Protein

Definition and Overview

Protein (protein) is the material basis of life, without protein there is no life. It is therefore a substance that is closely associated with life and with all forms of vital activity. Every cell and all the important components of the organism have proteins involved. Protein accounts for 16.3% of the body weight, i.e. a 60kg adult has about 9.8kg of protein in his body. there are many types of proteins in the human body, with different properties and functions, but all of them are made up of more than 20 kinds of amino acids in different ratios, and are constantly metabolized and renewed in the body. Protein is ingested in the body after digestion and decomposition into amino acids, absorbed in the body is mainly used to re-combine into human proteins in a certain ratio, while the new protein is constantly metabolized and decomposed, always in a dynamic balance. Therefore, the quality and quantity of food protein, the proportion of various amino acids, related to the amount of protein synthesis in the human body, especially the growth and development of adolescents, maternal eugenics, elderly health and longevity, have a close relationship with the amount of protein in the diet.

When the dietary protein source is appropriate, the body's protein metabolism is in a state of balance, the nitrogen intake and nitrogen discharge is equal to the nitrogen balance (nitrogen balance), should be supplied to children and adolescents more protein, so that the body has more nitrogen storage to ensure growth and development. That is to say, the intake of nitrogen is greater than the amount of nitrogen discharged, to achieve a positive nitrogen balance.

Protein is the material basis of all life, is an important part of the muscle cells, is the main raw material for the renewal and repair of human tissues, there is no life without protein. Protein is composed of more than 20 kinds of amino acids, with the number of amino acid composition and arrangement of different order, so that the human body protein up to more than 100,000 kinds. Their structures and functions are very different, forming the diversity and complexity of life.

Protein physiological function

1, the construction of the human body: protein is the material basis of all life, is an important part of the muscle cells, is the main raw material for the renewal and repair of human tissue. Every tissue of the human body: hair, skin, muscle, bone, viscera, brain, blood, nerves, endocrine, etc. are composed of protein, so that the diet creates the person itself. Protein is very important for human growth and development.

For example, brain development is characterized by a one-time completion of cell proliferation, the growth of human brain cells have two peak periods. The first is when the fetus is three months old; the second is after birth to one year old, especially 0 - 6 months of infants is the period of violent growth of brain cells. By the age of one year brain cell proliferation is basically complete, its number has reached 9/10 of the human. so 0 to 1 year old children on the protein intake requirements are very distinctive, on the intellectual development of children is particularly important.

2, repair human tissue: the human body is composed of 100 trillion cells, cells can be said to be the smallest unit of life, they are in the never-ending process of aging, death, newborn metabolism. For example, the epidermis of a young man is renewed once every 28 days, while the gastric mucosa is completely renewed in two or three days. So a person if the protein intake, absorption, utilization are very good, then the skin is glossy and elastic. On the contrary, the person is often in a sub-healthy state. Tissue damage, including trauma, can not get timely and high-quality repair, it will accelerate the decline of the body.

3. Maintaining the normal metabolism of the body and the transportation of all kinds of substances in the body. Carrier proteins are essential for maintaining the normal life activities of the human body. It can transport various substances in the body. For example, hemoglobin - transport oxygen (red blood cell renewal rate of 2.5 million / sec), lipoprotein - transport fat, cell membrane receptors and transport proteins.

4, albumin: to maintain the balance of osmotic pressure in the body and fluid balance.

5, maintain the acid-base balance of body fluids.

6, immune cells and immune proteins: there are white blood cells, lymphocytes, macrophages, antibodies (immunoglobulin), complement, interferon and so on. Renewed once every seven days. This force is strong when there is enough protein and can increase 100 times in hours when needed.

7, constitute the body's essential catalytic and regulatory functions of various enzymes. Our body has thousands of enzymes, each of which can only participate in one biochemical reaction. The human body cells to carry out more than a hundred biochemical reactions per minute. Enzymes have the function of promoting the digestion, absorption and utilization of food. When the corresponding enzymes are sufficient, the reactions go on smoothly and quickly, and we are energized and less likely to get sick. Otherwise, the reaction slows down or is blocked.

8, the main raw material of hormones. Has the physiological activity of regulating organs in the body. Insulin is synthesized from 51 amino acid molecules. Growth hormone is synthesized from 191 amino acid molecules.

7, constitute the neurotransmitter acetylcholine, pentazocine and so on. Maintain the normal function of the nervous system: taste, vision and memory.

8, collagen: 1/3 of the body's proteins, generating connective tissue, constituting the body skeleton. Such as bones, blood vessels, ligaments, etc., determines the elasticity of the skin, to protect the brain (in the brain brain cells, a large part of the collagen cells, and the formation of the blood-brain barrier to protect the brain)

9, to provide heat energy.

Protein and health

Protein was discovered by the Dutch scientist Griet in 1838. He observed that living things cannot survive without proteins. Protein is an extremely important macromolecular organic substance in living organisms, accounting for 54% of the body's dry weight. Proteins are mainly composed of amino acids, which are arranged in different combinations to form various types of proteins. There are estimated to be more than 100,000 types of proteins in the human body. Life is an advanced form of material movement, and this mode of movement is realized through proteins, so proteins have extremely important biological significance. Proteins are indispensable for all life activities such as growth, development, movement, heredity and reproduction in the human body. The movement of life needs proteins and cannot be separated from proteins.

Some physiologically active substances in the human body, such as amines, neurotransmitters, peptide hormones, antibodies, enzymes, nuclear proteins, as well as cell membranes, blood, play a role in the "carrier" role of proteins can not be separated from proteins, which regulate physiological function, maintain metabolism plays an extremely important role. The human body movement system in the muscle composition and muscle in the contraction, work, complete the action process of metabolism is not related to protein, leaving the protein, physical exercise can not begin.

In biology, protein is interpreted as a polypeptide formed by amino acids linked by peptide bonds, and then formed by the polypeptide connected to the substance. In layman's terms, it is the scaffolding and main substance that constitutes the tissues and organs of the human body, and plays an important role in human life activities, and it can be said that there is no life activity without protein. Daily dietary protein is mainly found in lean meat, eggs, beans and fish.

Protein deficiency: adults: muscle wasting, muscle immunity, anemia, severe cases will produce edema. Minors: growth and development stagnation, anemia, poor intellectual development, poor vision. Protein excess: protein in the body can not be stored, more muscle can not be absorbed, excessive intake of protein, will be due to metabolic disorders produce protein poisoning and even death.

Essential amino acids and non-essential amino acids

Protein in food must be digested through the gastrointestinal tract, broken down into amino acids to be absorbed and utilized by the human body, the human body's need for protein is actually the need for amino acids. The body's need for protein is actually the need for amino acids. The absorbed amino acids can only be used to synthesize their own proteins if the quantity and type of amino acids can meet the body's needs. Nutritional amino acids are divided into two categories: essential amino acids and non-essential amino acids.

Essential amino acids are amino acids that the body cannot synthesize on its own or at a rate that does not meet the body's needs, and must be consumed from food. For adults, there are eight such amino acids, including lysine, methionine, leucine, isoleucine, threonine, valine, tryptophan and phenylalanine. For infants, histidine is also an essential amino acid.

Non-essential amino acids do not mean that the human body does not need these amino acids, but that the human body can synthesize its own or be converted from other amino acids, not necessarily from food directly. These amino acids include glutamic acid, alanine, arginine, glycine, aspartic acid, cystine, proline, serine and tyrosine. Some of the non-essential amino acids such as cystine and tyrosine can also save on the essential amino acids methionine and phenylalanine if they are supplied in sufficient quantities.

The classification of protein

Nutritional science according to the type and amount of amino acids contained in food protein will be divided into three categories of food protein: 1, complete protein which is a class of high-quality protein. They contain a complete range of essential amino acids, sufficient quantity, and appropriate proportion of each other. This type of protein can not only maintain human health, but also promote growth and development. Milk, eggs, fish, meat in the protein belongs to the complete protein. 2, semi-complete protein this kind of protein contains amino acids although complete, but the number of some amino acids can not meet the needs of the human body. They can sustain life, but cannot promote growth and development. For example, wheat gluten is a semi-complete protein, containing little lysine. The amino acid or amino acids contained in food that fall short of the body's needs are called limiting amino acids. Cereal protein lysine content is mostly less, so their limiting amino acid is lysine. 3, incomplete proteins These proteins can not provide the body with all the essential amino acids, rely solely on them neither to promote growth and development, but also can not maintain life. For example, the collagen in the meat skin is incomplete protein.

The role of proteins

Proteins are the material basis of life: they are one of the three major components of the human body (proteins, fats, and carbohydrates). As the body's indispensable nutrients accounted for about 20% of human tissue, about 3% of the protein every day to participate in the metabolism to complete a variety of physiological activities in the human body.

Structure of proteins

The biological activity of proteins is not only determined by the primary structure of the protein molecule, but also closely related to its specific spatial structure. Abnormal spatial structure of proteins is likely to lead to the reduction and loss of their biological activity, and even lead to disease, mad cow disease, Alzheimer's disease and so on are due to abnormal protein folding caused by disease. How do proteins fold correctly within a cell? Why does this process sometimes fail? Research on protein folding over the past four decades has focused on the question of how denatured proteins refold when the denaturant is diluted with buffer. But such in vitro studies are far removed from the real intracellular situation. Studies emphasizing normal and abnormal protein folding in living cells, especially the involvement of folding catalysts, molecular chaperones and macromolecules, are the current research hotspots in this field. Elucidating the process about protein folding in functional and structural details will be important for the prevention and treatment of related diseases.

The peptide unit, also known as the peptide group, is a repeating structure on the main chain of a peptide bond. It is a flat unit composed of nitrogen atoms involved in the formation of the peptide chain, carbon atoms and their four substituents: the carbonyl oxygen atom, the acyl hydrogen atom and two neighboring α-carbon atoms.

Protein primary structure (primary structure): refers to the order of amino acid residues connected by *** valence in a protein.

Protein secondary structure (protein in the protein molecule in the localized area of the regular arrangement of amino acid residues. Common secondary structures are α-helix and β-fold. The secondary structure is maintained by hydrogen bonds formed between carbonyl and amide groups in the backbone.

protein tertiary structure: The three-dimensional conformation of a protein molecule in its natural folded state. The tertiary structure is further coiled and folded on the basis of the secondary structure. The tertiary structure is maintained by hydrophobic interactions between amino acid side chains, hydrogen bonds, van der Waals forces, and salt bonds.

Protein quaternary structure: the three-dimensional structure of a multisubunit protein. It is actually the three-dimensional structure of a polypeptide (subunit) with a tertiary structure that has been polymerized in an appropriate manner.

Super-secondary structure: also called motif. In proteins, especially globular proteins, it is common to see a combination of several adjacent secondary structure units that interact with each other to form a regular, spatially recognizable secondary structure assemblage.

Structural domain: an independent folding unit within the tertiary structure of a protein. Structural domains are usually combinations of several super-secondary structural units.

disulfide bond: A ****valent bond formed by the oxidation of two (cysteine) sulfhydryl groups. Disulfide bonds play an important role in stabilizing the three-dimensional structure of certain proteins.

Van der Waals force:A weak intermolecular force between neutral atoms produced by instantaneous electrostatic interactions. The van der Waals force is strongest when the distance between two atoms is the sum of their van der Waals radii. The repulsive effect of the strong van der Waals force prevents the atoms from moving closer to each other.

Alpha-helix (α-helix):A common secondary structure in proteins, where the main chain of the peptide chain is coiled into a helix around an imaginary central axis, generally a right-handed helical structure, and the helix is maintained by intrachain hydrogen bonding. The carbonyl group of each amino acid residue (the nth) forms a hydrogen bond with the amide nitrogen of the 4th residue (the 4th + nth) toward the C-terminal end of the peptide chain. In the classical right-handed alpha-helical structure, the pitch is 0.54 nm, and each turn contains 3.6 amino acid residues, each of which rises 0.15 nm along the long axis of the helix.

Beta-folding (beta-sheet): A common secondary structure in proteins, consisting of a stretched polypeptide chain. The conformation of the folded sheet is maintained by hydrogen bonds formed between the carbonyl oxygen of one peptide bond and another acyl hydrogen located in the same peptide chain. The hydrogen bonds are almost always perpendicularly stretched peptide chains, and these peptide chains can be either in parallel arrangement (in the N to C direction) or in antiparallel arrangement (with the peptide chains aligned in the reverse direction).

β-turn: also a common secondary structure in peptide chains, is a non-repeating polypeptide region that connects secondary structures (α-helix and β-folding) in protein molecules, changing the direction of the peptide chain, generally containing 2 to 16 amino acid residues. Corners containing more than 5 amino acid residues are also often called loops. There are two common types of loops containing four amino acid residues: loop I is characterized by the formation of a hydrogen bond between the carbonyl oxygen of the first amino acid residue and the acyl nitrogen of the fourth residue; and loop II tends to have a glycine as the third residue. The second residue in both of these corners is mostly proline.

Common proteins

Fibrous proteins: a major class of water-insoluble proteins, usually containing polypeptide chains that exhibit the same secondary structure Many fibrous proteins are tightly bound and provide mechanical strength, protection, or structure to individual cells or entire organisms.

Globular protein: A compact, nearly spherical group of proteins containing tightly folded polypeptide chains, many of which are soluble in water. Canonical globular proteins contain depressions or cleft sites that specifically recognize other compounds.

Keratin: A protective or structural protein consisting of parallel polypeptide chains in an alpha-helical or beta-folded conformation that is insoluble in water.

Collagen (protein) (collagen): is the most abundant type of protein in animal connective tissue, it is composed of procollagen molecules. Procollagen is a protein with a right-handed superhelical structure. Each procollagen molecule is formed by the right-handed rotation of a polypeptide chain with three special left-handed helices (pitch 0.95 nm, containing 3.3 residues per turn).

Chaperone: Forms a complex with a newly synthesized polypeptide chain and assists it in folding correctly into a biologically functional protein. Chaperones prevent the formation of incorrectly folded intermediates and the incorrect aggregation of unassembled protein subunits, and assist in the transmembrane transport of polypeptide chains as well as the assembly and disassembly of large multisubunit proteins.

Myoglobin:A binding protein consisting of a peptide chain and a heme cofactor, myoglobin is the protein that stores oxygen in the muscle, and it has a hyperbolic oxygen saturation curve.

Hemoglobin: is a binding protein consisting of four subunits containing heme cofactors. Hemoglobin is responsible for transporting oxygen from the lungs to peripheral tissues and has an S-shaped oxygen saturation curve.

Protein denaturation: A phenomenon in which the natural conformation of a biomolecule is disrupted, resulting in the loss of its biological activity. When proteins are exposed to light, heat, organic solvents, and some denaturing agents, the secondary bonds are damaged, resulting in the destruction of the natural conformation and the loss of the biological activity of the protein.

Renaturation: The phenomenon in which a denatured biomolecule is restored to its biologically active natural conformation under certain conditions.

Allosteric effect: also known as deformation effect, is a phenomenon in which oligomeric proteins combine with ligands to change the conformation of proteins, resulting in the loss of their biological activity.

The main sources of protein are meat, eggs, milk, and legumes. Generally speaking, proteins from animals are of high quality and contain sufficient essential amino acids. There are about 8 types of essential amino acids, which cannot be synthesized by the body itself and must be consumed in food. If the body does not have enough of one of these essential amino acids, it will not be able to synthesize enough protein to supply the various tissues of the body, and other excess protein will be metabolized by the body and wasted, so it is very important to make sure that you have enough of these essential amino acids. Plant-based proteins are usually deficient in 1-2 essential amino acids, so vegetarians need to eat a variety of foods to get enough essential amino acids from all combinations. A piece of cooked meat the size of a playing card contains about 30-35 grams of protein, a large glass of milk about 8-10 grams, and a half-cup of assorted beans about 6-8 grams. So a day of eating a piece of meat the size of a poker, drink two cups of milk, some beans, plus a small amount of fruit and vegetables from the rice, you can get about 60-70 grams of protein.