.................. And ................. Are two common secondary structures of protein.

Explanation

To answer the question regarding the two common secondary structures of proteins, we can follow these steps:

### Step-by-Step Solution:

1. **Understanding Protein Structure**: 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 interactions between the backbone atoms.

2. **Identifying Common Secondary Structures**: The two most common types of secondary structures in proteins are:
- **Alpha Helix**: This structure resembles a coiled spring and is stabilized by hydrogen bonds between the carbonyl oxygen of one amino acid and the hydrogen of another amino acid four residues down the chain.
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