Enzymes are biological catalysts which speed up a biological reaction through lowering the activation energy required for a reaction to start. They are classified based on the reactions they catalyse (e.g. hydrolases catalyse hydrolytic reactions).
Enzyme-catalysed reactions can be classified into two types – synthesis of substances (anabolic reactions) and breaking down of substances (catabolic reactions).
The energy required to start a chemical reaction is known as the activation energy of the reaction.
In the presence of enzymes, a greater number of reactant particles possess more energy than the activation energy and the reaction occurs at a faster rate. The reactant molecules on which enzyme act on are called substrates.
Characteristics of enzymes:
- Highly efficient – Remained unchanged in the reactions they catalyse, this means that only a small amount of the enzyme is required and these enzymes can be reused over and over again.
- Highly specific – Each enzyme only interact with one particular type of substrate (enzyme specificity) to form a unique enzyme-substrate complex. This is dependent on the 3D surface configuration of the enzyme. The depressions or ‘pockets’ on the surface of an enzyme which the substrate fits into is known as the active site of the enzyme. There are two main forms of models which explains how the shape of an enzyme affects the way it functions – Lock and key hypothesis and induced fit model.
==> Lock and key hypothesis – Enzyme is the lock and substrate is the key, substrate fits perfectly into the active sites of the enzymes as the shape of the substrate is complementary to the shape of the active site of the enzyme
==> Induced fit model – Original active site is not exactly complementary to the substrate, enzyme molecules undergoes slight adjustments in its shape to fit more tightly around the substrate molecule
- Catalyse reversible reaction – Enzyme catalyse both the forward and reverse reaction until a state of equilibrium is reached
- Affected by temperature changes – Temperature at which an enzyme is most active (catalyses the greatest number of reactions per second) is called the optimum temperature and most human enzymes perform best at temperatures between 37 to 40 degree Celcius
==> When temperature rises: Increase in kinetic energy of reacting particles, particles are more likely to collide with each other and substrates fitting into active sites, enzyme-substrate complexes formed at a faster rate and products are formed at a faster rate
==> When temperature exceeds optimum: High temperature cause hydrogen bonds in the enzymes to break, resulting in the loss of its unique three-dimensional shape and active site (denaturation), irreversible and the performance and rate of enzyme catalysed reactions drop rapidly
- Affected by pH changes – Optimum pH at which an enzyme works the best is not the same for each enzyme, slight changes in pH may change the electrostatic charges on the surface of enzyme active site and substrate resulting in electrostatic repulsion and affecting interaction between substrate and active site, extreme changes in pH of the solution will denature an enzyme
- Affected by substrate & enzyme concentration – When the substrate concentration is kept at a high level, increasing the enzyme concentration will increase the rate of reaction proportionally; For a fixed enzyme concentration, as the substrate concentration increases, the rate of reaction increases till it reaches the point of saturation (all active sites of the enzyme is taken up and the current reactions must be completed before further reactions can start)
Temperature, pH and enzyme and substrate concentrations become limiting factors when they are above or below the optimum required for the enzyme to be the most active.