Question

How do metal salts affect catalase activity? Why are some metal salts considered to be activators and others are considered to be inhibitors? How do you know if a metal salt is an activator or inhibitor? For example, between K, Mg, Zn, Ca, and Fe, which metal salt would be the activator and which would be the inhibitor? How do you know?

Answer 1

How do metal salts affect catalase activity?

Basic Mechanism:

Catalase is an important enzyme found in the skin, which scavenges H2O2 and dismutases it into the water and molecular oxygen thus reducing the damaging effects of H2O2 in the biological system.

While the 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[18]

Here Fe()-E represents the iron center of the heme group attached to the enzyme. Fe(IV)-E(.+) is a mesomeric form of Fe(V)-E, meaning the iron is not completely oxidized to +V but receives some stabilizing electron density from the heme ligand, which is then shown as a radical cation (.+).

As hydrogen peroxide enters the active site, it interacts with the amino acids Asn148 (asparagine at position 148) and His75, causing a proton (hydrogen ion) to transfer between the oxygen atoms. The free oxygen atom coordinates, freeing the newly formed water molecule and Fe(IV)=O. Fe(IV)=O reacts with a second hydrogen peroxide molecule to reform Fe(III)-E and produce water and oxygen. The reactivity of the iron center may be improved by the presence of the phenolate ligand of Tyr358 in the fifth coordination position, 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 His75 and Asn148 with reaction intermediates. The decomposition of hydrogen peroxide by catalase proceeds according to first-order kinetics, the rate is proportional to the hydrogen peroxide concentration.

Catalase can also catalyze the oxidation, by hydrogen peroxide, of various metabolites and toxins, including formaldehyde, formic acid, phenols, acetaldehyde and alcohols. It does so according to the following reaction:

H2O2 + H2R → 2H2O + R

The exact mechanism of this reaction is not known.

Role of metal ion on the activity:

Any heavy metal ion (such as copper cations in copper(II) sulfate) can act as a noncompetitive inhibitor of catalase. Furthermore, the poison cyanide is a noncompetitive inhibitor of catalase at high concentrations of hydrogen peroxide. Arsenate acts as an activator. Metal salts affect the catalase activity. Previous studies have elaborated that catalase can be inhibited by certain metal ions (including Cu2+, Zn2+, and Ag+), a process depends on the metal, concentration, the tissue, and species. Lee et al. compared several divalent metal ions on catalase‐peroxidase (KatG) activity, only the manganese ion revealed some inhibitory effects on the recombinant KatG activity, and EDTA could relieve partly inhibited activity. This implies that manganese may competitively bind to near the heme group and be involved in the enzyme reaction.

Although, the metal ions of Na+, K+ can act as activator depending on the type of catalase enzyme.