![]() ![]() In both cases, NADPH is the ultimate electron donor, reducing FAD back to FADH 2 in each catalytic cycle. GSH rather than GSSG) by a flavin-dependent enzyme called glutathione reductase.ĭisulfide bridges in proteins can also be directly reduced by another flavin-dependent enzyme called 'thioredoxin'. ![]() Inside the cell, cysteines are kept in their reduced (free thiol) state by a high intracellular concentration of GSH, which in turn is kept in a reduced state (ie. In its reduced (thiol) state, glutathione can reduce disulfides bridges in proteins through the reverse of the above reaction.ĭisulfide bridges exist for the most part only in proteins that are located outside the cell. The end result is that a new cysteine-cysteine disulfide forms at the expense of the disulfide in GSSG. In its oxidized form, glutathione exists as a dimer of two molecules linked by a disulfide group, and is abbreviated 'GSSG'.Ī new disulfide in a protein forms via a 'disulfide exchange' reaction with GSSH, a process that can be described as a combination of two S N2-like attacks. In its reduced (free thiol) form, glutathione is abbreviated 'GSH'. Recall that the important functional group in glutathione is the thiol, highlighted in blue in the figure below. The redox agent that mediates the formation and degradation of disulfide bridges in most proteins is glutathione, a versatile coenzyme that we have met before in a different context ( section 14.2A). Notice that in the oxidized (disulfide) state, each sulfur atom has lost a bond to hydrogen and gained a bond to a sulfur - this is why the disulfide state is considered to be oxidized relative to the thiol state. The interconversion between thiols and disulfide groups is a redox reaction: the thiol is the reduced state, and the disulfide is the oxidized state. Table 18.1, below, provides a quick comparison of oxygen-containing and sulfur-containing organic compounds.ĭisulfide (sulfur-sulfur) linkages between two cysteine residues are an integral component of the three-dimensional structure of many proteins. You need not memorize the methods used to carry out these oxidations. sulfoxides and sulfones are obtained by oxidizing organic sulfides. Later we shall see examples of tetraalkylammonium ions, R 4N +, which again may be regarded as being similar to hydronium ions. If you have trouble understanding why trialkylsulfonium ions are formed, think of them as being somewhat similar to the hydronium ions that are formed by protonating water: Thiolate anions are better nucleophiles than are alkoxy anions (see Section 11.5, pages 389-394 of the textbook). Thiolate anions, RS-, are analogous to alkoxy anions, RO. ![]() For example, SO 4 2 − is the sulfate ion while S 2O 3 2 −, in which one of the oxygen atoms of a sulfate ion has been replaced by a sulfur atom, is called thiosulfate. Notice that the term “thio” is also used in inorganic chemistry. The nomenclature of sulfides can be easily understood if one understands the nomenclature of the corresponding ethers. (Organic) sulfides have the structure R-S-R′, and are therefore the sulfur analogues of ethers. A disulfide is a compound containing an -S-S- linkage. The -SH group itself is called a mercapto group. ![]() A thiol is a compound which contains an SH functional group. However, we have included a short section on these compounds, not for the sake of increasing the amount of material to be digested, but because much of the chemistry of these substances can be predicted from a knowledge of their oxygen-containing analogues. The chemistry of sulfur-containing organic compounds is often omitted from introductory organic chemistry courses.
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