Geometry of Globular Proteins

Because only two types of secondary structures (a and p) exist, proteins can be divided into three main structural classes. These are mainly a pro-teins, mainly p proteins,and mixed a-p proteins. A fourth class includes proteins with little or no secondary structures at all that are stabilized by metal ions and/or disulphide bridges. A significant effort has been made by 

The a-p protein class is the largest of the three classes. It is subdivided into proteins having an alternating arrangement of a helices and p strands 

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