Topic 7.6: Enzymes

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

There are two different types of metabolic pathways; linear chain and cyclic pathways. There are two aims; catabolic pathways breakdown molecules, while anabolic pathways build up molecules.

Linear Chain

e.g. Glycolysis

Cyclic Pathway

e.g. Krebs cycle or Calvin cycle

7.6.2 Describe the induced-fit model

The lock and key hypothesis does not explain the broad specificity of some enzymes. Sometimes the active site isn't complementary to the subtrate. The induced fit model overcomes these difficulties. At the complexing of the enzyme and substrate, the active site changes to accommodate the substrate. The structure of the enzyme allows a certain amount of adaptation to the substrate,.

7.6.3 Explain that enzymes lower the activation energy of the chemcial reaction that they catalyse

Every reaction requires a certain amount of energy to proceed - this is the activational energy (Ea). By lowering the activation energy, the rate of biochemical reaction increases.

Endogonic is energy lost from the system. Similar to Endothermic. These reactions are anabolic as the energy is being used up to create the bonds between the two substrates

Exergonic is energy released into the system. Similar to Exothermic. These reactions are catabolic as the energy is released from the broken bonds between the two substrate.


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

Competitive Inhibition

• A molecule (Inhibitor) which is structurally/chemically similar to the substrate and binds to the active site of the enzyme
• This serves to block the active site and thus preventing substrate binding (competes for the active site)
• Its effect can be reduced by increasing substrate concentration.

Example: Oxygen competing with carbon dioxide for active site of Rubisco

Non-competitive Inhibition

• A molecule which is not structurally or chemically similar to the substrate and binds to a site other than the active site (allosteric site)
• This causes a conformational change in the active site, meaning the substrate cannot bind.
• Its effect cannot be reduced by increasing substrate concentration as it is not competing for the active site.

Example: Cyanide inhibits enzymes in the electron transport chain by breaking disulphide bonds within the enzyme

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

End-product inhibition is a form of negative feedback in which increased levels of product decreases the rate of product formation.

The product binds to the allosteric sites of an enzyme, causing a conformational change in the active site. As the enzyme couldn't function properly, this will decrease the rate of products formations.