Choose the statement.

Options

A. All proteins have 20 amino acids
B. Both ends of a protein are similar
C. All proteins are soluble
D. Proteins are formed by peptide bonds

Answer

To solve the question, we need to evaluate each of the four statements provided and determine which one is correct regarding proteins.

### Step-by-Step Solution:

1. **Evaluate Statement 1**: "All proteins have 20 amino acids."
- **Analysis**: While there are 20 standard amino acids that can be used to build proteins, not all proteins contain all 20 amino acids. Each protein has a unique sequence and number of amino acids. Therefore, this statement is **incorrect**.

2. **Evaluate Statement 2**: "Both ends of the protein are similar."
- **Analysis**: Proteins have distinct ends; one end has an amino group (NH2) and the other end has a carboxyl group (COOH). The end with the amino group is known as the N-terminal, and the end with the carboxyl group is known as the C-terminal. Thus, this statement is also **incorrect**.

3. **Evaluate Statement 3**: "All proteins are soluble."
- **Analysis**: Proteins can vary in their solubility. Many proteins are colloidal and not soluble in water, especially larger macromolecules. Therefore, this statement is **incorrect** as well.

4. **Evaluate Statement 4**: "Proteins are formed by the peptide bond."
- **Analysis**: Proteins are indeed formed by peptide bonds, which are covalent bonds that link amino acids together. This bond forms through a dehydration synthesis reaction between the amino group of one amino acid and the carboxyl group of another, releasing a molecule of water. This statement is **correct**.

### Conclusion:
The correct statement is: **"Proteins are formed by the peptide bond."**