|MadSci Network: Biochemistry|
Thanks for your question. The precise answer to it is probably pretty complicated, because many things about a reaction change as the pH changes. I'll give it a shot, though, and give you some references for more information. Catalase is an enzyme (a protein) composed of four identical units. All proteins are made of amino acids, molecules which are sensitive to pH. As pH changes, the amino acids can gain or lose hydrogens. This affects the charge of the amino acid, and the resulting charge of the protein. If the charge of a protein changes, the protein can change "conformation" or shape. In its new shape, it can have a harder (or easier) time interacting with other proteins, or with substrates. Also, each unit of catalase holds onto a structure called a "heme" group. You may have heard of the "heme" in hemoglobin, the molecule that carries oxygen in the blood. Heme groups are complicated structures that hold onto an iron molecule, a charged atom that can be used to provide the correct interactions for a chemical reaction. Catalase binds to two H2O2 molecules at a time, and converts them to 2 waters and molecular oxygen. The kinetics, or how fast catalase can convert H2O2, changes with pH. This could be because of changes in affinity for H2O2, or changes in the interactions of amino acids with the H2O2 that actually convert it to water and oxygen. Although I did find several articles that indicated that catalase was inhibited (less active) at acid pH, and one article that had maximum activity at a pH around pH 10, it is unusual for cells in the body to have such an alkaline pH. Therefore, catalase also does work at lower pH. Here are two articles about catalase: LeHir etal Histochemistry Nov. 1979 64(1):51-66 Abe, K etal Journal of Biochemistry Feb. 1979 85(2):473-9 And two really good web sites: bioinf.leed.ac.uk/promise/CATALASE.html www.facstaff.buknell.edu/toner/gb/lab121/labs34.html Hope that is helpful. Erin
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