In `beta`- pleated protein, polypeptide chains lie parallely and held together by

Options

A. S-S bond
B. CONH bond
C. H-bond
D. none of these

Answer

To solve the question regarding the bonds that hold the polypeptide chains together in a beta-pleated sheet of a protein, we can follow these steps:

### Step-by-Step Solution:

1. **Understand the Structure**: Recognize that a beta-pleated sheet is a type of secondary structure in proteins. In this structure, polypeptide chains (or strands) are arranged parallel to each other.

2. **Identify the Bonds**: In beta-pleated sheets, the polypeptide chains are stabilized by specific types of bonds. The most significant of these are hydrogen bonds.

3. **Explain the Role of Hydrogen Bonds**: Hydrogen bonds form between the carbonyl oxygen (C=O) of one amino acid and the amide hydrogen (N-H) of another amino acid in the adjacent polypeptide chain. This interaction helps to maintain the structure of the beta-pleated sheet.

4. **Eliminate Other Options**:
- **SS bonds (disulfide bonds)**: These are covalent bonds that occur between cysteine residues but are not the primary stabilizing force in beta-pleated sheets.
- **CO-NH bonds**: While these are part of the peptide backbone, they do not specifically refer to the interactions that stabilize the beta-pleated sheet.
- **None of these**: This option can be eliminated since hydrogen bonds are indeed present.

5. **Conclusion**: Therefore, the correct answer to the question is that polypeptide chains in beta-pleated sheets are held together primarily by hydrogen bonds.

### Final Answer:
The polypeptide chains in beta-pleated proteins are held together by hydrogen bonds.