Proteins (proteins, polypeptides) are the most numerous, most diverse and of primary importance biopolymers. Protein molecules contain atoms of carbon, oxygen, hydrogen, nitrogen and sometimes sulfur, phosphorus and iron.
Monomers of proteins are amino acids, which (having in their composition carboxyl and amino groups) have the properties of an acid and a base (amphoteric).
Thanks to this, amino acids can combine with each other (their number in one molecule can reach several hundred). In this regard, protein molecules are large, and they are called macromolecules.
Protein molecule structure
The structure of a protein molecule is understood as its amino acid composition, the sequence of monomers and the degree of curl of the protein molecule.
Only 20 types of different amino acids are found in protein molecules, and a huge variety of proteins is created due to their different combinations:
- The sequence of amino acids in a polypeptide chain is the primary structure of a protein. It is unique to any type of protein and determines the shape of its molecule, its properties and functions.
- A long protein molecule folds and first acquires the form of a spiral as a result of the formation of hydrogen bonds between the —CO and —NH groups of different amino acid residues of the polypeptide chain (between the carbon of the carboxyl group of one amino acid and the nitrogen of the amino group of another amino acid). This helix is the secondary structure of the protein.
- The tertiary structure of a protein is a three-dimensional spatial “packing” of the polypeptide chain in the form of a globule (ball). The strength of the tertiary structure is provided by a variety of bonds arising between amino acid radicals (hydrophobic, hydrogen, ionic and disulfide S – S bonds).
- Some proteins (for example, human hemoglobin) have a quaternary structure. It arises from the combination of several macromolecules with a tertiary structure into a complex complex. The quaternary structure is held together by fragile ionic, hydrogen and hydrophobic bonds.
The structure of proteins can be disrupted (denatured) when heated, treated with certain chemicals, irradiation, etc. With a weak effect, only the quaternary structure decomposes, with a stronger one, the tertiary structure, and then the secondary one, and the protein remains in the form of a polypeptide chain. As a result of denaturation, the protein loses its ability to perform its function.
Violation of the quaternary, tertiary and secondary structures is reversible. This process is called renaturation.
The destruction of the primary structure is irreversible.
In addition to simple proteins, consisting only of amino acids, there are also complex proteins, which may include carbohydrates (glycoproteins), fats (lipoproteins), nucleic acids (nucleoproteins), etc.
- Catalytic (enzymatic) function. Special proteins – enzymes – are capable of accelerating biochemical reactions in the cell tens and hundreds of millions of times. Each enzyme speeds up one and only one reaction. The enzymes contain vitamins.
- Structural (building) function is one of the main functions of proteins (proteins are part of cell membranes; keratin protein forms hair and nails; proteins collagen and elastin – cartilage and tendons).
- Transport function – proteins provide active transport of ions across cell membranes (transport proteins in the outer membrane of cells), transport of oxygen and carbon dioxide (hemoglobin of blood and myoglobin in muscles), transport of fatty acids (serum proteins promote the transfer of lipids and fatty acids, various biologically active substances).
- Signal function. The reception of signals from the external environment and the transmission of information to the cell occurs due to proteins built into the membrane, which are capable of changing their tertiary structure in response to the action of environmental factors.
The contractile (motor) function is provided by contractile proteins – actin and myosin (thanks to contractile proteins, cilia and flagella in protozoa move, chromosomes move during cell division, muscles contract in multicellular organisms, other types of movement in living organisms are improved).
- Protective function – antibodies provide the body’s immune defense; fibrinogen and fibrin protect the body from blood loss by forming a blood clot.
- The regulatory function is inherent in proteins – hormones (not all hormones are proteins!). They maintain constant concentrations of substances in the blood and cells, participate in growth, reproduction and other vital processes (for example, insulin regulates blood sugar).
Energy function – during prolonged fasting, proteins can be used as an additional source of energy after carbohydrates and fats have been consumed (with complete breakdown of 1 g of protein to end products, 17.6 kJ of energy is released). The amino acids released by the breakdown of protein molecules are used to build new proteins.