Enzyme Catalysis -A Brief Introduction
Author: Mankaran SinghIntroduction
Many biological processes have a slow rate of reaction or might be going through a longer path in which they might consume more energy. So we have special biocatalysts, called enzymes. Enzymes are biomolecules of protein with high relative molecular mass generally derived from living organisms with their catalytic properties; they take these slow reaction processing in a way that the rate of the reaction alters as per the need of the process by lowering the activation energy. They ensure that the reaction processing is done in a compatible manner with life. The substrate or the reactants get bound to the enzyme's special sites and process them to new products by breaking the bonds between them. For example, yeast produces enzyme zymase which converts glucose to ethanol and releases carbon dioxide and energy as byproducts.
Enzymes vs Catalysts
Enzymes are used to increase the rate of biological reactions whereas inorganic catalysts are employed to alter the rate of chemical reactions. Enzymes are more efficient than catalysts, and the reason behind this is hidden in the geometry of the enzymes; they have special arrangements that help the growth of the reaction in a faster way and hence making the transition state more stable.
The other reason behind enzymatic efficiency is the formation of substrate-enzyme complexes which form during the mechanism of the process. The changes in the geometry of the substrate due to complex formation makes the reaction free energy efficient to proceed with the reaction.
An inorganic catalyst usually can be used for various chemical reactions, the same catalyst when used against various reactants at different conditions of temperature and pressure gives different products whereas enzymes are specific. Each substrate needs a specially designed enzyme for the catalytic action.
Interaction of the substrate and enzyme
Emil Fischer, a German Scientist proposed a theory on the mechanism of enzyme activity- Lock and key theory which tells how the substrate (or reactant) reacts to the enzyme and concept of active sites.
The enzymes have a special structure that has some vacant spaces on it. These spaces are called active sites and substrates get accumulated at these active sites. After getting successfully attached to the enzyme the substrate starts converting to the products, the bonds get dissociated and products get detached from the enzymes. Substrate and enzyme must interact in a way that there must be no hindrance in their binding, the binding must be in a proper orientation and there must be proper groups for binding.
The Mechanism and kinetics of Enzyme catalyzed reactions.
Biochemists L. Michelis and Mary Menten in the early 1900s given a mechanism for the kinetics of the enzyme-catalyzed reactions anticipate following steps;
Step 1: Enzyme-Substrate Complex formation
E + S ⇆ ES …(step 1)
Step 2: Complex Decomposition
ES → P + E …(step 2)
ES: Enzyme-Substrate complex
The reaction in step 1 utilizes the enzyme with rate constant K₁ for the forward reaction and K₋₁ for the backward reaction. It gives back the enzyme in step 2 with rate constant K₂. If the total amount of enzyme present is assumed to be [Eo], [E] being the concentration of enzyme which is unbound and [ES] represent the amount of enzyme-bound by substrate then by conservation equation of enzyme:
[Eo] = [E] + [ES]
This equation gives the equilibrium between the bound and unbound the enzyme.
Rate, r = K₂[Eo][S] / (Km + [S])
Km is known as the Mi
This is known as the Michaelis-Menten equation.
The rate will be maximum when the substrate is present at higher concentration, thus
Rmax = K₂[EO]
And the graph between the concentration of substrate and rate of the reaction can be plotted on the basis of the above equation as:
From the graph, it can be observed that at a low concentration of substrate the rate of the reaction increases gradually and follows first-order kinetics. The active sites at low concentration of substrate mostly remain unoccupied and adding more substrate increases the substrate enzyme binding and hence more complex forms and the reaction increases but at a very high concentration of substrate no more binding can occur and complex formation stops and the rate becomes zero-order kinetics. Hence the enzyme-catalyzed reactions depend on the substrate concentration.
Enzymes, the biocatalyst has proven to be efficient in enhancing the rate of the bio reactions. The enzymes when successfully attracts the substrate successfully catalyzes it by providing essential necessities to it which efficiently converts them into products. The enzyme reaction rate is proportional to the concentration of substrate shows dual kinetics- first and zero-order.
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