Decomposition of Hydrogen Peroxide with Catalase

Demonstration Experiment on Video

Decomposition of Hydrogen Peroxide with Catalase

Objectives: Enzyme Catalysis, Denaturation of Enzymes by Heavy Metal Salts

Peter Keusch

German version

Chemicals:
hydrogen peroxide, 30% aqueous solution
mercury(II) chloride, 0.1% aqueous solution

Glass wares:

4 petri dishes (d = 10 cm)
3 volumetric pipettes 30 mL
3 pipette bulbs
metal forceps

Hazards and safety precautions:

Mercury(II) chloride is a poison. May be fatal if swallowed or inhaled. Chronic exposure may lead to systemic effects and build-up of mercury in the brain, liver and kidneys. May cause memory loss, tremors and other serious effects.

Hydrogenperoxide 30% is toxic, corrosive - can cause serious burns. Eye contact can cause serious injury, possibly blindness. Harmful by inhalation, ingestion and skin contact.

Safety goggles and protective gloves must be worn.

Experimental procedure:

Four petri dishes are set up as follows:
Dish A: 30 mL of water + 1 slice of uncooked potato
Dish B: 30 mL of hydrogen peroxide solution
Dish C: 30 mL of hydrogen peroxide solution
Dish D: 30 mL of mercury(II) chloride solution + 1slice of uncooked potato

Using metal forceps, the two potato chunks (from dish A and D, respectively) are transfered to the H₂O₂ solutions of dish B and C, respectively.

Video clip (Download RealPlayer .rm file)

Results:

The potato slice soaked with HgCl₂ solution remains nearly unchanged (dish B). Only some few bubbles can be observed after some time. In the other petri dish (C) immediately an intense gassing occurs on the surface of the untreated potato slice. A foam filled with bubbles is formed around the potato sliver.

Discussion:

Catalase in the damaged potato cells decomposes hydrogen peroxide into water and oxygen. The potato slice which was treated with HgCl₂ solution has become poisoned. The enzyme catalase is deactivated. Any heavy metal ion (such as mercury cations in mercury(II) chloride or copper cations in copper(II) sulfate) will act as a noncompetitive inhibitor on catalase.

Catalase contains four porphyrin heme (iron) groups which allow the enzyme to react with the hydrogen peroxide. While complete mechanism of catalase is not currently known, the reaction is believed to occur in two stages:

Fe ( ) - E represents the iron centre of the heme group attached to the enzyme E. Fe (IV) - E (.+) ís a mesomeric form of Fe (V) - E, meaning that iron is not completely oxidized to + V but receives some "supporting electron" from the heme ligand. This heme has to be drawn then as radical cation (.+).

Hydrogen Peroxide, upon entering the heme cavity, is severely sterically hindered and must interact with His74 and Asn147. It is in this position that the first stage of catalysis takes place. Transfer of a proton from one oxygen of the peroxide to the other, via His74, elongates and polarizes the O-O bond, which eventually breaks heterolytically. This reaction causes the release of one water molecule and coordination of the second peroxide oxygen atom to the iron center of the entyme. The resulting intermediate O = Fe (IV) - E (.+) is two oxidizing equivalents above the resting state. Two electrons are transferred from the enzyme to the coordinated oxygen atom, one from the iron and one from the porphyrin. During the second stage, in a similar two electron transfer reaction, O = Fe (IV) - E (.+) reacts with a second hydrogen peroxide to reform Fe(III) - E and produce water and oxygen.

Reference:
Computer-Interfaced Experiments Enzyme Kinetics: Enzymatic Decomposition of Hydrogen Peroxide

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