In this article we will discuss about the process of metabolism in amino acids.
We know that ammonia can be formed and then be found in the form of amino group of glutamic acid. We have studied the reactions — transamination and deamination — by which amino groups can be added to or removed from amino acids. The forms in which ammonia resulting from the amino groups of amino acids, can be eliminated by living organisms.
As for the carbon skeleton of the amino acid, it can be used for regenerating the corresponding amino acid (e.g, by transamination). It can also be used, either as such, or after some modifications for synthesizing other amino acids or for participating in reactions leading to the synthesis of compounds important for the organism (nucleotides, pigments, hormones, porphyrins, etc). Lastly, it can be degraded and thus join the carbohydrate or lipid metabolisms whence the notion of glucogenic and ketogenic amino acids.
The first experiments on this point were carried out in rather abnormal physiological conditions as they consisted in administering the amino acids to be studied, to animals made diabetic, either by removal of the pancreas, or by the action of phlorizin (which prevents the reabsorption of the glucose in the tubules by lowering the threshold of renal elimination); it was thus observed that in these conditions some amino acids cause an increase of glucosuria (glucogenic amino acids), and other amino acids, an increase of ketonuria (ketogenic amino acids).
The glucogenic character of some amino acids was also tested by studying the accumulation of hepatic glycogen, after administration of these amino acids to fasting rats.
These experiments were confirmed, in normal physiological conditions, by the use of amino acids containing a radioactive isotope of carbon (14C). There are glucogenic amino acids whose carbon atoms are found in glucose and glycogen upon neoglucogenesis.
The reactions explain the fact that among the glucogenic amino acids, one finds for example, glutamic acid (as it gives α-keto glutaric acid), aspartic acid (which gives oxalo-acetic acid), alanine, serine and cysteine (which all three give pyruvic acid); other amino acids are also glucogenic, particularly glycine and proline and we will understand the reason after studying the metabolism of these amino acids.
Among the ketogenetic amino acids whose catabolism joins that of fatty acids we may cite phenylalanine and tyrosine, the metabolism of which leads to the formation of acetoacetic acid as we shall see in the following. In fact these two aromatic amino acids can also be considered as glucogenic amino acids since their catabolism also yields fumaric acid which is an intermediate of Krebs cycle and can thus lead to glucose by neoglucogenesis.
Isoleucine is also a glucogenic and ketogenic amino acid since its catabolism (see fig. 5-15) leads to, on the one hand, succinyl-coenzyme A which can permit the formation of glucose by neoglucogenesis and on the other hand, acetyl coenzyme A which can be used either for the ketogenesis or the biosynthesis of lipids.
But the metabolism of the carbon skeleton, because of the wide variety of the structures of amino acids, is extremely different from one amino acid to another. And just as we had to exclude a description of the biosynthesis pathways of all amino acids, we will not be able to study here the metabolism of all amino acids.
We will examine the metabolism of glycine and serine (two closely related amino acids), sulphur-containing amino acids, glutamic and aspartic acids and the derived amino acids, phenylalanine and tyrosine (also closely related).