Which among the following is not an example of secondary structure of protein?

Options

A. Keratin protein of hair
B. Myoglobin protein in muscles
C. Fibroin protein of silk
D. Collagen helix

Answer

To determine which among the given options is not an example of the secondary structure of protein, we need to understand the definitions and characteristics of protein structures.

### Step-by-Step Solution:

1. **Understand Protein Structures**: Proteins have four levels of structure: primary, secondary, tertiary, and quaternary. The secondary structure refers to the local folded structures that form within a protein due to hydrogen bonding between the backbone atoms in the polypeptide chain. The most common types of secondary structures are alpha helices and beta-pleated sheets.

2. **Analyze Each Option**:
- **Option 1: Keratin (Creatinine protein of hair)**: Keratin is a fibrous protein that primarily consists of alpha helices, making it a clear example of secondary structure.
- **Option 2: Myoglobin (Protein in muscles)**: Myoglobin is a globular protein that has a tertiary structure. It contains alpha helices but is primarily characterized by its overall three-dimensional shape, which is not classified as secondary structure.
- **Option 3: Fibrin (Protein of cells)**: Fibrin is involved in blood clotting and has a fibrous structure, which includes beta-pleated sheets, thus representing secondary structure.
- **Option 4: Collagen (Helix)**: Collagen is a structural protein that forms a triple helix, which is also a type of secondary structure.

3. **Identify the Correct Answer**: Based on the analysis, Myoglobin is the only protein listed that does not primarily exhibit secondary structure but rather tertiary structure.

### Conclusion:
The answer to the question "Which among the following is not an example of secondary structure of protein?" is **Myoglobin protein in muscles**.