SULPHHYDRYL GROUPS (synonym: thiol groups, mercapto-groups) — functional groups in molecules of organic compounds, including proteins, in to-rykh these groups belong to the remains of cysteine; play an essential role in creation and maintenance of the native structure of proteins defining their specific functional properties. Functions This year in proteins (see), in particular in enzymes (see), are very various. In nek-ry enzymes — papain (see), a dehydrogenase of a 3-fosfoglitseraldegid, etc. they play a catalytic role, i.e. directly participate in formation of intermediate compounds during the reaction catalyzed by enzyme. Reversible transformation This year into disulfide groups (— S — S —) is revealed in active centers nek-ry oxidoreductases (see), napr, in a glutationreduktaza (KF 1. 6. 4. 2), in a lipoamid-dehydrogenase (KF 1. 6. 4. 3), in a tioredoksinreduktaza (KF 1. 6. 4. 5); this transformation plays an important role in electron transfer and protons from substrates to acceptors. In many proteins and enzymes — the ferredoksena, the rubredoksena, the metallo-tioneena, cytochrome with (see. Tsitokhroma ), monoamine oxidase (see), to alcohol dehydrogenase (see), etc. SH groups participate in formation of bonds between protein and an ion of metal or a coenzyme (see. Coenzymes ). This year play also important role in education and stabilization of native three-dimensional structure of proteins, forming in them intramolecular bonds: hydrogen, coordination (with participation of an ion of metal) and — S — S-bonds. The last provide a rigid fastening or separate polypeptide chains, napr, in immunoglobulins (see), or various sites of the same chain, napr, in lysozyme (see), pepsin (see), papain, albumine of blood serum (see. Albumine ). Contents — S — S-bonds in proteins of hair, wool, horns, hoofs and nails — keratins is especially big (see); — S — S-bonds are found also in proteinaceous and peptide hormones (see) — insulin (see), vasopressin (see), oxytocin (see), somatostatin (see) and somatotropic hormone (see). Splitting (e.g., recovery) — S — S-bonds in these hormones (except for somatotropic hormone) leads to falloff them biol. activities.
Disturbance of a row fiziol. and biochemical processes (e.g., cell fissions, muscular contraction, oxidizing phosphorylation, photosynthesis, etc.) under the influence of thiol reagents (SH reagents), i.e. substances which are selectively reacting with SH groups is explained by blocking of enzymes and other proteins This year, and also This year low-molecular, functionally important thiols — a coenzyme And, 4 '-fosfopantoteina (see. Vitamins ), lipoic acid (see) and glutathione (see), the cofactors which are carrying out a role in various fermental systems. Successful use of dithiols, such as the BALL (2,3 dimercaptopropyl alcohol) and Unithiolum (2,3 dimercaptopro-pan-sodium sulphonate), at treatment of the poisonings caused by connections arsenic (see) or heavy metals (see. Antidotes of OV ), is explained by ability of dithiols to deblock This year
This year are distinguished from other functional groups of proteins with high reactivity and variety of chemical reactions, in to-rye they enter. On reactivity in proteins distinguish three types This year: easily available modifications which are partially disguised and completely disguised to-rye are available to modification only after a denaturation of protein. The most important chemical properties are This year: relative ease of their oxidation with formation of a disulfide bridge (— S — S-bonds); formation of acylmonothioesters at interaction with the acylating reagents, napr, anhydrides organic to - t; education of S-carbamile group at reaction with cyanate; alkylation haloid-acids and connections with the polarized or easily polarized double bond, e.g. N-etilimidom maleic to - you; arylating ftordinitrobenzoly and quinones; interaction with aldehydes (see) and ketones (see) with formation of hemimercaptols and semi-mercaptols; interaction with ions of heavy metals and organo-mercuric compounds with formation of slabodissotsiiruyushchy mercaptides; reaction with compounds of trivalent arsenic with education mono - and ditioarseni-comrade react also tioldisulfidny exchange This year: 2RS - + R'S-SR' <-> 2R'S - + RS — SR, where R and R' — organic radicals. In this and in the majority of other reactions participate in a form of a merkaptidny ion of RS-This year. On reaction of tioldisulfidny exchange between — S — S-groups of a keratin of hair and inorganic sulfides (e.g., barium sulfide) is based use of the last with the cosmetic purposes in quality of hairremovers (see. Epilation ). Exchange reaction especially easily proceeds with aromatic disulfides, napr, with 5,5 '-ditiobis-(2 nitrobenzoate) — Ellman's reagent, and quite often stops at an intermediate stage of the mixed disulfide:
This reaction, and also reaction with n-hlormerkuribenzoatom is most widely used for quantitative spektrofotometrichesky definition in proteins This year. This year determine also by amperometrichesky titration by solutions of AgNO silver nitrate 3 or HgCl mercury bichloride 2 .
This year are poorly ionized; they are capable to form hydrogen bindings with electronegative groups, however much weaker, than the similar bonds formed by hydroxylic groups (IT - groups).
Bibliography: Torchinsky Yu. M. It is gray in proteins, M., 1977; Friedman M. Chemistry and biochemistry of the sulf-hydryl group in amino acids, peptides and proteins, Oxford — N. Y., 1973; Jocelyn P. C. Biochemistry of the SH group, L. — N. Y., 1972.
Yu. M. Torchinsky.