Question

Which among the following represents the process of reductive deamination?

• A. ketoglutaric acid + $NH_4^+$ + $NADPH$ $\to$ Glutamic acid + $H_2O$ + $NADP^+$
• B. Glutamic acid + Oxaloacetic acid $\to$ $\alpha$-ketoglutaric acid + Aspartic acid
• C. Glutamic acid + $NH_4^+$ + $ATP$ $\to$ Glutamine + $ADP$ + $Pi$
• D. Aspartic acid + $NH_4^+$ $\to$ Asparagine + $ADP$

Hint:

{Memory Trick:}

• {Reductive} → Involves a reducing agent (\(NADPH\))
• {Amination} → Addition of an amino group

Thus, reductive amination means {adding an amino group to a keto acid using reducing power}. \(\alpha\)-ketoglutaric acid acts as a major entry point through which inorganic nitrogen becomes incorporated into amino acids.

Answer 1

The Correct Option is A

Concept:
In plants, nitrogen assimilation involves the incorporation of inorganic nitrogen (mainly ammonia) into organic molecules such as amino acids. One of the key processes involved in this pathway is reductive amination. Reductive amination is the process in which ammonia (\(NH_4^+\)) combines with a keto acid to form an amino acid in the presence of a reducing agent and a specific enzyme. This reaction plays an essential role in incorporating nitrogen into organic compounds within plant cells.
Step 1: General reaction involved in reductive amination.
The typical reaction can be represented as: \[ \alpha\text{-keto acid} + NH_4^+ + NADPH \xrightarrow{\text{enzyme}} \text{Amino acid} + H_2O + NADP^+ \] Here:

• \(\alpha\)-keto acid acts as the carbon skeleton.
• \(NH_4^+\) provides the amino group.
• \(NADPH\) acts as the reducing agent.
• The reaction is catalyzed by the enzyme Glutamate Dehydrogenase.

Step 2: Example of reductive amination in plants.
A classical example of this reaction occurs when \(\alpha\)-ketoglutaric acid reacts with ammonium ions in the presence of NADPH: \[ \alpha\text{-ketoglutaric acid} + NH_4^+ + NADPH \rightarrow \text{Glutamic acid} + H_2O + NADP^+ \] In this reaction:

• \(\alpha\)-ketoglutaric acid serves as the keto acid.
• The product formed is glutamic acid, an important amino acid involved in nitrogen metabolism.

Step 3: Distinguishing from other reactions.

• Transamination: Transfer of an amino group from one amino acid to a keto acid.
• Amide formation: Addition of another amino group to amino acids such as glutamate or aspartate to form glutamine or asparagine.

Among the given reactions, the one involving \(\alpha\)-ketoglutaric acid, \(NH_4^+\), and \(NADPH\) represents reductive amination. Final Answer:
\[ \alpha\text{-ketoglutaric acid} + NH_4^+ + NADPH \rightarrow \text{Glutamic acid} + H_2O + NADP^+ \]