Enzymes are biologial catalysts.
This means that they speed up the rate of reaction.
Enzymes are proteins.
They work by allowing molecules (known as substrate) bind to them, and either break down the molecule into smaller mocules (a catabolic enzyme) or build larger molecules from smaller ones (an anabolic enzyme).
Each enzyme has a specific substrate that it can bind with.
To facilitate this, an enzyme has an active site.
The active site is a region on the enzyme with a complimentary shape to its substrate.
The idea that an enzyme's active site is a complimentary shape to its substrate is known as the lock and key theory.
Another theory, the Induced Fit theory, suggests that an enzyme's active site is not complimentary to a substrate, but rather it moulds to fit the substrate once it binds to it.
The rate at which an enzyme speeds up a reaction depends on several factors:
TemperatureIf temperature is too high, it will provide enough kinetic energy to break the weak hydrogen and ionic bonds in the enzyme, changing the specific shape of the active site so that the substrate can no longer bind to it.
If temperature is too low, substrate molecules will not have enough kinetic energy to collide with the active site, lowering the rate of reaction.
Note: heat-stable enzymes exist. They have an optimum rate of reaction in an extremely hot environemnt. They have more disulphide bonds that do not break with heat to achieve this.
pHIf pH differs too far from te optimum pH, the hydrogen bonds and ionic bonds in the enzyme will be broken, changing the specific shape of the active site so that the substrate can no longer bind to it, lowering the rate of reaction.
Concentration of SubstrateIf substrate concentration is high, then there is more substrate available to collide with enzymes' active sites, increasing the rate of reaction until all enzymes are saturated, at which point the rate plateaus, making enzyme concentration the limiting factor.
If substrate concentration is low, then there is less substrate available to collide with enzymes' active sites, leading to fewer successful collisions, therefore the rate decreases.
Concentration of EnzymeIf enzyme concentration is high, then there are more enzymes available to collide with substrate, increasing the rate of reaction until all substrate is used up.
If enzyme concentration is low, then there are less enzymes available to collide with substrate, resulting in fewer collisions, and therefore the rate decreases.
Some enzymes have an allosteric site.
An allosteric site is an area on an enzyme which a non-competitive inhibitor binds to.
When a non-competitive inhibitor binds to an enzyme, it changes the shape of the active site, preventing substrate from binding to it.
A competitive inhibitor binds to the active site of an enzyme, blocking substrate from binding to it.
Cofactors are non-protein helpers (like metal ions or vitamins) that bind to enzymes.
Some cofactors activate enzymes.
Many enzymes are inactive until a cofactor binds, which changes the shape of the active site, forming a complete, active enzyme.
Cofactors can be divided into two main types: coenzymes and prosthetic groups.
Coenzymes are organic cofactors.
They are often derived from vitamins.
Coenzymes bind temporarily to the enzyme during the reaction.
They transfer chemical groups or electrons between enzymes.
An example of a coenzyme is NAD, which transfers hydrogen ions and electrons in respiration.
Prosthetic groups are cofactors that are permanently attached to the enzyme.
They are tightly bound and form part of the enzyme’s structure.
Without the prosthetic group, the enzyme cannot function.
An example of a prosthetic group is the haem group in catalase.