There are two proposed models for explain the enzyme substrate interactions.
- Lock and key model
- Induced fit model
1. Lock and key model.
This model is proposed by Emil Fisher in 1984 as a result of studies carried out on enzyme specificity. This hypothesis states that the enzymes are structurally complementary to their substrates, as they can fit together like a lock and key. Active sites of the enzyme is perfectly shaped space for the shape of the substrate.
According to this hypothesis active sites of the enzymes are inflexible hence cannot change their shape complementary to variety of substrates. They can bind only with a particular type of substrate having complementary shape. So the enzymes are specific to their substrates. This is known as the enzyme – substrate.
The substrates bind with the active sites having complementary shape on the enzyme. Then the enzyme – substrate is formed and reaction is performed resulting the final products. Enzymes are not consumed during the reactions and can participate again for catalyze the reaction.
The results of some experiments could not be explained using this key and lock model. So in 1958, Daniel Koshland suggested a modification for lock and key model.
2 .Induced fit model.
This model is a further modification of the lock and key model. It states that the active sites of the enzymes are more flexible than it was thought. These active sites continuously change with the substrate as substrate interact with the enzyme. Substrate fits in to a general shape of the active site in the enzyme. This is not a perfect fit. Substrate – enzyme interaction cause a shape change in the active site. The change of protein configuration leads to near perfect fit of enzyme to the substrate.
Example for induced fit model – Glucose hexokinase activity.
Glucose hexokinase is an enzyme that catalyzes the ATP dependent phosphorylation of glucose and form glucose – 6 phosphate, When glucose binds to the active site of the glucose hexokinase enzyme,
Non covalent interactions take place between the enzyme and substrate. The shape of the active site is changed which then fit perfectly to the substrate transition state. Glucose hexokinase put the glucose molecule to correct orientation to make the reaction proceed. The catalysis is stated by the active sites of the enzyme. Especially R groups of the amino acids play a vital role in catalysis.
The transition state of the glucose / glucose -6- phosphate is formed due to the orientation of the glucose and releasing of the binding energy. The transition state is highly energetic compared to substrate or to the product. Therefor this has to be held more tightly to make it stabilize.
When reaction proceed, the phosphate bond is formed and glucose -6- phosphate is formed as a result. Glucose -6- phosphate is no longer fits to the active site hence it is released from the enzyme.