Two types of secondary structures of proteins are

Options

A. `alpha`- helix and `beta` -helix
B. `alpha ` - helix and `beta`-helix
C. `alpha`-helix and `beta`- pleated sheet
D. Helix and rod.

Answer

**Step-by-Step Solution:**

1. **Understanding Protein Structure**: Proteins have four levels of structure: primary, secondary, tertiary, and quaternary. The primary structure is the linear sequence of amino acids.

2. **Defining Secondary Structure**: The secondary structure refers to the local folded structures that form within a protein due to interactions between the backbone of the amino acids. These structures are stabilized by hydrogen bonds.

3. **Identifying Types of Secondary Structures**: The two main types of secondary structures in proteins are:
- **Alpha Helix**: This structure is coiled or spiral in shape. It is formed when the backbone of the protein folds into a helical shape, stabilized by hydrogen bonds between the NH group of one amino acid and the CO group of another amino acid that is four residues earlier in the sequence (N and N+4).
- **Beta Pleated Sheet**: This structure is sheet-like and can be arranged in parallel or anti-parallel configurations. In this structure, hydrogen bonds form between the NH and CO groups of adjacent strands of the polypeptide chain.

4. **Stabilization of Structures**: Both alpha helices and beta sheets are stabilized by hydrogen bonds. In alpha helices, the hydrogen bonds form between the backbone atoms, while in beta sheets, hydrogen bonds form between different strands of the polypeptide chain.

5. **Conclusion**: The two types of secondary structures of proteins are the alpha helix and the beta pleated sheet.

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