Which metabolic cycle is directly linked to the action of glutamine synthetase?

The metabolic cycle that is directly linked to the action of glutamine synthetase is indeed the urea cycle. Glutamine synthetase catalyzes the conversion of glutamate and ammonia into glutamine, a process that plays a crucial role in nitrogen metabolism. This is particularly important in the context of the urea cycle, which is responsible for the detoxification of ammonia, a byproduct of amino acid catabolism.

In the urea cycle, ammonia is converted into urea for excretion. Glutamine serves as a transport form of ammonia in the body and can be broken down back into glutamate and ammonia by the enzyme glutaminase, thus linking the action of glutamine synthetase to the urea cycle. The production of glutamine provides a reserve of ammonia that is processed through the urea cycle to prevent ammonia toxicity in the body.

While the citric acid cycle, fatty acid cycle, and gluconeogenesis are important metabolic pathways, they do not directly involve glutamine synthetase in the same manner as the urea cycle. The urea cycle specifically requires the regulation of nitrogen and ammonia, highlighting the essential role of glutamine synthetase in this process.