MadSci Network: Biochemistry |
Catalases normally utilize a ferric ion (Fe (III)) as part of their catalytic machinery. The ferric ion is at the center of a heme group. You have probably heard of hemes before – hemoglobin is the compound that makes your blood red. You can see a nice picture of a heme, with its bound iron, at: http://www.madsci.org/posts/archives/dec96/847573455.Bc.r.html Other metal ions can substitute for the ferric ion in the heme, and carry out the role usually performed by the ferric ion in catalysis, but as you have observed, they are not as effective at catalyzing the reaction. When an enzyme is in a solution containing other metal ions, these ions can displace the ferric ion from the heme. There will be an equilibrium between heme with bound ferric ion and that with the other competing ion; in other words, some of the enzyme molecules will have a heme with a ferric ion bound, others will have a copper ion in the heme. In most solutions the concentration of the enzyme is very low compared to the concentration of other species (in your case, copper sulfate). Enzyme concentrations are typically in the nanomolar range. Thus when enzyme containing ferric ion is added to a solution containing a micromolar or millimolar concentration of copper ions (which would be 1,000 and 1,000,000 times more concentrated than nanomolar), the equilibrium will lie far to the side of the copper bound form.
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