Amino acids and their derivatives
Amino acids and their derivatives are a group of compounds that contain both amino and carboxyl groups in their molecules. They occur in the free or bound state in living organisms. Free amino acids are found in all animal cells and body fluids, while bound amino acids are mainly the basic components of proteins and peptides.
Natural amino acids are colourless crystalline substances with a high melting point, mostly above 200°C. They are usually soluble in water and insoluble in water. They are usually easily soluble in water and insoluble in non-polar organic solvents. However, tyrosine and cystine are insoluble in water, and proline and carboxyproline are soluble in ethanol and ether. All amino acids are soluble in strong acid and base solutions.
The 20 common amino acids that make up proteins can be divided into four groups according to the polar nature of the R group of the side chain in alpha-amino acids:
① Amino acids with a non-polar R-group. There are 8 species, 5 with aliphatic side chains, namely alanine, leucine, isoleucine, valine and proline, 2 are aromatic amino acids, namely phenylalanine and tryptophan, and one is a sulphur-containing amino acid, namely methionine; the solubility of this group of amino acids in water is smaller than that of the polar R-group amino acids; proline differs from the common α-amino acids in that it is a side chain on the α-amino acid that replaces a formed by replacing a hydrogen atom on an α-amino acid, which is actually a subamino acid.
②Amino acids with a polar R-group but without an electrical charge. There are seven types of amino acids, namely serine, threonine and tyrosine, which have hydroxyl groups in the R group, cysteine, which has a sulfhydryl group in the R group, glutamine and asparagine, which have an amide group in the R group, and another amino acid, glycine; the glycine molecule does not have an R group, but has some polarity, so it is classified in this group; the side chains of amino acids in this group contain non-dissociated polar groups, which can form hydrogen bonds with water and are more soluble in water.
(iii) Amino acids with a positive charge in the R group. There are three types, namely lysine, arginine and histidine; they carry a positive charge at pH 7.0 and are also known as basic amino acids.
④Amino acids with a negative charge in the R group. There are 2 types, namely glutamic acid and aspartic acid; at pH 7.0 the molecules are negatively charged, also known as acidic amino acids. 20 amino acids are listed in the table with their structural formulae, abbreviated symbols and relevant constants. In addition to the 20 common amino acids mentioned above, there are also diiodotyrosine, thyroxine, hydroxyproline and hydroxylysine, which are found in certain proteins. In addition to the amino acids involved in protein composition, more than 200 other amino acids are found in various tissues and cells, most of which are derivatives of those alpha-amino acids that make up proteins. However, some are β-, γ- or δ-amino acids and some are D-amino acids such as β-alanine, γ-aminobutyric acid as well as phenylalanine in the antibiotic short bacillus peptide-S and D-alanine and D-glutamic acid in the cell wall of gram-positive bacteria. Some non-protein amino acids are metabolically important as precursors or intermediates, including β-alanine as a precursor to the vitamin pantothenic acid, citrulline and ornithine as precursors for the synthesis of arginine, and γ-aminobutyric acid as a chemical for nerve conduction. Plants contain a very large number of non-protein amino acids, which are plant secondary substances, such as theanine, nanine, cutanine, rhodanine and β-cyanoalanine.
In addition to the 20 amino acids that make up the proteins of the animal body and its products, nearly 200 have been found to exist in nature to date, most of which occur in plants and have complex molecular structures unrelated to protein metabolism, while fewer occur in animals, some of which are formed by chemical modification of amino acids already bound within specific proteins, such as proline and For example, the proline and lysine in collagen proteins are often partially re-hydroxylated to hydroxyproline and hydroxylysine; a small amount of the lysine and histidine in actin and myosin are often methylated to εN-methyl-lysine and 3N-methyl-histidine respectively; acetylation of the ∈ amino group of lysine and phosphorylation of the OH group of serine are also found in histone proteins; and thyroglobulin contains iodinated tyrosine and iodinated methionine. The heavy chain of τ globulin and the N terminus of some proteins have pyroglutamic acid formed from glutamine; cystine, formed from two molecules of cysteine, is also present in general proteins.