What are the classifications and sources of protein?

Classification by source
Proteins can be categorized by source into animal proteins and plant proteins, which contain different amino acids.
Generally speaking, there is not much difference between plant and animal proteins in nature, but there are some differences in the composition and quantity of amino acids.
Despite the fact that plant proteins are taken from a wide range of sources, there is a gap between the types and relative amounts of their proteins and the requirements of the human body.
For example, there is a lack of immunoglobulin in plant proteins and a relative lack of lysine in cereals. Plant proteins are less well digested and absorbed than animal proteins, but they have the advantage of not containing cholesterol. Animal proteins are relatively more compatible with the nutritional structure of human beings, and their protein types and structures are closer to the structure and quantity of human proteins, and generally contain the eight essential amino acids (especially egg products and dairy products), so animal proteins have a higher nutritional value than plant proteins.
Classification according to composition
According to the chemical composition, proteins can usually be divided into simple proteins, binding proteins and derived proteins. Simple proteins are hydrolyzed to obtain amino acids and amino acid derivatives; bound proteins are hydrolyzed to obtain amino acids, non-protein cofactors and others (the non-amino acid part of bound proteins is called cofactors); and proteins are modified by denaturation and modification to obtain derived proteins.
Simple proteins (simpleproteins), according to the solubility of different can be divided into: ① clear protein (albumins): soluble in water and dilute salt, dilute acid or dilute alkali solution, can be saturated ammonium sulfate precipitation, can be coagulated by heating. Widely found in organisms, such as serum protein, whey protein, egg white protein.
② globulins (globulins): insoluble in water but soluble in dilute salt, dilute acid and dilute alkali solution, can be half-saturated and ammonium sulfate precipitation. Prevalent in living organisms, such as serum globulin, myosin and plant seed globulin.
Glutelins: insoluble in water, ethanol and neutral salt solution, but soluble in dilute acid or dilute alkali. Such as rice gluten and wheat gluten.
Alcohol soluble gluten (prolamines): insoluble in water and anhydrous ethanol, but soluble in 70% ~ 80% ethanol, dilute acid and dilute alkali. There are more proline and amide in the molecule, and the non-polar side chain is much more than the polar side chain. These proteins are mainly found in cereal seeds, such as corn soluble proteins, wheat soluble proteins and so on.
⑤ Histones: soluble in water and dilute acid, but precipitated by dilute ammonia. Molecule histidine, lysine more, molecular alkaline, such as calf thymus histone.
(6) Protamines: soluble in water and dilute acid, insoluble in ammonia. Molecules of alkaline amino acids (arginine and lysine) is particularly high, so alkaline, such as salmon sperm protein.
(7) Scleroproteins (scleroprotein): insoluble in water, salt, dilute acid or dilute alkali. These proteins are proteins that serve as connective tissue and protective functions in animals, such as keratin, collagen, reticulin and elastin. According to the different cofactors, conjugated proteins (conjugated proteins) can be divided into:
① nuclear proteins (nucleoproteins): cofactor is nucleic acid, such as deoxyribonucleoproteins, ribosomes, tobacco mosaic virus and so on.
② lipoproteins (1ipoproteins): proteins bound to lipids. Lipid components are phospholipids, sterols and neutral lipids, such as β1-lipoproteins in the blood, yolk globulin and so on.
(iii) Glycoproteins and mucins (glycoproteins): cofactor components are one or more of galactose, mannose, hexosamine, hexanedioic acid, sialic acid, sulfuric acid, or phosphoric acid. Glycoproteins are soluble in alkaline solutions, such as ovalbumin, γ-globulin, serum mucin-like proteins.
④ phosphoproteins (phosphoproteins): the phosphate group is connected to the hydroxyl group of the side chain of serine or threonine residues in proteins through ester bonding, such as casein, pepsin, and so on.
⑤ Hemoglobin proteins (hemoproteins): the cofactor group is hemoglobin. Iron-containing such as hemoglobin, cytochrome c, magnesium-containing chlorophyll protein, copper-containing hemocyanin.
(6) Flavoprotein (flavoproteins): cofactor for flavin adenine dinucleotide, such as succinate dehydrogenase, D-amino acid oxidase.
(vii) Metalloproteins (metalloproteins): proteins that bind directly to metals, e.g. ferritin contains iron, ethanol dehydrogenase contains zinc, xanthine oxidase contains molybdenum and iron.
Derived protein, natural protein denaturation or modification, modification and decomposition products. ① primary derived proteins: insoluble in all solvents, such as denatured proteins. ② secondary derived proteins: soluble in water, not solidified by heat, such as peptones, peptides. ③ Tertiary derived proteins: functional improvement, such as phosphorylated proteins, acetylated proteins, succinamide proteins.
Classification according to molecular shape
According to the different molecular shapes, proteins can be divided into two major categories: globular proteins and fibrous proteins. Taking the ratio of the long axis to the short axis as a standard, globular proteins are less than 5 and fibrous proteins are more than 5. Fibrous proteins are mostly structural proteins, which are indispensable proteins for the structure of tissues, and are connected by long amino acid peptide chains to become fibrous or curled up into a disk-like structure, which become the pillars for a variety of tissues such as collagen in the skin, tendons, cartilage, and bone tissues; globular proteins are similar in shape to a sphere or oval shape. Many physiologically active proteins, such as enzymes, transport proteins, protein hormones and immunoglobulins, complement and so on belong to globular proteins.
Classification by structure
Proteins can be categorized according to their structure: monomeric proteins, oligomeric proteins, polyproteins. Monomeric proteins: proteins consist of a peptide chain, the highest structure for the tertiary structure. Including proteins formed by several peptide chains connected by disulfide bonds, the highest structure is also tertiary. Most hydrolases are monomeric proteins.
Oligomeric protein: contains 2 or more subunits of tertiary structure. It can be a polymerization of the same subunits or different subunits.
Multimeric proteins: super multimeric proteins polymerized from tens of subunits or more, or even hundreds of subunits.
Classification by function
Proteins are categorized into two main groups, active and inactive proteins, according to their functions. Active proteins include regulatory proteins, contractile proteins, antibody proteins, and so on. Inactive proteins include structural proteins.
Structural proteins: proteins that make up human tissues, such as ligaments, hair, nails and skin. Regulatory proteins: proteins with regulatory functions, such as insulin, thyroxine, etc. Contractile proteins: proteins involved in the contractile process, such as myosin, actin, etc. Antibody proteins: proteins that constitute antibodies in the body, such as immunizer proteins.
Classification of proteins according to their nutritional value
The nutritional value of food proteins depends on the type and quantity of amino acids contained, so nutritionally it can be divided into three categories: complete proteins, semi-complete proteins and incomplete proteins according to the amino acid composition of food proteins.
1. Complete proteins contain a complete range of essential amino acids, sufficient quantity, appropriate ratio, not only to maintain the health of adults, and can promote the growth and development of children, such as dairy casein, lactalbumin, eggs in the egg albumin, ovalbumin, meat in the albumin, myosin, soybeans in the soybean protein, wheat in the wheat gluten, corn in the gluten and so on.
2. Semi-complete proteins contain a complete range of essential amino acids, but some amino acids are insufficient in number and inappropriate proportion, which can sustain life but cannot promote growth and development, such as wheat glutenin.
3. Incomplete proteins contain incomplete types of essential amino acids, which can neither maintain life nor promote growth and development, such as corn in the corn gliadin, animal connective tissue and meat skin in the gliadin, peas in the soybean globulin and so on.
Recommended intake of protein and main food sources
The Chinese Nutrition Society suggests that the recommended protein intake for adult men is 70g/day, which is equivalent to 525g of eggs, and for women is 65g/day, which is equivalent to 490g of eggs. The protein requirements for different ages and labor intensities are shown in the attached table.
Food sources of protein can be divided into two categories: plant proteins and animal proteins. Among plant proteins, cereals contain about 10% protein, which is generally the main source of dietary protein due to energy needs. Beans are rich in protein, soybeans contain up to 36%~40% protein, amino acid composition is also more reasonable, the utilization rate is also higher, is the quality of plant protein.
Eggs contain 11%~14% protein, which is an important source of high quality protein. Eggs are known as the ideal protein because their amino acid composition is closest to the amino acid pattern of human protein. Milk (cow’s milk) generally contains 3.0% to 3.5% protein, and is the best source of protein for infants and young children other than breast milk. Meat includes muscle from poultry, livestock and fish. Fresh muscle contains 15%~22% protein. The nutritional value of muscle protein is better than that of plant protein, and it is one of the sources of protein for human body.
In order to improve the quality of dietary protein, a certain amount of high-quality protein should be guaranteed in the diet. General requirements for animal protein and soy protein should account for 30% to 50% of the total dietary protein.