Which statement about proteins is correct ?

Options

A. Collagen is an example of fibrous protein
B. Myoglobin is an example of a globular protein and contains a prosthetic group
C. Proteins are naturally occurring polypeptides
D. Hydrogen bonding between C = O and H - N groups

Explanation

To determine which statement about proteins is correct, particularly focusing on the zwitterionic form of an amino acid, we can analyze the options provided. Here’s a step-by-step breakdown of the solution:

### Step-by-Step Solution:

1. **Understanding Amino Acids**:
- Amino acids are organic compounds that serve as the building blocks of proteins. Each amino acid contains a central carbon atom (C) bonded to an amino group (NH2), a carboxyl group (COOH), a hydrogen atom (H), and a variable side chain (R group).

**Hint**: Remember that the structure of an amino acid includes both an amino group and a carboxyl group.

2. **Formation of Zwitterion**:
- In a zwitterionic form, the amino acid has both a positive and a negative charge, resulting in a net charge of zero. The carboxyl group (COOH) donates a proton (H+) and becomes negatively charged (COO−), while the amino group (NH2) accepts a proton and becomes positively charged (NH3+).

**Hint**: Zwitterions are unique because they have both positive and negative charges but are overall neutral.

3. **Evaluating the Options**:
- **Option A**: "The zwitter ion acts as only a base."
- This is false. The zwitterion can act as both an acid and a base due to the presence of both acidic (COO−) and basic (NH3+) groups.

**Hint**: Consider the dual nature of zwitterions in terms of acid-base chemistry.

- **Option B**: "The zwitter ion carries an overall charge which can be positive or negative."
- This is also false. The zwitterion is neutral overall because the positive and negative charges cancel each other out.

**Hint**: Think about the net charge of the zwitterion.

- **Option C**: "The zwitter ion is neutral overall."
- This is correct. The zwitterion has equal positive and negative charges, resulting in a net charge of zero.

**Hint**: Focus on the balance of charges in the zwitterion.

- **Option D**: "The zwitter ion only acts as an acid."
- This is false for the same reason as Option A; it can act as both an acid and a base.

**Hint**: Remember that zwitterions can participate in both acid-base reactions.

4. **Conclusion**:
- The correct statement about the zwitterionic form of an amino acid is that it is neutral overall.

### Final Answer:
The correct statement about proteins, specifically regarding the zwitterionic form of an amino acid, is: **"The zwitter ion is neutral overall."**