RELEVANT LEARNING OUTCOME:

(a) Explain the mode of action of enzymes in terms of an active site, enzyme-substrate complex, lowering of activation energy and enzyme specificity using the lock-and-key and induced-fit hypotheses.

Summary of Overall Mechanism of Enzyme Action

1. When the substrate molecule happens to collide with the active site of the enzyme at the right orientation, enzyme’s active site will bind with the substrate. 

2. This binding of substrate with active site causes the conformational change of the enzyme’s active site to better fit the substrate (induced fit hypothesis).

3.     The enzyme’s active site is now complementary in shape to its substrate. The amino acid R-groups that form the active site interact specifically with the substrate in a chemically attractive manner.

4. An enzyme-substrate complex will thus be formed.

5. The enzyme’s contact amino acids will hold the substrate molecules in an orientation that will favour the reaction to occur.

6. In addition, the specific spatial arrangement of the substrate molecules at the active site may cause physical stress to certain bonds in the substrate molecules. This will increase the likelihood that the bonds will break.

7. The R groups of the enzyme’s catalytic amino acids can also change the charge on the substrate molecules, alter the distribution of electrons within the bonds of the substrate molecules, or cause other changes that will increase the reactivity of the substrate molecules.

8. Hence, in the presence of the enzyme, the initial amount of energy (activation energy) required for product formation is lower. Its reaction rate is thus faster.

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