenty different biosynthetic precursors of peptide hormones by processing enzymes. The prediction of the probability for forming secondary structures around the basic amino acids, constituting the cleavage sites, was made using the modified method of Chou and Fasman. The results indicate that the processing sequences which are cleaved in vivo, are in all cases located inside regions with high β-turn formation probability or else immediately adjacent to these structures. The β-turn forming region at the cleavage locus, is flanked on both sides by amino acid sequences with a high probability for forming highly ordered structures, either β-sheet or α-helix. These conformational features are not found in precursors around dibasic pairs, i.e. putative cleavage loci, but which are not cleaved in vivo and appear to be conserved. We hypothesize that β-turns including the basic amino acids doublets, flanked by highly ordered secondary structures (either β-sheet or α-helix) may constitute a minimal requirement for the recognition by the endoproteases involved in the processing of these precursors.
