The covalent bond between the carboxyl group of one amino acid and the amino group of another amino acid is called peptide bond. In protein molecules, amino acids are linked by peptide bonds to form peptide chains.

The simplest peptide consists of two amino acids, called dipeptides. Peptides that contain three amino acids are called tripeptides, and so on. Amino acids in peptide chains are not complete amino acids because of dehydration when peptide bonds are formed, so they are called residues. The naming of peptides is based on the amino acid residues that make up the peptides. Generally, it starts from the amino end of peptide, which is called certain acyl certain acyl An amino acid. Peptide writing also starts from the amino end:

H2N-Ala-Tyr-Gly-COOH

The English name of peptide chain is also written continuously. The above three peptides are "alanyltyrosylglycine". So, a long peptide chain will have a long name. It is said that myonectin contains more than 25000 amino acids. Its chemical name is the longest English word with 189819 letters. It takes three and a half hours to read it once.

The atoms in peptide bond show high stability because of resonance. In peptide bond, C-N single bond has about 40% double bond property, while C = O double bond has 40% single bond property. There are two important results: first, the imino of peptide bond is at pH

In the range of 0-14, there is no obvious tendency of dissociation and protonation; secondly, the C-N single bond in peptide bond can not rotate freely, so that the protein can fold into various three-dimensional conformations.

Resonance formula of peptide bond

In addition to the partial hydrolysis of proteins, which can produce a variety of simple peptides, there are also small peptides with different lengths in nature, which have special physiological functions. Animal and plant cells contain a kind of tripeptide, called glutathione, namely δ - glutamylcysteine. Among them, "δ -" means that glutamic acid forms peptide bond with cysteine by carboxyl group of side chain. Because these peptides are synthesized by special enzymes, there will be nonstandard peptide bonds and special amino acids.

glutathione

Glutathione contains sulfhydryl group, expressed as GSH, which is called reduced glutathione. The two reduced glutathiones are linked together by disulfide bonds after being oxygenated, known as oxidized glutathione (GSSG). Glutathione, an important antioxidant, plays a role in the process of oxidation-reduction in vivo. Glutathione peroxidase

GSH is used in the removal of peroxides.

Glutathione peroxidase

Enkephalin is a natural analgesic. Carnosine in muscle can be used as a buffer in muscle to buffer the effect of lactic acid produced by muscle on pH. An antibiotic is called Brevibacterium casein, which is composed of 12 amino acids. Many hormones are also polypeptides, such as oxytocin, vasopressin and bradykinin.

The physical properties of small peptides are similar to amino acids. They are also ionic crystals and exist as dipole ions in aqueous solution. The imino group of peptide bond does not dissociate, so the acidity and basicity of peptide depend on terminal amino group, carboxyl group and side chain group. The titration curve of peptide is similar to that of amino acid, and the isoelectric point can also be determined according to the pK value of each group.

The chemical properties of peptides are similar to those of amino acids, but some reactions related to peptide bonds are not found in amino acids, such as biuret reaction. Generally, compounds containing two or more peptide bonds can react with CuSO4 alkaline solution to form purplish red or blue purple complexes. This reaction does not require biuret, so it is named because the structure of biuret is similar to peptide bond and similar reaction can occur. This reaction can be used to determine protein content.

Biuret, formed by condensation of two molecules of urea

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Chengdu Shengnuo Biotechnology Co., Ltd. has "Chengdu polypeptide drug engineering technology research center" in Chengdu, mainly engaged in polypeptide, polypeptide drug and beauty peptide research. Our zero defect has passed the FDA certification, and now it has become the first-class professional peptide drug and product development, technology transfer, technical service and peptide drug industry in the scale production and export of China's parks.