Structure and classification of amino acids

Basic amino acids

The 20 amino acids that make up proteins are called basic amino acids. They are all -amino acids except proline, that is, they have an amino group on the alpha carbon atom. The basic amino acids all conform to the general form, and all have monograms and trigrams.

According to the side chain structure of amino acids, they can be divided into three categories: aliphatic amino acids, aromatic amino acids and heterocyclic amino acids.

1. There are 15 aliphatic amino acids.

Side chains are only hydrocarbon chains: Gly, Ala, Val, Leu and Ile have the latter three kinds of branched chains, which cannot be synthesized by human body and are essential amino acids.

Side chains contain hydroxyl groups: Ser, Thr many of the active centers of proteases contain serine, which also plays an important role in the binding of proteins to sugars and phosphoric acids.

Sulfur atoms in side chain: Cys,

The two homocysteines of Met bind to each other to form a cysteine by forming a disulfide bond. Disulfide bonds are important for maintaining the higher structure of proteins. Cysteine is also frequently found in the active centers of proteins. The sulfur atoms of methionine are sometimes involved in the formation of coordination bonds. Methionine can be used as a universal methyl donor to participate in the methylation of various molecules.

Side chains containing carboxyl groups: Asp (D), Glu (E)

Side chain containing amide group: Asn (N), Gln (Q)

Alkaline side chain: Arg (R), Lys (K)

2. Aromatic amino acids include Phe (F) and tyrosine (Tyr,Y). Tyrosine is the raw material for the synthesis of thyroxine.

3. Heterocyclic amino acids

There are three kinds of tryptophan (Trp,W), histidine (His) and proline (Pro). Both tryptophan and aromatic amino acids contain benzene ring and have uv absorption (280nm). So protein content can be measured by measuring uv absorption of the protein. Histidine is also a basic amino acid, but its alkalinity is weak. Whether it is charged under physiological conditions is related to the surrounding environment. It often ACTS as a transfer charge in the active center. Histidine can coordinate with metal ions such as iron. Proline, the only secondary amino acid, is the destroyer of alpha - helix.

B means Asx, that is, Asp or Asn; Z is for Glx, Glu or Gln.

Basic amino acids can also be classified by side chain polarity:

Non-polar amino acids: Ala, Val, Leu, Ile, Met, Phe, Trp, Pro

Polarity without charge: Gly, Ser, Thr, Cys, Asn, Gln, Tyr total seven kinds

Positively charged: Arg, Lys, His

Negative charge: Asp, Glu

(2) uncommon protein amino acids

Some proteins contain some uncommon amino acids, which are derived from basic amino acids after protein synthesis through modification such as hydroxylation, carboxylation and methylation. Also called rare or special amino acids. Such as 4- hydroxyproline, 5- hydroxylysine, chain element. Among them, hydroxyproline and hydroxylysine were more abundant in collagen and elastin. It also has 3, 5-diiodotyrosine in it.

(3) non-protein amino acids

There are more than 150 amino acids in nature that are not involved in building proteins. They are mostly derivatives of basic amino acids, but some are d-amino acids or,, delta-amino acids. Some of these amino acids are important metabolite precursors or intermediates, such as citrulline and ornithine, which are intermediates in the synthesis of arginine; -alanine, which is A precursor of pantothenic acid (A coenzyme precursor); and -aminobutyric acid, which is A chemical mediator for nerve impulses.