Proteins (polypeptides, proteins) are macromolecular substances, which include alpha-amino acids connected by a peptide bond. The composition of proteins in living organisms is determined by the genetic code. As a rule, the synthesis uses a set of 20 standard amino acids.
Protein classification
Separation of proteins is carried out according to different criteria:
- The shape of a molecule.
- Composition.
- Functions.
According to the last criterion, proteins are classified:
- On structural.
- Nutritious and spare.
- Transport.
- Contractors.
Structural proteins
These include elastin, collagen, keratin, fibroin. Structural polypeptides are involved in the formation of cell membranes. They can create channels or perform other functions in them.
Nutritious, storage proteins
The nutrient polypeptide is casein. Due to it, the growing organism is provided with calcium, phosphorus andamino acids.
Reserve proteins are seeds of cultivated plants, egg white. They are consumed during the developmental stage of the embryos. In the human body, as in animals, proteins are not stored in reserve. They must be regularly obtained with food, otherwise the development of dystrophy is likely.
Transport polypeptides
Hemoglobin is a classic example of such proteins. Other polypeptides involved in the movement of hormones, lipids and other substances are also found in the blood.
Cell membranes contain proteins that have the ability to transport ions, amino acids, glucose and other compounds across the cell membrane.
Contractile proteins
The functions of these polypeptides are related to the work of muscle fibers. In addition, they provide the movement of cilia and flagella in protozoa. Contractile proteins carry out the function of transporting organelles within the cell. Due to their presence, a change in cellular forms is ensured.
Examples of contractile proteins are myosin and actin. It is worth saying that these polypeptides are found not only in muscle fiber cells. Contractile proteins perform their tasks in almost all animal tissues.
Features
An individual polypeptide, tropomyosin, is found in cells. The contractile muscle protein myosin is its polymer. It forms a complex with actin.
Contractile muscle proteins do not dissolve in water.
Rate of polypeptide synthesis
It is regulated by thyroid andsteroid hormones. Penetrating into the cell, they bind to specific receptors. The formed complex penetrates into the cell nucleus and binds to chromatin. This increases the rate of polypeptide synthesis at the gene level.
Active genes provide increased synthesis of certain RNA. It leaves the nucleus, goes to the ribosomes and activates the synthesis of new structural or contractile proteins, enzymes or hormones. This is the anabolic effect of genes.
Meanwhile, protein synthesis in cells is a rather slow process. It requires high energy costs and plastic material. Accordingly, hormones are not able to quickly control metabolism. Their key task is to regulate the growth, differentiation and development of cells in the body.
Muscle contraction
It is a prime example of the contractile function of proteins. In the course of research, it was found that the basis of muscle contraction is a change in the physical properties of the polypeptide.
The contractile function is performed by the actomyosin protein, which interacts with adenosine triphosphoric acid. This connection is accompanied by contraction of myofibrils. Such an interaction can be observed outside the body.
For example, if soaked in water (macerated) muscle fibers, devoid of excitability, are exposed to a solution of adenosine triphosphate, their sharp contraction will begin, similar to the contraction of living muscles. This experience is of great practical importance. He proves the fact thatmuscle contraction requires a chemical reaction of contractile proteins with an energy-rich substance.
The action of vitamin E
On the one hand, it is the main intracellular antioxidant. Vitamin E protects fats and other easily oxidized compounds from oxidation. At the same time, it acts as an electron carrier and participates in redox reactions, which are associated with the storage of released energy.
Vitamin E deficiency causes atrophy of muscle tissue: the content of the contractile protein myosin is sharply reduced, and it is replaced by collagen, an inert polypeptide.
Specificity of myosin
It is considered one of the key contractile proteins. It accounts for about 55% of the total content of polypeptides in muscle tissue.
Filaments (thick filaments) of myofibrils are made of myosin. The molecule contains a long fibrillar part, which has a double-helix structure, and heads (globular structures). Myosin contains 6 subunits: 2 heavy and 4 light chains located in the globular part.
The main task of the fibrillar region is the ability to form bundles of myosin filaments or thick protofibrils.
On the heads are the active site of ATPase and the actin-binding center. This ensures ATP hydrolysis and binding to actin filaments.
Varieties
Subtypes of actin and myosin are:
- Dynein of flagella and ciliaprotozoa.
- Spectrin in erythrocyte membranes.
- Neurostenin of perisynaptic membranes.
Bacterial polypeptides responsible for the movement of various substances in a concentration gradient can also be attributed to the varieties of actin and myosin. This process is also called chemotaxis.
The role of adenosine triphosphoric acid
If you put actomyosin filaments in an acid solution, add potassium and magnesium ions, you can see that they are shortened. In this case, the breakdown of ATP is observed. This phenomenon indicates that the breakdown of adenosine triphosphoric acid has a certain relationship with a change in the physicochemical properties of the contractile protein and, consequently, with the work of the muscles. This phenomenon was first identified by Szent-Gyorgyi and Engelhardt.
The synthesis and breakdown of ATP are essential in the process of converting chemical energy into mechanical energy. During the breakdown of glycogen, accompanied by the production of lactic acid, as in the dephosphorylation of adenosine triphosphoric and creatine phosphoric acids, the participation of oxygen is not required. This explains the ability of an isolated muscle to function under anaerobic conditions.
Lactic acid and products formed during the breakdown of adenosine triphosphoric and creatine phosphoric acids accumulate in muscle fibers that are tired when working in an anaerobic environment. As a result, the reserves of substances are exhausted, during the splitting of which the necessary energy is released. If a tired muscle is placed in an environment containing oxygen, it willconsume it. Some of the lactic acid will begin to oxidize. As a result, water and carbon dioxide are formed. The released energy will be used for the resynthesis of creatine phosphoric, adenosine triphosphoric acids and glycogen from decay products. Due to this, the muscle will again acquire the ability to work.
Skeletal muscle
Individual properties of polypeptides can only be explained by the example of their functions, i.e. their contribution to complex activities. Among the few structures for which a correlation has been established between protein and organ function, skeletal muscle deserves special attention.
Her cell is activated by nerve impulses (membrane-directed signals). Molecularly, contraction is based on the cycling of cross-bridges through periodic interactions between actin, myosin, and Mg-ATP. Calcium-binding proteins and Ca ions act as mediators between effectors and nerve signals.
Mediation limits the speed of response to "on/off" impulses and prevents spontaneous contractions. At the same time, some oscillations (fluctuations) of the flywheel muscle fibers of winged insects are controlled not by ions or similar low-molecular compounds, but directly by contractile proteins. Due to this, very fast contractions are possible, which, after activation, proceed on their own.
Liquid crystal properties of polypeptides
When shortening muscle fibersthe period of the lattice formed by protofibrils changes. When a lattice of thin filaments enters a structure of thick elements, the tetragonal symmetry is replaced by the hexagonal one. This phenomenon can be considered a polymorphic transition in a liquid crystal system.
Features of mechanochemical processes
They boil down to the transformation of chemical energy into mechanical energy. ATP-ase activity of mitochondrial cell membranes is similar to the act of the iosin system of skeletal muscles. Common features are also noted in their mechanochemical properties: they are reduced under the influence of ATP.
Consequently, a contractile protein must be present in the mitochondrial membranes. And he really is there. It has been established that contractile polypeptides are involved in mitochondrial mechanochemistry. However, it also turned out that phosphatidylinositol (membrane lipid) also plays a significant role in the processes.
Extra
The myosin protein molecule not only contributes to the contraction of various muscles, but can also participate in other intracellular processes. This, in particular, is about the movement of organelles, the attachment of actin filaments to membranes, the formation and functioning of the cytoskeleton, etc. Almost always, the molecule interacts in one way or another with actin, which is the second key contractile protein.
It has been proven that actomyosin molecules can change length under the influence of chemical energy released when a phosphoric acid residue is cleaved from ATP. In other words, this processcauses muscle contraction.
The ATP system thus acts as a kind of accumulator of chemical energy. As needed, it turns directly into a mechanical one through actomyosin. At the same time, there is no intermediate stage characteristic of the processes of interaction of other elements - the transition to thermal energy.
