A large number of organic substances that make up a living cell are characterized by large molecular sizes and are biopolymers. These include proteins, which make up from 50 to 80% of the dry mass of the entire cell. Protein monomers are amino acids that are linked together by peptide bonds. Protein macromolecules have several levels of organization and perform a number of important functions in the cell: building, protective, catalytic, motor, etc. In our article, we will consider the structural features of peptides, and also give examples of globular and fibrillar proteins that make up the human body.
Shapes of organization of polypeptide macromolecules
Amino acid residues are sequentially connected to each other by strong covalent bonds calledpeptide. They are quite strong and keep the primary structure of the protein in a stable state, which has the form of a chain. The secondary form occurs when the polypeptide chain is twisted into an alpha helix. It is stabilized by additionally emerging hydrogen bonds. The tertiary, or native, configuration is of fundamental importance, since most of the globular proteins in a living cell have just such a structure. The spiral is packed in the form of a sphere or globule. Its stability is due not only to the appearance of new hydrogen bonds, but also to the formation of disulfide bridges. They arise due to the interaction of sulfur atoms that make up the amino acid cysteine. An important role in the formation of the tertiary structure is played by hydrophilic and hydrophobic interactions between groups of atoms within the peptide structure. If a globular protein combines with the same molecules through a non-protein component, for example, a metal ion, then a quaternary configuration arises - the highest form of organization of the polypeptide.
Fibrillar proteins
The contractile, motor and building functions in the cell are performed by proteins, the macromolecules of which look like thin threads - fibrils. The polypeptides that make up the fibers of the skin, hair, and nails are classified as fibrillar species. The most famous of them are collagen, keratin and elastin. They do not dissolve in water, but can swell in it, forming a sticky and viscous mass. Peptides of a linear structure are also part of the fission spindle filaments, forming the mitotic apparatus of the cell. They areattach to chromosomes, contract and stretch them to the poles of the cell. This process is observed in the anaphase of mitosis - the division of somatic cells of the body, as well as in the reduction and equational stages of division of germ cells - meiosis. Unlike globular protein, fibrils are able to quickly stretch and contract. The cilia of ciliates-shoes, the flagella of euglena green or unicellular algae - chlamydomonas are built from fibrils and perform the functions of movement in the simplest organisms. The contraction of muscle proteins - actin and myosin, which are part of muscle tissue, determine the various movements of skeletal muscles and maintain the muscular skeleton of the human body.
Structure of globular proteins
Peptides - carriers of molecules of various substances, protective proteins - immunoglobulins, hormones - this is an incomplete list of proteins, the tertiary structure of which has the form of a ball - globules. There are certain proteins in the blood that have certain areas on their surface - active centers. With their help, they recognize and attach to themselves the molecules of biologically active substances produced by the glands of mixed and internal secretion. With the help of globular proteins, hormones of the thyroid and sex glands, adrenal glands, thymus, pituitary gland are delivered to certain cells of the human body, equipped with special receptors for their recognition.
Membrane polypeptides
The fluid-mosaic model of the structure of cell membranes is best adapted to their important functions: barrier,receptor and transport. The proteins included in it carry out the transport of ions and particles of certain substances, such as glucose, amino acids, etc. The properties of globular carrier proteins can be studied using the sodium-potassium pump as an example. It carries out the transition of ions from the cell to the intercellular space and vice versa. Sodium ions are constantly moving into the middle of the cell cytoplasm, and potassium cations are constantly moving out of the cell. Violation of the desired concentration of these ions leads to cell death. To prevent this threat, a special protein is built into the cell membrane. The structure of globular proteins is such that they carry the cations Na+ and K+against a concentration gradient using the energy of adenosine triphosphoric acid.
Structure and function of insulin
Soluble proteins of spherical structure, which are in tertiary form, act as regulators of metabolism in the human body. Insulin is produced by the beta cells of the islets of Langerhans and controls blood glucose levels. It consists of two polypeptide chains (α- and β-forms) connected by several disulfide bridges. These are covalent bonds that arise between the molecules of the sulfur-containing amino acid - cysteine. The pancreatic hormone mainly consists of an ordered sequence of amino acid units organized in the form of an alpha helix. A small part of it has the form of a β-structure and amino acid residues without strict orientation in space.
Hemoglobin
A classic example of globular peptidesThe protein in the blood that causes the red color of blood is hemoglobin. The protein contains four polypeptide regions in the form of alpha and beta helices, which are connected by a non-protein component - heme. It is represented by an iron ion that binds polypeptide chains into one confirmation related to the quaternary form. Oxygen particles are attached to the protein molecule (in this form it is called oxyhemoglobin) and then transported to the cells. This ensures the normal course of dissimilation processes, since in order to obtain energy, the cell oxidizes the organic substances that have entered it.
Role of blood protein in gas transport
In addition to oxygen, hemoglobin is also able to attach carbon dioxide. Carbon dioxide is produced as a by-product of catabolic cellular reactions and must be removed from the cells. If the inhaled air contains carbon monoxide - carbon monoxide, it is able to form a strong bond with hemoglobin. In this case, a toxic substance without color and odor in the process of breathing quickly penetrates to the cells of the body, causing poisoning. Particularly sensitive to high concentrations of carbon monoxide are the structures of the brain. There is paralysis of the respiratory center located in the medulla oblongata, which leads to death from suffocation.
In our article, we examined the structure, structure and properties of peptides, and also gave examples of globular proteins that perform a number of important functions in the human body.
