Amino acid
The general formula – H2N – CHR – COOH. The structure of individual amino acids differs by radicals. Based on them, three main groups of amino acids are distinguished: aliphatic, aromatic, and heterocyclic. Aliphatic amino acids are further divided into monoaminomonocarboxylic and monoaminodicarboxylic acids. The molecules of three amino acids – cysteine, cystine, and methionine contain a sulfur atom. Amino acids are colorless crystalline substances. Except for glycine, they all have an asymmetric carbon atom and are optically active. The composition of human proteins includes 20 individual amino acids. Some of them are essential, while others are replaceable as they can be synthesized.
Replaceable and irreplaceable amino acids
* – partially replaceable amino acids
During the catabolism of all amino acids, six substances are formed, which are involved in the common catabolism process. These substances are pyruvate, acetyl-CoA, alpha-ketoglutarate, succinyl-CoA, fumarate, and oxaloacetate. Amino acids, from which intermediate Krebs cycle products are formed during catabolism (alpha-ketoglutarate, succinyl-CoA, fumarate), are later converted to the final product oxaloacetate and can be used for glycogenesis, called glycogenic amino acids. Some amino acids during catabolism are converted to acetoacetate (Lys, Leu) or acetyl-CoA (Leu) and can be used in the synthesis of acetone substances. These are called ketogenic. Many amino acids are used in the synthesis of glucose and acetone substances because during catabolism two products are formed – the corresponding Krebs cycle metabolite and acetoacetate (Tyr, Phe, Trp) or acetyl-CoA (Ile). Such amino acids are called mixed or glucogenic ketogenic.
Almost all natural amino acids (an exception being methionine) react with alpha-ketoglutaric acid. In this transaminase-catalyzed reaction, glutamic acid and the corresponding alpha-keto acid are formed. The resulting glutamic acid, catalyzed by glutamate dehydrogenase, is deaminated by oxidation:
The overall equation for this process would be:
Classification of amino acids based on the resulting non-nitrogenous residue:
Amino acid symbols
Source | Glossary of Most Commonly Used Biomedical Terms and Concepts | Lithuanian University of Health Sciences | Academician Professor Antanas Praškevičius, Professor Laima Ivanovienė