Amino acids source:

There are three major sources:

-- dietary proteins: usual in high animals;

-- storage proteins: as for example during the germination of storaging proteins seeds;

-- endogen turnover: that occurs in all cells - the proteins are degraded into amino acids and then they can be oxyzed or recycled.

The proteins catabolism begins by the hydrolysis of the peptide-bond (proteolysis), releasing small amino acids chain - peptides. The proteolysis of dietary proteins occurs at stomach and duodenum.

Excess dietary amino acids of those needed for the synthesis of prtoteins and other biomolecules are neither stored for future use in contrast with fatty acids and glucose, nor are they excreted. Rather, surplus amino acids are converted to common metabolic intermediates.

Amino acids degradation

The major site of amino acid degradation in mammals is the liver. The first reaction in the breakdown of an amino acid is lmost always removal of it's a-amino group with the object of excreting excess nitrogen and degrading the remaining carbon skeleton. The strategy of amino acid degradation is to form major metabolic intermediates that can be converted into glucose or be oxyzed by the citric acid cycle.

Fates of the carbon skeleton of amino acids

The carbon skeletons of the diverse set of 20 fundametal amino acids are funneles into only seven molecues: pyruvate, acetyl-CoA, acetoacetyl-CoA, 2-oxoglutarate, succinyl-CoA, fumarate and oxaloacetate.

Amino acids that are degraded to acetyl-CoA or acetoacetyl-CoA are termed ketogenic because they give rise to ketone bodies. Recall that mammals lack a pathway for the net synthesis of glucose from acetyl-CoA. In contrast, amino acids that are degraded to pyruvate or any citric acid cycle intermediate are termed glicogenic. Net synthesis of glucose from theses amino acids are feasible because they can be converted into phosphoenolpyruvate (gluconeogenesis from oxaloacetate) and then into glucose.

Removal and fates of the nitrogen

As said, this removal is the first step in the breakdown of amino acids. After that, the amino group can be recycled or transported to the liver for excretion. This removal can release ammonia, wich is very toxic to animals tissues. The fates of ammonia are discussed below.

There are two ways of removing the amino group:

Transamination: It's the transfer of an amino acid amino group to an a-keto acid to yield the a-keto acid of the original amino acid and a new amino acid, in reactions catalyzed by aminotransferases (see the scheme). The predominant aceptor is a-ketoglutarate, producing glutamate as the new amino acid. It doens't result in any net deamination, it just transfer the amino group from one amino acid to another that will carry it to the liver for excretion or to another a-keto acid (recycling).

In the muscloes, there's an alternative route of transporting the amni groups to the liver: the pyruvate-alanine cycle.

Deamination: In the mitochondria of the hepatic cells, the amino group is finally released as ammonia, that is rapidly converted in urea, and so does not cause any damage to the cell. It can also be used in diverse biosynthetic pathways. The most important deaminase is Glutamate dehydrogenase, that catalyzes the reaction:

glutamate + NAD⁺/NADP⁺ + H₂O a-keto acid + NH₄⁺ + NADH/NADPH + H⁺

The glutamate can actually receive one more amino group and be converted to glutamine, that is more important in transporting the amino group through the organism.

Ammonia fate

Urea cycle.