Why DsbA is Such an Oxidizing Disulfide Catalyst Essay -- Biology Enzy

"Why is DsbA Such an Oxidizing Disulfide Catalyst?" Introduction The importance of the DsbA enzyme is due to its protein-folding ability in Escherichia coli bacteria. Protein is an essential part of all forms of life. Genes code for proteins, both structural and enzymes responsible for the utilization of other macromolecules. All bodily functions, therefore, are dependent upon proteins. Every protein, or polypeptide chain, uses the amino acid cysteine, which contain dithiol molecules. IN E. coli, this reversible redox reaction takes place between the endoplasm in the center of the cell and the periplasm surrounding it (see figure A below). The endoplasm is a reduced environment which contains thioredoxin, the catalyst responsible for reducing disulfide molecules to dithiol and maintaining the reduced state of the endoplasm. The periplasm, on the other hand, is oxidized. It contains DsbA, which catalyzes the oxidation of dithiol to disulfide. Proteins essential for cell function are manufactured within the reduced endoplasm. After the polypeptide c hain has been formed, however, the macromolecule must fold into its tertiary structure, which is essential for function. This structure utilizes disulfide and hydrogen bonds. Hydrogen bonds require the presence of oxygen, so the protein must enter an oxidized environment. The protein is exported into the periplasm, therefore, where disulfides are able to aid in the folding process. The finished protein is now able to function normally. Question While studying this process, the researchers noted that the structures of DsbA and thioredoxin are remarkably similar. What then, they wondered, is responsible for the difference in function between the two? Why does DsbA oxidize and thio... ..., has been disproved by several observations. One of these is that there is no evidence of strain in the active site disulfide of DsbA because all bond angles are close to optimal and when superimposed on the active site of the much less oxidizing thioredoxin, the position of the carbons match. His-32 also seems to play an important role in determining the oxidizing power of DsbA. His-32 is found in DsbA, and not in the less oxidizing thioredoxin. The effectiveness of DsbA as a catalyst depends on its redox potential, which can be easily measured, however, it also depends on the kinetics at which it participates in the disulfide interchanges, which is much more difficult to measure. The factors determining the redox potential of DsbA are becoming clear, but many questions still remain unanswered about how DsbA reoxidizes and how it kinetically interacts with proteins.