Which of the following statement is not correct ?

Options

A. amino acid can exist as inner salt
B. each polypeptide has one C - terminal and other N - terminal
C. Enzymes are naturally occurring simple proteins
D. The union of two amino acids produces two peptide linkages

Answer

To determine which statement is not correct, we will analyze each statement provided in the question:

1. **Amino acids can exist as inner salts.**
- Amino acids contain both an amino group (-NH2) and a carboxyl group (-COOH). In solution, they can exist in a form known as a zwitterion, where the amino group is positively charged and the carboxyl group is negatively charged. This zwitterionic form can be considered an inner salt because it has both positive and negative charges within the same molecule, making it neutral overall. Therefore, this statement is **correct**.

2. **Each polypeptide has one C terminal and another N terminal.**
- Polypeptides are chains of amino acids linked by peptide bonds. Each polypeptide chain has a specific orientation, with one end having a free amino group (N-terminal) and the other end having a free carboxyl group (C-terminal). Thus, this statement is also **correct**.

3. **Enzymes are naturally occurring simple proteins.**
- Enzymes are biological catalysts that speed up chemical reactions in living organisms. They are typically proteins, and many enzymes are indeed simple proteins (composed solely of amino acids). Therefore, this statement is **correct**.

4. **The union of two amino acids produces two peptide linkages.**
- When two amino acids join together, they form a single peptide bond (or peptide linkage) between them. Therefore, the statement that the union of two amino acids produces two peptide linkages is **incorrect**. Instead, it produces **one** peptide linkage.

Based on the analysis, the statement that is **not correct** is: **The union of two amino acids produces two peptide linkages.**

### Summary of the Solution:
- **Correct Statements:**
1. Amino acids can exist as inner salts (True).
2. Each polypeptide has one C terminal and one N terminal (True).
3. Enzymes are naturally occurring simple proteins (True).
- **Incorrect Statement:**
4. The union of two amino acids produces two peptide linkages (False - it produces one peptide linkage).

### Answer:
The statement that is not correct is: **The union of two amino acids produces two peptide linkages.**

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