Lock and Key hypothesis

 Lock and Key hypothesis

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This hypothesis was given by Emil Fischer (1894). According to this hypothesis, both enzyme and substrate molecules have specific geometrical shapes. It is similar to the system of lock and key, which have, special geometrical shapes in the region of their activity. Just as a lock can be opened by its specific key, a substrate molecule can be acted upon by a particular enzyme.

This also explains the specificity of enzyme action. After coming in contact with the active site of the enzyme, the substrate molecules or reactants form a complex called enzyme substrate complex. In the enzyme substrate complex, the molecules of the substrate undergo chemical change and form products.

This theory explains how a small concentration of enzyme can act upon a large amount of the substrate. It also explains how the enzyme remains unaffected at the end of chemical reaction.

Enzyme + Substrate → Enzyme - Substrate Complex Enzyme - Substrate → Complex Enzyme + End Products

Next >> Induced fit hypothesis. 

Induced fit hypothesis

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This hypothesis was proposed by Koshland (1960). According to this hypothesis the active site of the enzyme does not initially exist in a shape that is complementary to the substrate but is induced to assume the complementary shape as the substrate becomes bound to the enzyme. There are two ends

 

(a) Buttressing group is meant for supporting the substrate 

(b) Catalytic group is meant for catalysing the reaction. When substrate comes in contact with the buttressing group, the active site changes to bring catalytic group opposite to the substrate.

Next >> Factors affecting Enzyme Activity