Enzyme Action: Lock-and-Key and Induced-Fit Models
Enzymes are proteins that serve as catalysts in chemical reactions. A catalyst is any substance that speeds the rate of a chemical reaction. Enzymes lower the activation energy needed to start a chemical reaction. Enzymes are typically uniquely structured to facilitate one specific reaction. The active site is where substances bind, and the chemical reaction is initiated, forming the product.
There are two main types of mechanisms of enzyme action:
- Lock-and-key model
- Induced-fit model
Lock-and-Key Model
This theory of enzyme action was hypothesized by Emil Fischer in 1899. According to Fischer, enzymes exhibit a high degree of specificity to the substances they react with.
He proposed that this is due to the shape of the enzyme fitting the shape of the substrate, similar to how a lock only fits a specific key. By this model, the substrate binds to the enzyme and forms an enzyme-substrate (ES) complex, which undergoes the reaction and forms the product.
Induced-Fit Model
The induced-fit model was proposed by Daniel Koshland in 1958. According to Koshland’s hypothesis, the active site is shaped similarly enough and has specific chemical properties that attract a substrate to bind. Once the substrate binds, the active site is induced or promoted to change shape.
This results in a more precise fit. The induced-fit model better explains the phenomenon of more than one type of substrate binding and reacting to an enzyme when similarly structured.
Factors Affecting Enzyme Activity
Enzyme activity is affected by various factors:
- Temperature: Enzyme activity increases up to a certain temperature point, after which the enzyme gets denatured completely. The optimal temperature for many enzymes is around 40°C.
- pH: Enzymes are sensitive to pH changes. The optimal pH varies for different enzymes. Changes in pH alter the charge distribution, decreasing enzyme activity.
- Substrate Concentration: Enzyme activity increases with substrate concentration, but only to a certain point, beyond which the enzyme becomes saturated, and its concentration decreases.
- Enzyme Concentration: As enzyme concentration increases, so does enzyme activity, up to a certain point where the reaction rate becomes independent of enzyme concentration.
- Inhibitors: These are specific molecules that bind to the enzyme’s active site, inhibiting its activity. They can be reversible or irreversible.
- Activators: Some enzymes require the presence of specific molecules called activators to function. They can also change the structure of the enzyme.
- Salinity: Changes in salt concentration can affect enzyme activity by influencing the ionic environment of the enzyme and its substrate.
Active Site: Structure and Mechanism
The active site is the region of an enzyme where substrate molecules bind, and a chemical reaction or catalysis takes place.
The active site is made up of amino acid residues that establish temporary bonds with the substrate at the binding site. The catalytic site is where the catalysis reaction takes place, and the substrate undergoes a reaction, forming an enzyme-substrate complex.
- Active sites mainly work based on substrate specificity.
- Binding specificity is determined by the precise arrangement of atoms in an active site.
- Clefts or crevices are active locations.
- Multiple weak attractions bind substrates to the enzyme.
Mechanism:
The mechanism of enzyme action at the active site involves the precise interaction between the enzyme and substrate, leading to catalysis and product formation.