Proteins (building block of life) represent the polymers of alpha-amino acids, which can exist in either dextro (D) or levo (L) form, also known as stereoisomers. Dextro and levo refer to the absolute confirmation of optically active components. Aside from glycine, all other amino acids can be described as mirror images not able to be superimposed. Major part of them, which are found in nature, appear to be of the L-type. That's why eukaryotic proteins turn out to be always composed of levo amino acids, despite the fact that dextro ones can be found in bacterial cell walls and a number of peptide antibiotics. Thus far, more than three hundred kinds of amino acids have been discovered in nature. However, only 20 of them are usually found as compounds of human peptides and proteins.
Molecule of an amino acid contains carboxyl (COOH) and amino (NH2) functional groups, and each molecule features a different side chain, also known as R group, which vary greatly in properties. Alanine is one of the standard amino acids.
Aside from playing an important role in protein and enzyme synthesis, amino acids are considered very crucial for your good health, since they contribute considerably to the health of the human nervous system, hormone production, and muscular structure. In addition, they are needed for vital organs and cellular structure. If a person experiences low levels of the essential amino acids, this may cause hormonal imbalances, lack of concentration, irritability, and even depression.
Amino acids are crystalline solids able to dissolve in water. Meanwhile, they only dissolve sparingly in organic solvents, and the extent of their solubility depends on the size and nature of the side chain. Amino acids feature very high melting points - up to 200-300°C, and other properties vary for each particular amino acid.
Experts classify amino acids based on lots of different features. One of them is whether or not people can acquire them through the diet. According to this factor, scientists recognize 3 types: the nonessential, essential, and conditionally essential amino acids. However, the classification as essential or nonessential doesn't actually reflect their importance, as all twenty of them are necessary for human health. Those 8 called essential (or indispensable) can't be produced by the body and therefore should be supplied by food: Leucine, Isoleucine, Lysine, Threonine, Methionine, Phenylalanine, Valine, and Tryptophan. One more amino acid, Histidine, can be considered semi-essential, as the human body doesn't always need dietary sources of it. Meanwhile, conditionally essential amino acids aren't usually required in the human diet, but are able to become essential under some circumstances. Finally, nonessential ones are produced by the human body either out of the essential ones or from normal proteins breakdown. These include Asparagine, Alanine, Arginine, Aspartic acid, Cysteine, Glutamic acid, Glutamine, Praline, Glycine, Tyrosine, and Serine.
One more classification depends on the side chain structure, and experts recognize 5 types in this classification:
Finally, there's another classification based on structure of the side chain that divides the list of twenty into 4 groups, two of which are main groups and two are subgroups: non-polar, polar, acidic and polar, basic and polar. For example, side chains having pure hydrocarbon alkyl or aromatic groups are considered non-polar, and their list includes Phenylalanine, Glycine, Valine, Leucine, Alanine, Isoleucine, Proline, Methionine, and Tryptophan. Meanwhile, if the side chain contains different polar groups like amides, acids, and alcohols, they are classified as polar. Their list includes Tyrosine, Serine, Asparagine, Threonine, Glutamine, and Cysteine. Further classification goes for acidic-polar (includes Aspartic Acid and Glutamic Acid), if the side chain has a carboxylic acid, and basic-polar (includes Lysine, Arginine, and Histidine), if the side chain contains an amino group.
Branched-Chain Amino Acids
You might already know that a carbon atom can form up to 4 covalent bonds. If one carbon bonds to more than 2 other carbons, a branch forms. That's why Valine, Leucine, and Isoleucine are called branched-chain amino acids - their side-chains have a branch. Meanwhile, the combination of these 3 amino acids represent around one-third of skeletal muscle in your body.