Chapter 3: Enzymes

Enzymes are globular proteins that increase the rate of reaction by lowering the activation energy of the reaction they catalyse. They can be defined as biological catalyst.

Active site: It is an area where substrate binds with the enzyme.

Substrate is the molecule that binds to the enzyme.

Enzymes speed up the reaction but it remains unchanged. They can be intracellular and extracellular both.

Lock and key and Induced fit hypothesis

Lock and key hypothesis

Proposed by Fischer in 1894.

It purposes the following points:

  • An enzyme has a cleft in its surface, called active site. The substrate molecule has complementary shape.
  • The R group of the amino acids in active site reacts with the substrate.
  • The enzyme binds with substrate forming an enzyme-substrate complex.
  • Only one substrate can fit each active site.
  • This shows the high specificity of enzymes.

Induced fit hypothesis

It is purposed by Koshland in 1958.

  • The shape of enzyme and active site are not complementary but they are moulded together and becomes complementary to each other.
  • Bonds form between oppositely charged groups on substrate and R groups to induce a better fit. This puts a strain on the substrate molecule so reactions occur more easily.
  • It is more acceptable hypothesis.

Activation Energy

It is the energy that the substrate needs to change into products.

Enzyme reduce Activation energy:

Factors affecting enzyme action

The factors are:

  • Temperature
  • pH
  • Enzyme concentration
  • Substrate concentration
  • Inhibitor concentration
  1. Temperature
    Rate of reaction increases up to the optimum temperature as kinetic energy increases. Each enzyme has an optimum temperature at which it works fastest. As temperature increases above the optimum temperature, the enzyme gradually denatures. When an enzyme is completely denatured, it ceases to function, denaturation is sometimes reversible.
  2. pH
    Most enzymes maintain their pH. The pH at which the enzyme has maximum activity is called optimum pH. Most enzyme has optimum pH range of 7.0 – 7.5. As pH moves away from optimum level the rate of reaction decreases.
  3. Enzyme concentration
    The rate of reaction increases if there is more enzyme concentration provided that there are enough substrate molecule present. Increasing the enzyme concentration beyond a certain point has not reaction as there are less substrate than enzyme.
  4. Substrate Concentration

    The greater the concentration of substrate, the faster the rate of reaction given that enough enzyme molecules are present. However, beyond a certain point the rate of reaction no longer increases as enzyme concentration becomes the limiting factor.

Inhibitor concentration

Inhibitor is a substance that slows down the rate at which the enzyme works.

  1. Competitive Inhibitor

They are the molecules which are similar in shape to normal substrate for active site of the enzyme. They are reversible. As the concentration of competitive reversible inhibitors increases, rate of reaction decreases as the active sites are temporarily blocked by inhibitors so substrates cannot bind to them.


  1. Non competitive inhibitors
    They are those inhibitors that bind permanently to the active site or bind at a site elsewhere on enzyme causing a change in shape of the active site. They may or may not be reversible. As concentration on non-competitive reversible inhibitors increases, rate of reaction decreases as the shape of the enzyme (not the active site) is altered by the inhibitor.

Michaelis – Menten Equation

The efficiency of the reaction can be measured by finding the value known as Michaelis – Menten constant, Km . To do this the maximum rate of reaction Vmax, must first be determined. Vmax represents the maximum rate of reaction achieved by the system at maximum substrate concentration.

Immobilising Enzyme

Enzymes can be immobilised and it is commercially useful as it can be reused and the product is uncontaminated. Therefore they are immobilised by attaching them to an insoluble, inert material e.g. calcium alginate which forms a gel capsule around them thus holding them in place during the reaction. This process enables enzymes to be reused as they can be easily separated from the products.