Most of the amino acids that make up proteins are biosynthesized using the intermediates of the Embden-Meyerhof pathway and the citric acid cycle as the carbon chain skeleton. The exceptions are aromatic amino acids and histidine. The biosynthesis of the former is related to the intermediate erythrose-4-phosphate of pentose phosphate, and the latter is synthesized from ATP and ribose pyrophosphate. Microorganisms and plants can synthesize all amino acids in the body, while animals cannot synthesize some amino acids in the body (essential amino acids). Essential amino acids are generally biosynthesized from intermediates of carbohydrate metabolism through multi-step reactions (more than 6 steps). The synthesis of non-essential amino acids requires about 14 enzymes, while the synthesis of essential amino acids requires more. About 60 enzymes are involved. In addition to being used as raw materials for protein synthesis, biosynthesized amino acids are also used in the synthesis of alkaloids, lignin, etc. On the other hand, amino acids are decomposed in vivo by transamination or oxidation to form keto acids, or decarboxylated into amines and then decomposed.
Amino acid: It is an organic compound containing an alkaline amino group and an acidic carboxyl group. The amino group is usually attached to the α-carbon. General structural formula of amino acids
It is the basic unit of biological functional macromolecule protein.
Classification of amino acids
Essential amino acids: refers to amino acids that humans (or other vertebrates) (lysine, threonine, etc.) cannot synthesize themselves and need to obtain them from food.
Nonessential amino acids: Refers to amino acids that humans (or other vertebrates) can synthesize from simple precursors and do not need to be obtained from food.
There are also acidic, alkaline, neutral and heterocyclic classifications, which are classified according to their chemical properties.
Detection:
Ninhydrin reaction: Under heating conditions, an amino acid or peptide reacts with ninhydrin to form a purple (reacts with proline to form a yellow) compound.
Protein:
Peptide bond: An amide bond formed by condensing the carboxyl group of one amino acid with the amino group of another amino acid and removing a molecule of water.
Peptide: A polymer formed by two or more amino groups covalently linked through peptide bonds.
It is a compound in which amino acids are connected through peptide bonds. The products of incomplete hydrolysis of proteins are also peptides. Peptides are called dipeptides, tripeptides and tetrapeptides respectively according to the number of amino acids they consist of: 2, 3 and 4. Generally, oligopeptides consisting of less than 10 amino acids are called oligopeptides. The above amino acid composition is called a polypeptide (polypeptide), and they are all simply called peptides. The amino acids in the peptide chain are no longer free amino acid molecules because their amino and carboxyl groups are combined in the formation of peptide bonds. Therefore, the amino acids in polypeptide and protein molecules are called amino acid residues.
Polypeptides include open-chain peptides and cyclic peptides. In the human body, it is mainly open-chain peptides. The open-chain peptide has a free amino terminus and a free carboxyl terminus, and retains free α-amino and α-carboxy groups respectively, so it is also called the N-terminal (amino-terminal) and C-terminal (carboxyl-terminal) of the polypeptide chain. When writing, the N-terminus is generally written on the left side of the molecule and is represented by (H), and the amino acid residues in the polypeptide molecule are numbered sequentially from this point, while the C-terminus of the peptide chain is written on the right side of the molecule and is represented by (OH). to express. At present, the amino acid composition and sequence of peptide segments in about 200,000 peptides and protein molecules have been determined, many of which are peptides closely related to medicine and have important physiological functions or pharmacological effects.
Polypeptides have extensive distribution and important physiological functions in the body. Among them, glutathione is abundant in red blood cells and has the function of protecting the cell membrane structure and keeping intracellular enzyme proteins in their original and active state. Among various polypeptides, glutathione has a special structure. In the molecule, glutamic acid is dehydrated and condensed with its γ-carboxyl group and the α-amino group of cysteine to form a peptide bond, and it can be reversible in cells. Oxidation-reduction reaction, so there are two types of glutathione: reduced type and oxidized type.
In recent years, some polypeptide molecules with powerful biological activities have been continuously discovered and identified. Most of them have important physiological functions or pharmacological effects. For example, some "brain peptides" are related to the body's learning and memory, sleep, diet and... Behaviors are closely related, which increases people's understanding of the importance of peptides, and peptides have become one of the most eye-catching research areas in biochemistry.
The difference between polypeptides and proteins is that on the one hand, the number of amino acid residues in polypeptides is less than that in proteins, generally less than 50, while most proteins are composed of more than 100 amino acid residues, but there is no strict number between them. In addition to molecular weight, it is now believed that polypeptides generally do not have a strict and relatively stable spatial structure, that is, their spatial structure ratioIt is more variable and plastic, while protein molecules have a relatively tight and stable spatial structure, which is also the basis for proteins to exert physiological functions. Therefore, insulin is generally classified as a protein. However, some books do not strictly call insulin a polypeptide because of its small molecular weight. However, both peptides and proteins are polycondensates of amino acids, and peptides are also products of incomplete hydrolysis of proteins.
Protein primary structure: refers to the order of covalently linked amino acid residues in a protein.
Amino acids refer to a class of organic compounds containing a carboxyl group and an amino group attached to the carbon base attached to the carboxyl group. It is the basic substance that constitutes the protein required for animal nutrition.
There are about 22 types of amino acids required by the human body, which are divided into non-essential amino acids and essential amino acids (must be supplied from food).
Essential amino acids refer to the amino acids that the human body cannot synthesize or the synthesis rate is far from meeting the needs of the body, and must be supplied by food protein. These amino acids are called essential amino acids. There are 10 functions in total:
(1) Lysine: Promotes brain development, is a component of liver and gallbladder, can promote fat metabolism, regulate pineal gland, mammary gland, corpus luteum and ovary, and prevent cell recovery;
(2) Tryptophan: Promotes the production of gastric juice and pancreatic juice;
(3) Phenylalanine: involved in eliminating the loss of kidney and bladder function;
(4) Methionine; participates in the composition of hemoglobin, tissue and serum, and has the function of promoting spleen, pancreas and lymph;
(5) Threonine: It has the function of converting certain amino acids to achieve balance;
(6) Isoleucine: Participates in the regulation and metabolism of the thymus, spleen and subcerebral glands; the subcerebral gland headquarters acts on (1) the thyroid (2) the gonads;
(7) Leucine: acts to balance isoleucine;
(8) Valine: Acts on the corpus luteum, breast and ovaries.
(9) Histidine: plays a role in the regulation of metabolism;
(10) Arginine: Promotes wound healing and is a component of sperm protein.
Its physical and chemical properties are roughly as follows:
1) They are all colorless crystals. The melting point is about 230. Above C, most of them have no exact melting point. They decompose and release CO2 when melted. They are all soluble in strong acid and strong alkaline solutions. Except for cystine, tyrosine and diiodothyronine, they are all soluble in water; except for proline Except acid and hydroxyproline, it is difficult toSoluble in ethanol and ether.