Complex protein: definition, composition, structure, structure, functions, classification and characteristics. What is the difference between simple proteins and complex ones?

Complex protein, in addition to the protein component proper, contains an additional group of a different nature (prosthetic). As this component are carbohydrates, lipids, metals, phosphoric acid residues, nucleic acids. What distinguishes simple proteins from complex ones, what kinds of substances these substances are divided into, and what are their characteristics, this article will tell. The main difference between the substances under consideration is their composition.

Complex proteins: definition

These are two-component substances, which include simple protein (peptide chains) and non-protein substance (prosthetic group). In the process of their hydrolysis, amino acids, a non-protein part and decay products are formed. What is the difference between simple proteins and complex ones? The first consist only of amino acids.

Classification and characterization of complex proteins

These substances are divided into species, depending on the type of supplementary group. Complex proteins include:

• Glycoproteins are proteins whose molecules contain a carbohydrate residue. Among them, proteoglycans (components of the intercellular space) are isolated, including mucopolysaccharides in their structure. Glikoproteidam include immunoglobulins.
• Lipoproteins include a lipid component. These include apolipoproteins performing the function of providing lipid transport.
• Metalloproteins contain metal ions (copper, manganese, iron, etc.), which are connected through a donor-acceptor interaction. This group does not include heme proteins, which include compounds of the profinine ring with iron and compounds similar in structure (chlorophyll, in particular).
• Nucleoproteins are proteins that have non-covalent bonds with nucleic acids (DNA, RNA). These include chromatin - a component of chromosomes.
• 5. Phosphoproteins, which include casein (a complex protein of cottage cheese), include covalently linked residues of phosphoric acid.

• Chromoproteins combine the staining of the prosthetic component. This class includes heme proteins, chlorophylls and flavoproteins.

Features of glycoproteins and proteoglycans

These proteins are complex substances. Proteoglycans contain a large proportion of carbohydrates (80-85%), in ordinary glycoproteins the content is 15-20%. Uronic acids are present only in the molecule of proteoglycans, their carbohydrates differ regular structure with repeating units. What is the structure and function of complex proteins of glycoproteins? Their carbohydrate chains include only 15 links and have an irregular structure. In the structure of glycoproteins, the bond of the carbohydrate to the protein component is usually carried out through the residues of amino acids such as serine or aspartic.

Functions of glycoproteins:

• They are part of the bacterial cell wall, bone connective and cartilaginous tissue, surround collagen fibers, elastin.
• They play a protective role. For example, this structure has antibodies, interferons, clotting factors (prothrombin, fibrinogen).
• They are receptors that interact with an effector - a small non-protein molecule. The latter, joining the protein, leads to a change in its conformation, which leads to a definite intracellular response.
• Perform a hormonal function. Glycoproteins include gonadotropic, adrenocorticotropic and thyrotropic hormones.
• Transport substances in the blood and ions through the cell membrane (transferrin, transcortin, albumin, Na +, K + -ATPase).

The glycoprotein enzymes include cholinesterase and nuclease.

More on proteoglycans

Typically, the complex protein proteoglycan includes in its structure large carbohydrate chains with repeated disaccharide residues consisting of some uronic acid and amino sugars. Oligo- or polysaccharide chains are called glycans. The former usually contain 2-10 monomer units.

Depending on the structure of the carbohydrate chains, various types are distinguished, for example, acid heteropolysaccharides with a large number of acid groups or glycosaminoglycans including amino groups. The latter include:

• Hyaluronic acid, which is actively used in cosmetology.
• Heparin, preventing coagulability of blood.
• Keratansulfates are components of cartilaginous tissue and cornea.
• Chondroitin sulfates are a part of cartilage and synovial fluid.

These polymers are components of proteoglycans that fill the intercellular space, retain water, lubricate the moving parts of the joints, are their structural components. Hydrophilicity (good solubility in water) of proteoglycans allows them in the intercellular space to create an obstacle for large molecules and microorganisms. With their help, a jelly-like matrix is created, into which the fibers of other important proteins, for example, collagen, are immersed. Its strands in the environment of proteoglycan have a tree-like shape.

Features and types of lipoproteins

Complex protein lipoprotein is distinguished by a well-expressed dual hydrophilic and hydrophobic nature. The nucleus of the molecule (hydrophobic part) forms non-polar esters of cholesterol and triacylglycerides.

Outside, the hydrophilic zone contains the protein part, phospholipids, cholesterol. There are several varieties of lipoprotein proteins depending on their structure.

The main classes of lipoproteins:

• High-density complex protein (HDL, α-lipoproteins). Moves cholesterol to the liver and peripheral tissues.
• Low density (LDL, β-lipoproteins). In addition to cholesterol, triacylglycerides and phospholipids are transported.
• Very low density (VLDL, pre-β-lipoproteins). Perform a function similar to LDL.
• Chylomicrons (HM). Fatty acids and cholesterol are transported from the intestine after ingestion of food.

Such vascular pathology, as atherosclerosis, arises as a result of an incorrect correlation of different types of lipoproteins in the blood. The characteristics of the composition can reveal several trends in the structure of phospholipids (from HDL to chylomicrons): a decrease in the proportion of protein (from 80 to 10%) and phospholipids, an increase in the percentage of triacylglycerides (20 to 90%).

Among metalloproteins there are many important enzymes

The metalloprotein can include ions of several metals. Their presence affects the orientation of the substrate in the active (catalytic) center of the enzyme. Metal ions are localized in the active center and play an important role in catalytic reaction. Often, the ion serves as an electron acceptor.

Examples of metals contained in the structure of enzyme metalloproteins:

• Copper is included in the composition of cytochrome oxidase, which along with the heme contains the ion of a given metal. The enzyme is involved in the formation of ATP during the work of the respiratory chain.
• Iron contains enzymes such as ferritin, which performs the function of depositing iron in the cell; Transferrin - the carrier of iron in the blood; Catalase is responsible for the hydrogen peroxide decontamination reaction.
• Zinc is a metal characteristic of alcohol dehydrogenase, which is involved in the oxidation of ethyl and similar alcohols; Lactate dehydrogenase - an enzyme in the metabolism of lactic acid; Carbonic anhydrase, which catalyzes the formation of carbonic acid from CO ₂ and H ₂ O; Alkaline phosphatase, which performs hydrolytic cleavage of phosphoric acid esters with various compounds; Α2-macroglobulin is an antiproteinic blood protein.
• Selenium is a part of thyreperoxidase, which participates in the process of formation of thyroid hormones; Glutathione peroxidase, which performs an antioxidant function.
• Calcium is characteristic of the structure of α-amylase, an enzyme of hydrolytic cleavage of starch.

Phosphoproteins

What is a part of complex proteins of phosphoproteins? This category is characterized by the presence of a phosphate group, which is linked to the protein part through amino acids with hydroxyl (tyrosine, serine or threonine). What are the functions of phosphoric acid in the structure of the protein? It changes the structure of the molecule, gives it a charge, increases solubility, affects the properties of the protein. Examples of phosphoproteins are milk casein and egg albumin, but mainly this category of complex proteins include enzymes.

The phosphate group plays an important functional role, since many proteins are not permanently associated with it. In the cell all the time there are processes of phosphorylation and dephosphorylation. As a result, regulation of proteins is performed. For example, if histones - proteins that are connected to nucleic acids go to a phosphorylated state, then the activity of the genome (genetic material) increases. The activity of enzymes such as glycogen synthase and glycogen phosphorylase depends on phosphorylation.

Nucleoproteins

Nucleoproteins are proteins connected to nucleic acids. They are an integral part of the storage and regulation of genetic material, the operation of ribosomes, which perform the function of protein synthesis. The simplest forms of life of viruses can be called ribo-and deoxyribonucleoproteins, since they consist of genetic material and proteins.

How does deoxyribonucleic acid (DNA) and histones interact? In chromatin, two types of proteins are associated with DNA (histone and non-histone). The former participate in the initial stage of DNA compactification. The nucleic acid molecule wraps around the proteins to form nucleosomes. The resulting filament is similar to beads, they form a supercoiled structure (chromatin fibril) and supercoiled (interphase chromone). Due to the action of histone proteins and proteins of higher levels, the DNA dimension is reduced by thousands of times. It is sufficient to compare the size of the chromosomes and the length of the nucleic acid in order to evaluate the importance of proteins (6-9 cm and 10-6 μm, respectively).

What are the chromoproteins

Chromoproteins contain very diverse groups, which unite only one - the presence of color in the prosthetic component. Complex proteins of this category are divided into: hemoproteins (contain in the structure of heme), retinalproteins (vitamin A), flavoproteins (vitamin B2), cobamidproteins (vitamin B12).

Hemoproteins are classified depending on the functions for non-enzymatic (hemoglobin and myoglobin protein) and enzymes (cytochromes, catalases, peroxidases).

Flavoproteins contain, as a prosthetic component, vitamin B2 derivatives, flavin mononucleotide (FMN) or flavin adenine dinucleotide (FAD). These enzymes also participate in oxidation-reduction transformations. These include oxidoreductase.

What is cytochrome?

As described above, gem is composed of porphyrin. Its structure includes 4 pyrrole rings and ferrous iron. A special group of heme enzymes - cytochromes, differing in the composition of amino acids and the number of peptide chains, are specialized in carrying out oxidation-reduction reactions, which ensure the transfer of electrons in the respiratory chain. These enzymes are involved in microsomal oxidation - the initial reactions of biotransformation of xenobiotics, leading to their neutralization, and the exchange of many exogenous and exogenous substances, for example, steroids, saturated fatty acids.

Effect of the prosthetic group

Prosthetic group, which is part of complex proteins, affects its properties: it changes its charge, solubility, thermoplasticity. For example, residues of phosphoric acid or monosaccharides possess such an action. The carbohydrate part, included in the complex protein, protects it from proteolysis (destruction as a result of the hydrolysis process), affects the penetration of molecules through the cell membrane, their secretion and sorting. The lipid fragment allows the creation of protein channels for the transport of poorly water-soluble (hydrophobic) compounds.

The structure and function of complex proteins completely depend on the prosthetic group. For example, by means of an iron-containing heme, hemoglobin binds oxygen and carbon dioxide. Due to the nucleoproteins formed as a result of the interaction of histones, protamines with DNA or RNA, the genetic material is protected, its compact storage, the binding of RNA during the synthesis of proteins. Nucleoproteins are called stable complexes of proteins and nucleic acids.

Conclusion

Thus, complex proteins perform a wide range of functions in the body. Therefore, the consumption of macro- and microelements is so important for maintaining health. Metals are part of many enzymes. Knowing biochemistry, your health and the ecological state of your place of residence, you can adjust your diet. For example, identify areas that are deficient in any element. Its additional addition in the diet in the form of additives makes it possible to fill the deficiency.