Question

How is metal ion catalysis an effective catalytic strategy? Give two possible roles for a metal ion in enzyme catalysis and talk about how each role helps to lower the activation energy of the reaction.

Answer 1

Answer:

Many enzymes require presence of certain metallic cations for their optimum activity. Metallic cations normally seen working as activators for enzyme include Mg2+, Mn2+, Zn2+, Ca2+, Co2+, Cu2+, Na+, K+, etc. Rarely anions are also needed for optimum enzyme activity as seen in case of Cl- needed for activity of enzyme amylase.

Metals seem to work through various mechanisms to activate enzymes and increase enzyme velocity, as such they combine with substrate, through formation of Enzyme-Substrate-Metal complex. They may also participate directly in the reaction by bringing about a conformational change in the enzyme.

There can e identified two categories of enzymes that depend on metallic ions:

a) Metal-activated enzymes: In this category metal is not tightly held by the enzyme and hence can be exchanged easily with other ions.

eg. ATPase with Mag2+ / Ca2+ and Enolase with Mg2+

b) Metalloenzymes: These enzymes make stronger bonds with metal, holding them tightly and do not exchange it readily.

eg. Phenol Oxidase with Cu and Pyruvate oxidase with Mn and Xanthine oxidase with Mo and Alcohol hydrogenase with Zn as well as Cytochrome Oxidase with either Fe or Cu.

The presence of metals or metal ions in these enzyme activate enzymes directly without using cell energy and affect the velocity of the reaction positively and hence reducing the activation energy required for the enzyme action.