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What is a key difference between alpha helices and beta sheets? An alpha helix is composed of two or more polypeptide strands, but a beta sheet is composed of a single polypeptide strand. In an alpha helix hydrogen-bonded amino acids are 4 amino acids apart in the primary sequence, but in a beta sheet hydrogen-bonded amino acids may be many amino acids apart in the primary sequence. In an alpha helix hydrogen bonding occurs between amino acid carbonyl and amino groups, but in a beta sheet covalent bonding between amino acid carbonyl and amino groups occurs. Turns and loops can link two alpha helices, but not two beta sheets