Beta sheet of alpha helix transition in the binding subunit of cholera toxin

Conformational and biological properties of glucagon fragments containing residues 1-17 and 19-29

The 29 amino acid polypeptide hormone glucagon was cleaved into two large fragments by the enzyme clostripain. The conformational properties of these two fragments were monitored by circular dichroism at pH 2 and 12 in both the presence and absence of sodium dodecyl sulfate. Both glucagon (1-17) and glucagon (19-29) have reduced abilities to fold in aqueous solution. However, both fragments can take on structure of higher apparent helical content in acidic solution in the presence of sodium dodecyl sulfate but only the glucagon (19-29) retains this conformation at high pH. Neither of the two fragments react with dimyristoylphosphatidylcholine as the intact peptide does. Only the carboxyl terminal fragment was capable of reacting with an antibody specific for glucagon. The glucagon (1-17) has markedly reduced affinity for binding to the glucagon receptor as well as markedly reduced ability to stimulate adenylate cyclase activity which is not affected by the presence of glucagon (19-29). It is proposed that the intact sequence provides specific groups required for activity as well as the potential for forming a stable amphipathic helix, both of which are necessary for full biological activity at low peptide concentrations.

Helix formation upon acidification of protein-dodecyl sulfate complexes

The pH dependence of circular dichroism spectra has been studied for dodecyl sulfate complexes formed by 25 proteins and for a random copolypeptide of glutamic acid and alanine. The pH range covered is that in which titration of side-chain carboxyl groups is to be expected. Circular dichroism spectra signify an increase in helical content upon acidification, although in many cases the increase is quite small. For all but three of the proteins studied, the spectral changes are in reasonable agreement with those expected because helix propagation by glutamyl and aspartyl residues is enhanced when the state of the side-chain carboxyl changes from COO- to COOH. This simple explanation seriously underestimates conformational changes reported for gastrin, Kunitz trypsin inhibitor and tropomyosin. Changes in charge density appear to play an important role in these proteins.