Enzymes are globular proteins that act as catalysts.Catalysts alter the rate of reaction without undergoing permanent changes themselves.They can be reused repeatedly and are therefore effective in small amounts.
An initial boost of energy is needed to kick start the reaction.The minimum amount of energy needed to activate the reaction is called the activation energy.
The activation energy level must initially be overcome before the reaction can proceed.
Enzymes lower activation energy by providing the substrates with an ideal environment(more stable) for the reaction.
Structure of an enzyme
An enzyme molecule is very large, but only a small region of it is functional. This part is called the active site.
The molecule on which the enzyme acts is called the substrate.This fits neatly into the active site to form an enzyme-substrate complex. The substrate molecule is held in the active site by bonds that form temporarily between certain amino acids of the active site and groups on the substrate molecule.
Lock and Key Model
This model is where the substrate fits into the enyme in the same way that a key fits into a lock.
The shapes are very precise, and are maintained by the tertiary structure of the enzyme.
Molecules that do not have the same shape as the active site cannot fit/bind to it.
Induced fit model
This model states that the enzyme changes it shape slightly to fit the shape of the substrate.In other words, the enzyme is flexible and can mould itself round the substrate.
Once the reaction is complete, the enzyme changes back to it’s original shape.
Effect of temperature on enzyme action
A rise in temperature increases the kinetic energy of molecules. As a result, the molecules move around more rapidly and collide with each other more often. In an enzyme-catalysed reaction, this means that the enzyme and substrate molecules come together more often in a given time, so that the rate of reaction increases.
When the temperature gets too high, the hydrogen ( and other) bonds start to break. This results in the enzyme, including it's active site to change shape.This means the substrate fits less easily into the active site, lowering the rate of reaction.
At some point (around 60 degrees C) the enzyme is so disrupted that it stops working all together. It is said to be denatured. This is a permanent change, and once it has occured the enzyme can not function again.
Effect of pH on enzyme action
In a similar way to a rise in temperature, a change in pH reduces the effectiveness of an enzyme and may eventually cause it to stop working altogether (denatured)
pH affects an enzyme in the following ways:
It alters the charges on the amino acids that make up the active site of an enzyme. As a result, the substrate can no longer become attached to the active site and so the enzyme-substrate complex cannot be formed.
A change in pH can cause the bonds that maintain the enzyme's tertiary structure to break.The enzyme therefore changes shape. The enzyme has been denatured.
Enzyme inhibitors are substances that directly or indirectly interfere with the functioning of the active site of an enzyme and so reduce its activity.Somes the inhibitor binds itself so strongly to the active site that it cannot be removed, so it permantly prevents the enzyme functioning.Most enzymes only make temporary attachments to the active site - these are called reversible inhibitors and there are two types:
Competitive inhibitors- these bind to the active site of the enzyme. They have a molecular shape similar to that of thr substrate. This allows them to bind with the active site of an enzyme. They therefore compete with the substrate for the active sites.The inhibitor is not permanently bound to the active site, so when the inhibitor is released another molecule can takes its place.
Non-competitive inhibitors- these attach themselves to a site which is not the active site (allosteric site) When the inhibitor attaches itself to the enzyme, the inhibitor alters the shape of the active site, meaning that substrate molecules can no longer occupy it, and so the enzyme cannot function.