Mechanism of Enzyme Catalysis
Lowering of activation energy
The enzyme promotes the given reaction, but it remains unchanged. In 1913 Leonar Michael is and Maud Menten proposed that an intermediate enzyme-substrate complex is formed during enzyme activity. This can be schematically represented as-
Enzyme (E) + Substrate(S) <—> Enzyme-Substrate Complex (ES) —» Enzyme (E) + Product (P)
Enzyme performs its work by lowering the activation energy (The energy required to bring a substance to its reactive state. Or The energy required for the reactants to undergo product). The reduction in activation energy causes the reaction to proceed at a lower temperature.
The enzyme combines with the substrate to produce a transition state requiring low energy. In other words, the combination of the substrate with the enzyme creates a new reaction pathway that has a transition state of lower energy than in the absence of the enzyme. Also, the enzyme does not alter the equilibrium constant, they only enhance the velocity of the reaction.
Example- Decomposition of hydrogen peroxide without enzyme requires 765 KJ‘mole, while in the presence of enzyme energy required is < 8 KJmole.
The mechanism of a reaction between enzyme and substrate can be explained by the following two theories-
a). Lock and Key model
It is the first model proposed to explain enzyme action. It was proposed by Emil Fischer; hence it is also called a ‘Fischer model’
According to this model, the structure or conformation of the enzyme is rigid. The substrate binds to the active site just like a key that fits into the proper lock.
Thus according to this concept, a structurally well-defined catalytic site will accept only those substrate molecules which have a matching shape and will repel others that differ structurally. This hypothesis is rather attractive since it provides a simple explanation for the specificity of enzymatic action.
- This model does not give any explanation regarding the flexible nature of the enzyme.
- This model fails to explain many facts of enzymatic reactions like allosteric modulations.
b). Induced fit theory
It is also called a ‘Koshland model’ having an essential feature of ‘flexibility. According to this model, the active site of an enzyme is not rigid and pre-shaped, instead of that, it is flexible.
In this induced-fit model, the substrate induces a conformational change in the enzyme resulting in the formation of a strong substrate binding site. Due to induced fit, the appropriate amino acids of the enzyme are repositioned to form an active site and bring about catalysis. This model has sufficient experimental evidence and also explains the allosteric modulations and competitive inhibition of the enzyme.