A complex protein, in addition to the protein component itself, contains an additional group of a different nature (prosthetic). Carbohydrates, lipids, metals, phosphoric acid residues, nucleic acids act as this component. This article will tell you how simple proteins differ from complex ones, what types these substances are divided into, and what are their features. 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, a non-protein part and decay products are formed. How do simple proteins differ from complex ones? 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. To complexproteins include:
- Glycoproteins are proteins whose molecules contain a carbohydrate residue. Among them, proteoglycans (components of the intercellular space) are distinguished, which include mucopolysaccharides in their structure. Glycoproteins include immunoglobulins.
- Lipoproteins include a lipid component. These include apolipoproteins, which perform the function of providing lipid transport.
- Metalloproteins contain metal ions (copper, manganese, iron, etc.) bound through a donor-acceptor interaction. This group does not include heme proteins, which include compounds of the prophyrin ring with iron and compounds similar to them 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 curd protein), include covalently linked phosphoric acid residues.
Chromoproteins are united by the color 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 conventional glycoproteins, the content is 15-20%. Uronic acids are present only in the proteoglycan molecule, their carbohydrates are distinguished by a regular structure with repeating units. What is the structure and function of complex glycoprotein proteins? Their carbohydrate chains include only 15 links and are irregular.structure. In the structure of glycoproteins, the connection of a carbohydrate with a protein component is usually carried out through residues of amino acids such as serine or aspargine.
Functions of glycoproteins:
- They are part of the bacterial cell wall, connective bone and cartilage tissue, surround collagen and elastin fibers.
- Play a protective role. For example, antibodies, interferons, blood coagulation factors (prothrombin, fibrinogen) have this structure.
- 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 certain intracellular response.
- Perform hormonal function. The glycoproteins include gonadotropic, adrenocorticotropic and thyroid-stimulating hormones.
- Transport substances in the blood and ions through the cell membrane (transferrin, transcortin, albumin, Na+, K+ -ATPase).
Glycoprotein enzymes include cholinesterase and nuclease.
More about proteoglycans
Usually, the complex protein proteoglycan includes in its structure large carbohydrate chains with repeating disaccharide residues, consisting of some kind of uronic acid and an amino sugar. Oligo- or polysaccharide chains are called glycans. The former usually contain 2-10 monomeric units.
Depending on the structure of carbohydrate chains, different types are distinguished, for example, sourheteropolysaccharides with a large number of acidic groups or glycosaminoglycans, including amino groups. The latter include:
- Hyaluronic acid, which is actively used in cosmetology.
- Heparin, which prevents blood clotting.
- Keratan sulfates are 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, and are their structural components. The hydrophilicity (good solubility in water) of proteoglycans allows them to create a barrier for large molecules and microorganisms in the intercellular space. With their help, a jelly-like matrix is created, in which fibers of other important proteins, such as collagen, are immersed. Its strands in the proteoglycan medium have a tree shape.
Features and types of lipoproteins
The complex protein lipoprotein has a well-defined dual hydrophilic and hydrophobic nature. The core of the molecule (hydrophobic part) is formed by non-polar cholesterol esters and triacylglycerides.
Outside in the hydrophilic zone are the protein part, phospholipids, cholesterol. There are several types of lipoprotein proteins depending on their structure.
Main classes of lipoproteins:
- High density complex protein (HDL, α-lipoproteins). Moves cholesterol to the liver and peripheral tissues.
- Low density (LDL, β-lipoproteins). Exceptcholesterol is transported by triacylglycerides and phospholipids.
- Very low density (VLDL, pre-β-lipoproteins). Perform a function similar to LDL.
- Chylomicrons (XM). Transport fatty acids and cholesterol from the intestines after food intake.
Such a vascular pathology 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 can include ions of several metals. Their presence affects the orientation of the substrate in the active (catalytic) site of the enzyme. Metal ions are localized in the active site and play an important role in the catalytic reaction. Often the ion functions 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 operation of the respiratory chain.
- Iron contains enzymes such as ferritin, which performs the function of deposition of iron in the cell; transferrin - a carrier of iron in the blood; catalase is responsible for the hydrogen peroxide neutralization reaction.
- Zinc is a metal characteristic ofalcohol dehydrogenase 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 CO2 and H2O; alkaline phosphatase, which performs the hydrolytic cleavage of phosphoric acid esters with various compounds; α2-macroglobulin is an anti-protease blood protein.
- Selenium is part of thyroperoxidase involved in the formation of thyroid hormones; glutathione peroxidase, which performs an antioxidant function.
- Calcium is characteristic of the structure of α-amylase, an enzyme for the hydrolytic breakdown of starch.
Phosphoproteins
What is included in the 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 is the function 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 enzymes are mostly in this category of complex proteins.
The phosphate group plays an important functional role, since many proteins are not permanently bound to it. Phosphorylation and dephosphorylation processes are constantly taking place in the cell. As a result, the regulation of the work of proteins is performed. For example, if histones are proteins connected to nucleic acids, they passinto a phosphorylated state, then the activity of the genome (genetic material) increases. The activity of such enzymes as glycogen synthase and glycogen phosphorylase depends on phosphorylation.
Nucleoproteins
Nucleoproteins are proteins linked to 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, as they are composed of genetic material and proteins.
How does deoxyribonucleic acid (DNA) and histones interact? In chromatin, 2 types of proteins associated with DNA are distinguished (histone and non-histone). The former are involved in the initial stage of DNA compaction. A nucleic acid molecule wraps around proteins to form nucleosomes. The resulting thread is similar to beads, they form a supercoiled structure (chromatin fibril) and a supercoil (interphase chromonema). Due to the action of histone proteins and proteins of higher levels, it is provided with a reduction in the dimension of DNA by thousands of times. It is enough to compare the size of chromosomes and the length of the nucleic acid to assess the importance of proteins (6-9 cm and 10-6 µm, respectively).
What are chromoproteins
Chromoproteins contain very diverse groups that have only one thing in common - the presence of color in the prosthetic component. Complex proteins of this category are divided into: hemoproteins (contain heme in the structure), retinal proteins (vitamin A), flavoproteins (vitamin B2),cobamide proteins (vitamin B12).
Hemoproteins are classified according to their functions into non-enzymatic (hemoglobin and myoglobin protein) and enzymes (cytochromes, catalase, peroxidase).
Flavoproteins contain as a prosthetic component derivatives of vitamin B2 flavin mononucleotide (FMN) or flavin adenine dinucleotide (FAD). These enzymes are also involved in redox transformations. These include oxidoreductases.
What are cytochromes?
As described above, heme 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 redox 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.
Influence of the prosthetic group
The prosthetic group, which is part of complex proteins, affects its properties: changes its charge, solubility, thermoplasticity. For example, phosphoric acid residues or monosaccharides have such an effect. The carbohydrate part included in the composition of a complex protein protects it from proteolysis (destruction as a result of the hydrolysis process), affects the penetration of molecules through the cellmembrane, 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, iron-containing heme in 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, stored compactly, and RNA is bound during protein synthesis. Nucleoproteins are stable complexes of proteins and nucleic acids.
Conclusion
Thus, complex proteins perform a wide range of functions in the body. Therefore, the intake of macro- and micronutrients is so important for maintaining he alth. Metals are part of many enzymes. Knowing the biochemistry, the characteristics of your he alth and the ecological state of the place of residence, you can adjust your own diet. For example, allocate territories that are characterized by a deficit of any element. Its additional introduction into the diet in the form of supplements allows you to make up for the deficiency.
