Biochemistry

Sulfur is a fairly common element on our planet (0.05% of the total mass of the earth's crust). Native sulfur is found in nature, but most of its reserves are in the form of compounds - sulfides and sulfates. The most characteristic oxidation states of sulfur are -2, +4 and +6, which allows it to form a variety of compounds with both electropositive and electronegative elements. In organisms, sulfur with an oxidation state of -2 is present in amino acids and proteins, as well as in lipids, vitamins (Bl) and hormones (insulin, vasopressin). The most important sulfur compound with an oxidation state of -2 is H 2 S hydrogen sulfide. In nature, it is formed during rotting as a result of the activity of disulfurizing and anaerobic sulfur bacteria. Hydrogen sulfide is highly toxic primarily because of its ability to firmly bind to copper cations, which are part of the respiratory chain enzymes. Important biological functions are performed by the sulfhydryl group -SH, which is part of proteins and some other compounds. Firstly, the -SH group strongly binds to the cations of many metals, forming metalloproteins that play the role of enzymes. Secondly, compounds with the -SH group are strong reducing agents, and therefore they perform a protective function in the cell and protect it from the action of oxidizing agents (they serve as antioxidants). Finally, -SH groups in the composition of protein molecules play another important role: undergoing oxidation, they participate in the formation of disulfide bridges — S-S-. As a result, the protein molecule acquires a different spatial structure (conformation). Especially a lot of disulfide bridges in the proteins of the hair, hair, nails, as well as in the nervous tissue, in cartilage and bones. The bonds — S - S — give the protein molecules of these substances the necessary rigidity. Selenium is an element not widely distributed in nature. Unlike sulfur, with which selenium is similar in chemical properties, this element is contained in biological quantities in micro quantities. Being an analogue of sulfur, selenium replaces it in the -SH group and in disulfide bridges — S- S-. Like sulfur, selenium performs a protective function in the body. It protects proteins from the action of such strong oxidizing agents as hydrogen peroxide, as well as from heavy metal cations - mercury and cadmium. The participation of selenium in the actions of some enzymes that regulate the oxidation of lipids and proteins is traced. Derivatives of selenium having a radioprotective (radioprotective) action are also known. At the same time, it has been proven that selenium has a damaging effect on tumor cells, and therefore its compounds are considered as potential anticarcinogens. 10