Which one of the following statements is not correct?

Options

A. Proteins are polyamides formed from amino acids.
B. Except glycine, all other amino acids show optical activity.
C. Natural proteins are made up of L- isomers of amino acids.
D. `-NH_2 and -COOH` groups are attached to different carbon atoms in `alpha`-amino acids.

Explanation

To determine which statement is not correct regarding amino acids and proteins, let's analyze each statement step by step.

### Step 1: Analyze the First Statement
**Statement:** Proteins are polyamides formed from amino acids.

**Explanation:**
Proteins are indeed polymers made up of amino acids linked by peptide bonds, which are a type of amide bond. Therefore, this statement is correct.

### Step 2: Analyze the Second Statement
**Statement:** Except glycine, all other amino acids show optical activity.

**Explanation:**
Glycine is the only amino acid that does not have a chiral center because its side chain is a hydrogen atom. All other amino acids have a chiral center and thus exhibit optical activity. Therefore, this statement is also correct.

### Step 3: Analyze the Third Statement
**Statement:** Natural proteins are made up of L-isomers of amino acids.

**Explanation:**
Natural proteins are indeed composed of L-isomers (levo) of amino acids. The L-isomers are the ones that are predominantly found in proteins. Therefore, this statement is correct.

### Step 4: Analyze the Fourth Statement
**Statement:** NH2 and COO- groups are attached to different carbon atoms in alpha amino acids.

**Explanation:**
In alpha amino acids, the amino group (NH2) and the carboxylic group (COOH) are both attached to the same carbon atom, known as the alpha carbon. Therefore, this statement is incorrect.

### Conclusion
The statement that is not correct is: **NH2 and COO- groups are attached to different carbon atoms in alpha amino acids.**