A-helices are sometimes referred to as "local structures", while b-sheets are usually not local structures. explain. do all proteins contain both a-helices and b-sheets

It is maybe because the alpha helixsecundary structure) characterized by intramolecular hydrogen bonds between amino acid residues and on the other hand, the beta sheet is composed of extended chains with hydrogen bonds between adjacent chains (between residues from two distinct strands).

Many proteins contain both alpha helices and beta sheets, but some contain just one type of secondary structure.