What are the common types of secondary structure of proteins?
The common types of secondary structure of proteins include the helix form and other folded forms. In the helix structure, the protein thread is folded similar to a revolving staircase. In proteins, only right-handed helices are observed.
Other regions of the protein thread are folded into other forms as part of the secondary structure. Not all portions of the protein thread exist as an extended rigid rod; only some portions are arranged in the helix form while others take different folded configurations.
Explanation
The textbook context describes secondary structure as the folding pattern of the protein thread. It specifically mentions the helix form (similar to a revolving staircase) with only right-handed helices observed in proteins. The context also refers to 'other forms' that other regions of the protein thread are folded into. Note that the provided context does not explicitly name alpha-helix or beta-sheet, so those specific terms should not be introduced in the answer.