The secondary structure of proteins can cause the polypeptides to form into two distinct shapes. One is called an alpha helix and the other is called a beta pleated sheet. An alpha helix forms when the polypeptides begin to twist. A beta pleated sheet happens when the polypeptides fold several times. The hydrogen bonds between the amino acids determine if the secondary structure of a protein will be an alpha helix or a beta pleated sheet.
Source: Concepts in Biology. 12th Edition. copywright 2007 McGraw-Hill.