Complex protein, in addition to the protein component itself, contains an additional group of a different nature (prosthetic). As this component are carbohydrates, lipids, metals, phosphoric acid residues, nucleic acids. How simple proteins differ from complex ones, what types these substances are divided into, and what their features are, this article will tell. The main difference between the considered substances is their composition.
Complex proteins: definition
These are two-component substances, which include a simple protein (peptide chains) and a non-protein substance (prosthetic group). In the process of their hydrolysis, amino acids, non-protein part and decay products are formed. What is the difference between simple and complex proteins? The former consist only of amino acids.
Classification and characterization of complex proteins
These substances are divided into types depending on the type of additional group. Complex proteins include:
- Glycoproteins are proteins whose molecules contain a carbohydrate residue. Among them are proteoglycans (components of the intercellular space), which include mucopolysaccharides in their structure. Glycoproteins include immunoglobulins.
- Lipoproteins include a lipid component. These include apolipoproteins that perform the function of providing lipid transport.
- Metalloproteins contain metal ions (copper, manganese, iron, etc.), bonded through donor-acceptor interaction. This group does not include heme proteins, including compounds of the profirin ring with iron and structurally similar compounds (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 phosphoric acid residues.
- Chromoproteins unite the coloration 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 in their regular structure with repeating units. What is the structure and function of complex glycoprotein proteins? Their carbohydrate chains include only 15 links and have an irregular structure. In the structure of glycoproteins, the connection of the carbohydrate with the protein component is usually carried out through the remains of amino acids such as serine or aspargin.
The functions of glycoproteins:
- They are part of the bacterial cell wall, bone connective and cartilage tissue, are surrounded by collagen, elastin fibers.
- Play a protective role. For example, antibodies, interferons, and coagulation factors (prothrombin, fibrinogen) have this structure.
- 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 specific intracellular response.
- Perform hormonal function. Glycoproteins include gonadotropic, adrenocorticotropic and thyroid-stimulating hormones.
- They transport substances in the blood and ions through the cell membrane (transferrin, transcortin, albumin, Na +, K + -ATPase).
Glycoprotein enzymes include cholinesterase and nuclease.
More on proteoglycans
Typically, the complex protein proteoglycan includes large carbohydrate chains in its structure with repeating disaccharide residues consisting of any uronic acid and amino sugar. Oligo or polysaccharide chains are called glycans. The former usually contain 2-10 monomer units.
Depending on the structure of the carbohydrate chains, their various types are distinguished, for example, acidic 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, which prevents blood coagulation.
- Keratan sulfates are the components of cartilage and cornea.
- Chondroitin sulfates are 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. The hydrophilicity (good solubility in water) of proteoglycans allows them to create an obstacle in the intercellular space for large molecules and microorganisms. With their help, a jelly-like matrix is created in which the fibers of other important proteins, for example, collagen, are immersed. Its cords in the proteoglycan environment are tree-like.
Features and types of lipoproteins
Complex protein lipoprotein is characterized by a pronounced dual hydrophilic and hydrophobic nature. The core of the molecule (the hydrophobic part) is formed by nonpolar cholesterol esters and triacylglycerides.
Outside in the hydrophilic zone are the protein part, phospholipids, cholesterol. Several types of lipoprotein proteins are distinguished, depending on their structure.
The main classes of lipoproteins:
- Complex high-density 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 (XM). Fatty acids and cholesterol are transported from the intestines after food intake.
A vascular pathology such as atherosclerosis occurs as a result of an incorrect ratio of different types of lipoproteins in the blood. According to the characteristics of the composition, several trends in the structure of phospholipids (from HDL to chylomicrons) can be identified: a decrease in the proportion of protein (from 80 to 10%) and phospholipids, an increase in the percentage of triacylglycerides (from 20 to 90%).
There are many important enzymes among metalloproteins.
Metalloprotein may include several metal ions. 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 the catalytic reaction. Often an ion acts as an electron acceptor.
Examples of metals contained in the structure of enzymatic metalloproteins:
- Copper is included in the composition of cytochrome oxidase, which along with heme contains an ion of this metal. The enzyme is involved in the formation of ATP during the respiratory chain.
- Iron contains enzymes such as ferritin, which performs the function of depositing iron in the cell; transferrin - a carrier of iron in the blood; catalase is responsible for the neutralization reaction of hydrogen peroxide.
- 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, catalyzing the formation of carbonic acid from CO 2 and H 2 O; alkaline phosphatase performing hydrolytic cleavage of phosphoric acid esters with various compounds; α2-macroglobulin is an antiprotease blood protein.
- Selenium is part of thyroperoxidase involved in the formation of thyroid hormones; glutathione peroxidase, performing an antioxidant function.
- Calcium is characteristic of the structure of α-amylase, an enzyme for the hydrolytic degradation 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 associated with the protein part through amino acids with hydroxyl (tyrosine, serine or threonine). What are the functions of phosphoric acid in the protein structure? 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 enzymes belong to this category of complex proteins.
The phosphate group plays an important functional role, since many proteins are not permanently bound to it. In the cell, phosphorylation and dephosphorylation occur all the time. As a result, regulation of protein work is performed. For example, if histones - proteins connected with nucleic acids become phosphorylated, 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 combined with nucleic acids. They are an integral part of the storage and regulation of genetic material, the work of ribosomes that perform the function of protein synthesis. The simplest life forms 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, 2 types of proteins associated with DNA (histone and nonhistone) are distinguished. The former are involved in the initial stage of DNA compaction. A nucleic acid molecule coils around proteins to form nucleosomes. The resulting thread is similar to beads, from which a supercoiled structure (chromatin fibril) and a supercoiled (interphase chromoneme) are formed. Due to the action of histone proteins and proteins of higher levels, it is provided by a reduction in the dimension of DNA by a thousand times. It is enough to compare the size of the chromosomes and the length of the nucleic acid to assess the importance of proteins (6-9 cm and 10-6 microns, respectively).
What are chromoproteins
Chromoproteins contain very diverse groups that are united by only one thing - the presence of color in the prosthetic component. Complex proteins of this category are divided into: hemoproteins (contain heme in the structure), retinalproteins (vitamin A), flavoproteins (vitamin B2), cobamideproteins (vitamin B12).
Hemoproteins are classified according to their functions into non-enzymatic (hemoglobin and myoglobin protein) and enzymes (cytochromes, catalases, peroxidases).
Flavoproteins contain, as a prosthetic component, derivatives of vitamin B2 flavin mononucleotide (FMN) or flavin adenine dinucleotide (FAD). These enzymes also participate in redox transformations. These include oxidoreductases.
What are cytochromes?
As described above, heme consists of porphyrin. Its structure includes 4 pyrrole rings and divalent 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 redox reactions, due to which electron transfer in the respiratory chain is ensured. These enzymes are involved in microsomal oxidation - the initial biotransformation reactions of xenobiotics, leading to their neutralization, and the exchange of many exogenous and exogenous substances, for example, steroids, saturated fatty acids.
The influence of the prosthetic group
The prosthetic group, which is part of complex proteins, affects its properties: it changes its charge, solubility, thermoplasticity. For example, phosphoric acid or monosaccharide residues have this effect. The carbohydrate part included in the complex protein protects it from proteolysis (destruction as a result of hydrolysis), 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 functions of complex proteins are completely dependent on the prosthetic group. For example, using iron-containing heme in hemoglobin, oxygen and carbon dioxide are bound. Due to the nucleoproteins formed as a result of the interaction of histones, protamines with DNA or RNA, the genetic material is protected, it is compactly stored, and RNA is bound in the process of protein synthesis. 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 micronutrients is so important for maintaining health. Metals are part of many enzymes. Knowing the biochemistry, the characteristics of your health and the ecological condition of the place of residence, you can adjust your diet. For example, they distinguish territories that are deficient in any element. Its additional introduction into the diet in the form of additives allows you to make up for the shortcoming.