Notes on Catalase for Biochemistry

by on January 30th, 2011
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Catalytic strategies:
serves to protect the cell from the toxic effects of hydrogen peroxide by catalyzing its decomposition into molecular oxygen and water without the production of free radicals. nonproteinaceous group, called the prosthetic group, that is important in the actual catalysis.
However, some enzymes that only have a single substrate do not fall into this category of mechanisms. Catalase is an example of this, as the enzyme reacts with a first molecule ofhydrogen peroxide substrate, becomes oxidised and is then reduced by a second molecule of substrate. Although a single substrate is involved, the existence of a modified enzyme intermediate means that the mechanism of catalase is actually a ping-pong mechanism, a type of mechanism that is discussed in the Multi-substrate reactions section below.
While complete mechanism of catalase is not currently known, the reaction is believed to occur in two stages: H2O2 + Fe(III)-E → H2O + O=Fe(IV)-E H2O2 + O=Fe(IV)-E → H2O + Fe(III)-E + O2[7] (where Fe()-E represents the iron centre of the heme group attached to the enzyme.) As hydrogen peroxide enters the active site it interacts with the amino acids Asn147 andHis74 , causing a proton (hydrogen ion ) to transfer between the oxygen atoms, polarizing and stretching the O-O bond, which breaks heterolytically. The free oxygen atom coordinates with the iron centre of the active site, freeing the newly formed water molecule and forming Fe(IV)=O. Next, the Fe(IV)=O reacts with a second hydrogen peroxide molecule to reform Fe(III)-E and produce water and oxygen.[7] The reactivity of the iron center may be improved by the presence of the phenolate ligand of Tyr357 in the fifth iron ligand , which can assist in the oxidation of the Fe(III) to Fe(IV). The efficiency of the reaction may also be improved by the interactions of His74 and Asn147 with reaction intermediates .[7] Generally, the rate of the reaction can be determined by the Michaelis-Menten equation .[4] Catalase can also oxidize different toxins, such as formaldehyde , formic acid , and alcohols . In doing so, it uses hydrogen peroxide according to the following reaction: H2O2 + H2R → 2H2O + R Again, the exact mechanism of this reaction is not known. Any heavy metal ion (such as Copper cations in Copper(II) sulfate ) will act as anoncompetitive inhibitor on catalase. Also, the poison cyanide is a competitive inhibitor of catalase, strongly binding to the heme of catalase and stopping the enzyme’s action. Three-dimensional protein structures of the peroxidated catalase intermediates are available at the Protein Data Bank . This enzyme is commonly used in laboratories as a tool for learning the effect of enzymes upon reaction rates.

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