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Amino acids and their derivatives
Amino acids and derivatives thereof are compounds that contain both amino groups and carboxyl groups in the molecule. Occurs in the body in a free or bound state. Free amino acids are found in all animal cells and body fluids, while bound amino acids are the basic components of proteins and peptides.
Natural amino acids are colorless crystalline substances with a high melting point, most of which is above 200°C. Usually soluble in water, 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 strong alkali solutions.
According to the polar nature of the R group in the α-amino acid side, the 20 common amino acids that make up the protein can be divided into 4 groups:
The 1R group is a non-polar amino acid. There are 8 species, 5 with aliphatic side chains, namely alanine, leucine, isoleucine, valine and valine, and 2 are aromatic amino acids, namely phenylalanine and color ammonia. Acid, a sulfur-containing amino acid, namely methionine; this group of amino acids solubility in water is smaller than the polar R-amino acid; proline is different from the general α-amino acid, it is a side chain on the α-amino acid What is formed by a hydrogen atom on an amino acid is actually an imino acid.
The 2R group has polar but non-charged amino acids. There are seven types, namely, serine, threonine and tyrosine with hydroxyl group in R group, cysteine ??with mercapto group in R group, glutamine and asparagine with R group as amide group, and glycine as another amino acid. The glycine molecule has no R group, but has a certain polarity, so it belongs to this group; this group of amino acid side chain contains a dissociating polar group, can form hydrogen bonds with water, and is more soluble in water.
3R is a positively charged amino acid. There are three kinds, namely lysine, arginine and histidine; carry a positive charge at pH 7.0, also called basic amino acid.
The 4R group is a negatively charged amino acid. There are two kinds, namely glutamic acid and aspartic acid; at pH 7.0, the molecules are negatively charged, also called acidic amino acids. The structural formulas, abbreviations and related constants of the 20 amino acids are shown in the table. In addition to the above 20 common amino acids, diiodotyrosine, thyroxine, hydroxyproline, hydroxylysine, etc. are also found in certain proteins. In addition to amino acids involved in protein composition, more than 200 other amino acids are found in various tissues and cells. These amino acids are mostly derivatives of those α-amino acids that make up proteins. However, some are β-, γ-, or δ-amino acids, and some are D-type amino acids, such as β-alanine, γ-aminobutyric acid, and phenylalanine in the antibiotic Gramicidin-S, D-alanine and D-glutamic acid in the cell wall of Gram-positive bacteria. Some non-protein amino acids are metabolically important precursors or intermediates, of which β-alanine is the precursor of vitamin pantothenate, citrulline and ornithine are precursors of synthetic arginine, γ-aminobutyric acid It is a nerve conduction chemical. Plants contain a large number of non-protein amino acids and belong to plant secondary biomass such as theanine, arginine, canavanine, valine, and β-cyanoalanine.
In addition to the 20 amino acids that make up the animal's body and its products' proteins, nearly 200 species have been discovered in nature, most of which occur in the plant kingdom, have complex molecular structures, have nothing to do with protein metabolism, and appear in the animal kingdom. Few, some of which are formed by chemical modification of amino acids that have been incorporated into a particular protein, such as collagen and lysine, which are often part of the rehydroxylation of hydroxyproline and hydroxylysine. Another example is that actin and myosin often have a small amount of lysine and histidine methylated to εN methyl lysine and 3 N methyl histidine; also found in histone The amino group of arginine is acetylated and the OH group of serine is phosphorylated; the thyroglobulin contains iodinated tyrosine and iodothyronine; the tau globulin heavy chain and the N-terminus of certain proteins There is pyroglutamic acid formed from glutamine; cystine formed from dicysteine ??is also present in general proteins.
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