Enzyme Structure and Functions:
Enzymes are biological catalysts. They increase the rate of reactions by a factor of between 106 to 1012 times, allowing the chemical reactions that make life possible to take place at normal temperatures Definition of enzyme: A protein with catalytic properties due to its power of specific activation is defined as an enzyme.
Structure
Enzymes are proteins their function depends on its complexity. The reaction takes place in a small part of the enzyme called the active site, while the rest of the protein acts as “scaffolding”.
The shape and the chemical environment inside the active site permits a chemical reaction to proceed more easily Many enzymes need cofactors (or coenzymes) to work properly. Tightly bound cofactors are called prosthetic groups Cofactors that are bound and released easily are called coenzymes These can be metal ions (such as Fe2+, Mg2+, Cu2+) or organic molecules (such as haem, biotin, FAD, NAD or coenzyme A). Many of these are derived from dietary vitamins, which is why they are so important. The complete active enzyme with its cofactor is called a holoenzyme, while just the protein part without its cofactor is called the apoenzyme.
How Does An Enzyme Work?
- Reaction Mechanism
- Molecular geometry reaction mechanism:
In any chemical reaction, a substrate (S) is converted into a product (P) In an enzyme-catalysed reaction, the substrate first binds to the active site of the enzyme to form an enzyme-substrate (ES) complex, then the substrate is converted into product whilst attached to the enzyme, and finally the product is released, thus allowing the enzyme to start all over again An example is the action of the enzyme sucrase hydrolysing sucrose into glucose and fructose.
Molecular Geometry
The substrate molecule is complementary in shape to that of the active site. It was thought that the substrate exactly fitted into the active site of the enzyme molecule like a key fitting into a lock (the now discredited ‘lock and key’ theory). This explains enzyme specificity This explains the loss of activity when enzymes denature The Induced Fit Hypothesis : * Some proteins can change their shape (conformation) When a substrate combines with an enzyme, it induces a change in the enzyme’s conformation * The active site is then moulded into a precise conformation * Making the chemical environment suitable for the reaction * The bonds of the substrate are stretched to make the reaction easier (lowers activation energy)
Energy Changes
Energy needed for initial reaction is known as activation energy. The larger the activation energy is, the slower the reaction will be.
This is because only a few substrate molecules will have sufficient energy to overcome the activation energy barrier. Enzymes reduce the activation energy of a reaction so that the kinetic energy of most molecules exceeds the activation energy required and so they can react. Factors affecting Enzymes substrate concentration pH temperature enzyme concentration inhibitors
Substarte Concentration
The rate of an enzyme-catalysed reaction is also affected by substrate concentration.
As the substrate concentration increases, the rate increases because more substrate molecules can collide with active sites, so more enzyme-substrate complexes form. At higher concentrations the enzyme molecules become saturated with substrate, and there are few free active sites, so adding more substrate doesn’t make much difference The maximum rate at infinite substrate concentration is called vmax, and the substrate concentration that gives a rate of half vmax is called KM.
These quantities are useful for characterising an enzyme. A good enzyme has a high vmax and a low KM. pH Enzymes have an optimum pH at which they work fastest. For most enzymes this is about pH 7-8 (normal body pH), but a few enzymes can work at extreme pH. The pH affects the charge of the amino acids at the active site, so the properties of the active site change and the substrate can no longer bind.
Temperature
Enzymes have an optimum temperature at which they work fastest.
For mammalian enzymes this is about 40°C. Up to the optimum temperature the rate increases geometrically with temperature. Above the optimum temperature the rate decreases as more of the enzyme molecules denature. The thermal energy breaks the hydrogen bonds holding the secondary and tertiary structure of the enzyme together, so the enzyme loses its shape Q10 (the temperature coefficient) = the increase in reaction rate with a 10°C rise in temperature.
Enzyme Concentration
As the enzyme concentration increases the rate of the reaction also increases, because there are more enzyme molecules (and so more active sites), available to catalyse the reaction, therefore, more enzyme-substrate complexes form inhibitors. Inhibitors inhibit the activity of enzymes, reducing the rate of their reactions.
2 types: Competitive and non competitive
- Competitive: A competitive inhibitor molecule has a similar structure to the substrate molecule, and so it can fit into the active site of the enzyme. It therefore competes with the substrate for the active site, so the reaction is slower. Increasing the concentration of substrate restores the reaction rate and the inhibition is usually temporary and reversible.
- Non competitive: A non-competitive inhibitor molecule is quite different in structure from the substrate and does not fit into the active site. It binds to another part of the enzyme molecule, changing the shape of the whole enzyme, including the active site, so that it can no longer bind substrate molecules. Non-competitive inhibitors therefore simply reduce the amount of active enzyme.
