7.6 – Enzymes

7.6 – Enzymes

7.6.1 – State that metabolic pathways consist of chains and cycles of enzyme catalysed reactions

Metabolic pathways form a series of reactions that regulate the concentration of substances within cells by enzyme-mediated linear and circular sequences. Respiration and photosynthesis are examples of a metabolic pathway (see below). All these reactions may be classified into two types.

Catabolic Reactions

This is the breaking down of larger molecules, releasing energy. This is exergonic. Enzymes in these reactions will break apart the chemical bonds, and two molecules are formed. These reactions include digestion and cellular respiration.

Anabolic Reactions

This is when smaller molecules form bonds and become larger molecules. This process requires energy, therefore it is endergonic. Enzymes will draw the smaller molecules in and help the new bonds to form. Examples include protein synthesis (build up of polypeptides from peptides) and cellular respiration.


Chain Pathways move from one reaction to the next. Each substrate has its own enzyme. The final product is called the end product.

Cyclic Pathways are when the initial substrate is fed into the cycle. The final product is reacted with the initial substrate. From here, the products are converted. The only difference in this pathway is the regeneration of the final intermediate. Examples of this type of pathway include Krebs cycle and the Calvin cycle.

7.6.2 – Describe the induced fit model

In the more accurate, induced fit model, there is modification. Enzymes are fairly flexible, and will reshape the active site by interactions with the substrate. Hence, the substrate does simply bind to a rigid active site, but the amino acid side chains will mould into positions for the enzyme to perform its function.

This change in shape is critical to momentarily raise the substrate molecule to the transitional state, when it can react. This model also accounts for the range of substrates that some enzymes can bind to.

7.6.3 – Explain that enzymes lower the activation energy of the chemical reactions that they catalyse

The activation energy is the minimum amount of energy required to raise substrate molecules to their transition state. The reaction cannot happen until this energy barrier has been overcome. The use of an enzyme reduces the amount of this energy that is needed.

The transition state is when the bonds in the reactant molecules break and begin to form the bonds of the products. Since breaking bonds is an endothermic process, the reactants require some energy to be added before they can start making the new bonds and begin the reaction. This amount of energy is the activation energy.

Enzymes work by providing an alternative reaction pathway that requires a lower activation energy. The frequency of collision between molecules increases, speeding up the reaction.

7.6.4 – Explain the difference between competitive and non-competitive inhibition, with reference to one example of each

Enzyme inhibitors deactivate enzymes. They come in two types:

Reversible Inhibitors are used to control enzyme activity.

Competitive inhibition is when the inhibitor and substrate must compete for the active site. The inhibitor is structurally similar to the substrate, and it prevents the substrate from binding. Examples are:

  • O₂ competing with CO₂ for the active site of RuBisCo
  • Malonate competing with succinate for the active site of succinate dehydrgenase

Non-competitive inhibitors slow down the rate of reaction because they distort the shape of the active site. When there is a build-up of the end product or lack of substrate, the enzyme may become deactivated. Examples are:


  • Cyanide ions blocking cytochrome oxidase in terminal oxidation in cell aerobic respiration
  • Nerve gas Sarin blocking acetyl cholinesterase in synapse transmission

Irreversible Inhibitors bind to the enzyme permanently and destroy their catalytic activity. These will covalently modify the enzyme.

Many drug molecules are enzyme inhibitors.


7.6.5 – Explain the control of metabolic pathways by end-product inhibition, including the role of allosteric sites

Allosteric inhibition is a process when metabolic pathways are switched off. Allosteric enzymes have two sites – the active site and the site where an additional substance can lock in. When this additional substance locks in, the entire enzyme is altered, and the active site is deactivated.

This process regulates and adjusts individual pathways in metabolism. In end product inhibition, as the product molecules accumulate, the steps in their production are switched off. This is because the final product inhibits the enzyme that catalyses the first step in the pathway. However, these products become substrates in subsequent reactions. They are used up, and production of more molecules with recommence.

In the pathway above, isoleucine is the end product, which inhibits the enzyme threonine deaminase. This occurs at the inhibition site. Thus, excess isoleucine switches off the production of more isoleucine. When it is used up, production will start again. This is very similar to non-competitive inhibition. This mechanism allows for self-regulation of production.