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Jul 15, 2008



Passage: Oxidative stress damages cell membranes, proteins, and other cellular structures. Antioxidant defenses such as ascorbate play a key role in reducing the effects of oxidative stress. An example of antioxidant defenses is the ascorbate-glutathione cycle which helps to process H2O2 to H2O in the cell, according to the following simplified scheme:

The red couplings indicate enzymes which catalyze the oxidation of one molecule and result in the subsequent reduction of another molecule.

Question 1: Catalase is another important enzyme which degrades H2O2 into less harmful byproducts. The catalase test is used to help differentiate bacterial species. In the catalase test, a drop of H2O2 is placed on a colony that is being tested. Which of the following would identify a catalase-positive colony?

(a) blackening of the colony
(b) formation of bubbles
(c) absorption of H2O2 to form a large mass
(d) H2O2 will have no observable effects on a catalase-positive colony


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