Читать книгу Snyder and Champness Molecular Genetics of Bacteria - Tina M. Henkin - Страница 226

Another characteristic of proteins that are exported to the periplasm or secreted outside the cell is that many of them have disulfide bonds between cysteines (see the inside front cover). In other words, two of the cysteines in the protein are held together by covalent bonds between their sulfides. The sulfur atom of a cysteine in a disulfide bond is in its oxidized form because one of its electrons is shared by the two sulfurs, while the sulfur atom of an unbound cysteine is in its reduced form because it has an extra electron. These disulfide bonds can be between two cysteines in the same polypeptide chain or between cysteines in different polypeptide chains. Exported proteins need the covalent disulfide bonds to hold them together in the harsh environments of the periplasm and outside the cell. Failure to form the correct disulfide bonds or formation of disulfide bonds between the wrong cysteines can result in inactivity of the protein.

The disulfide bonds in proteins are formed by enzymes called disulfide oxidoreductases (DsbA, DsbB, etc.) as the proteins pass through the oxidizing environment in the periplasmic space between the inner and outer membranes of bacteria that contain outer membranes (or at the outer cell surface of bacteria that lack the outer membrane and periplasm). Proteins that are found inside the cell in the cytoplasm lack disulfide bonds because of the “reducing atmosphere” inside the cytoplasm due to the presence of high concentrations of small reducing molecules, such as glutathione, thioredoxin, and bacillithiol. In fact, the appearance of disulfide bonds in some cytoplasmic regulatory proteins is taken as a signal by the cell that oxidizing chemicals are accumulating in the cell and that proteins should be made to combat the potentially lethal oxidative chemical stress.