Primary assimilation of ammonia (NH4+) into organic molecules What is the major biochemical route by which free ammonia is incorporated into organic compounds in microbes?

Introduction / Context:
Nitrogen assimilation is essential for biosynthesis of amino acids, nucleotides, and cofactors. Cells must convert inorganic or reduced nitrogen into organic forms. Two central enzyme systems mediate the bulk of ammonia assimilation under physiological conditions.

Given Data / Assumptions:

• Available nitrogen: free ammonia (NH4+).
• Organisms possess glutamate dehydrogenase (GDH) and/or the glutamine synthetase–glutamate synthase (GS–GOGAT) pathways.
• Goal: incorporate nitrogen into carbon skeletons.

Concept / Approach:
Ammonia is primarily channeled into glutamate and glutamine, which then donate amino groups via transamination. Two routes dominate: (1) reductive amination of alpha-ketoglutarate by GDH to form glutamate; and (2) GS–GOGAT, where glutamine synthetase forms glutamine from glutamate + NH4+, and glutamate synthase converts glutamine + alpha-ketoglutarate into two glutamate molecules. Pyruvate is not the central acceptor for ammonia, and nitrogen fixation occurs only in specialized organisms/conditions.

Step-by-Step Solution:
Identify primary amino acceptors: alpha-ketoglutarate → glutamate; glutamate → glutamine.Recognize GDH and GS–GOGAT as main enzymatic systems.Conclude that reductive amination of alpha-ketoglutarate (and the GS–GOGAT cycle) is the major route.

Verification / Alternative check:
Biochemistry texts emphasize glutamate/glutamine as the universal nitrogen donors in biosynthesis; labeling studies trace ammonia into these pools first.

Why Other Options Are Wrong:

• Atmospheric nitrogen fixation is restricted to diazotrophs with nitrogenase, not a general route in most cells.
• Oxidation of pyruvate is not a standard pathway for ammonia assimilation.
• All of these: incorrect because only the reductive amination/GS–GOGAT route is generally true.
• Non-enzymatic reactions with DNA are damaging, not assimilatory.

Common Pitfalls:
Confusing nitrogen fixation (N2 → NH4+) with assimilation (NH4+ → amino acids); they are distinct processes performed by different enzyme sets.

Final Answer:
Reductive amination of alpha-ketoglutarate (and related ketoacids) by specific enzymes