We have seen before that, during digestion in the gut, proteins are broken down to their constituent amino acids. Proteins contain twenty standard amino acids. Nitrogen balance and tracer studies indicate that protein oxidation and net protein Muscle Protein Amino Acid Metabolism Muscle Glycogen Human Muscle. Jump to Amino acid metabolism - Dietary proteins are first broken down to individual amino acids by various enzymes and hydrochloric acid present in the gastro-intestinal tract. These amino acids are further broken down to α-keto acids which can be recycled in the body for generation of energy, and production of glucose or fat or other amino acids.‎Protein catabolism · ‎Amino acid synthesis · ‎Proteolysis.


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Glutaminase gets the ball rolling by releasing ammonia from glutamine. Quite unusually, ammonia exercises positive rather than negative feedback on glutaminase, which causes a rapid accumulation of both glutamate and ammonia. High levels of glutamate then promote the incorporation of ammonia into urea in several ways.


Some glutamate is converted to N-acetylglutamate. This is a regulatory molecule that allosterically activates carbamoylphosphate synthetase. Glutamate is also the biosynthetic precursor of ornithine.

A high level of glutamate will also raise protein and amino acid metabolism level of ornithine, which will increase the flow through the urea cycle. As we have seen, glutamate itself feeds nitrogen into urea synthesis via glutamate dehydrogenase or via aspartate. Note that all these regulatory events amplify the flow through the urea cycle; they are the ones that run in the periportal zone of the liver lobule.

Amino acid metabolism

As stated above, the urea cycle is shut down in the pericentral zone through the capture of remaining ammonia by glutamine synthetase as well as by ornithine degradation. Another aspect is the deficiency of arginine.

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In healthy individuals, arginine can be diverted from the urea cycle toward protein synthesis as required, but this supply is lacking if the cycle is disrupted. This may induce protein catabolism, thereby exacerbating the symptoms.

The problem is addressed by the addition of arginine to the diet.

Protein and amino acid metabolism and requirements in older persons.

If the enzyme defect is not between citrulline and protein and amino acid metabolism, it is possible to supply citrulline instead; this has the advantage of picking up one nitrogen equivalent en route to arginine.

The rationale for treating these enzyme defects with a protein-restricted diet is fairly obvious. Another, more intriguing approach is known as alternate pathway therapy.

Here, the patients are given several innocuous organic acids that are substrates for conjugation with amino acids.


These conjugates then serve as an alternative vehicle for the renal elimination of surplus nitrogen. This ingenious form of treatment is further discussed in slide As we had seen, some amino acids only require a single protein and amino acid metabolism step; on the other hand, others have lengthy degradation pathways with intriguing catalytic mechanisms.

Metabolism of Amino Acids

We will here consider some selected examples; several others are discussed in a later chapter slides Asparaginase is mentioned here not for any notable chemistry, but rather because of some interesting medical context. Asparagine is a non-essential amino acid, which means that it can be synthesized by human protein and amino acid metabolism the enzyme responsible for this, asparagine synthetase, uses glutamine as its amide group donor.

Nevertheless, in some forms of leukemia, the leukemic cells lack the synthetic capacity for asparagine. This can be exploited for therapy—the leukemia patients are treated with intravenous application of asparaginase.

Only the first step is protein and amino acid metabolism catalyzed; the aminoacrylate produced is unstable and spontaneously hydrolyzes to pyruvate.

Protein and amino acid metabolism and requirements in older persons.

The second step protein and amino acid metabolism ammonia, which must be disposed of. It seems that in humans the reaction occurs only in the liver, where the ammonia can directly enter the urea cycle.

Like the transaminases, the enzyme uses pyridoxal phosphate, and the role of the coenzyme is often presented along the lines of the usual electron sink mechanism see slide However, based on the crystal structure of the enzyme, a different mechanism has been proposed, in which no electron sink appears and instead the phosphate group of PLP plays a prominent role [ 73 ].

Protein and amino acid metabolism am not enough of a chemist to judge how plausible this mechanism may be.