## **Core Concept**
The conversion of glutamate to glutamine is a critical reaction in amino acid metabolism, particularly in the context of nitrogen metabolism and detoxification of ammonia in the brain. This reaction involves the enzyme glutamine synthetase.

## **Why the Correct Answer is Right**
The correct answer, **Glutamine Synthetase**, is the enzyme responsible for catalyzing the condensation of glutamate and ammonia (or ammonium ion) to form glutamine. This reaction is crucial for removing ammonia, a toxic product of nitrogen metabolism, from the brain and other tissues. The reaction requires energy in the form of ATP.

## **Why Each Wrong Option is Incorrect**
- **Option A:** Not provided, but any incorrect enzyme would not catalyze the specific conversion of glutamate to glutamine.
- **Option B:** Similarly, without specifics, any incorrect enzyme listed would not be responsible for this conversion.
- **Option C:** This option is also incorrect for the same reason; it does not correspond to glutamine synthetase.

## **Clinical Pearl / High-Yield Fact**
A key point to remember is that **glutamine synthetase** is predominantly found in astrocytes of the brain, where it plays a vital role in the detoxification of ammonia by converting it into glutamine. This process is essential for maintaining the health and function of the central nervous system.

## **Correct Answer:** D. Glutamine Synthetase.