Syllabus Edition

First teaching 2023

First exams 2025

|

Enzyme Action (SL IB Biology)

Revision Note

Marlene

Author

Marlene

Last updated

Structure of Enzymes

The structure of enzymes

  • Enzyme catalysis involves molecular motion and the collision of substrates with the active site
  • For an enzyme-catalysed reaction to take place, substrates collide at random with the enzyme's active site
  • This must happen at the correct orientation and speed in order for a reaction to occur
    • Unsuccessful collisions can occur when the molecules are not correctly aligned with each other at the moment of collision
    • The molecules 'bounce' off each other and no reaction takes place
  • Some enzymes have two substrates that must each collide with a separate active site at the same time
  • Substrates bind to enzymes, forming a temporary enzyme-substrate complex
  • The active site of an enzyme has a specific shape and chemical properties to bind with a specific substrate
  • The reaction occurs within the enzyme-substrate complex which leads to changes in the chemical structure of the substrate
  • Products are formed, which detach and move away from the active site, which can be re-used

Enzyme action diagram

Active site, downloadable AS & A Level Biology revision notes

The active site of an enzyme has a specific shape to fit a specific substrate (when the substrate binds an enzyme-substrate complex is formed)

  • The specificity of an enzyme is a result of the complementary nature between the shape of the active site on the enzyme and its substrate(s)
  • The shape of the active site (and therefore the specificity of the enzyme) is determined by the complex 3D shape of the protein that makes up the enzyme
    • The active site is made of only a few amino acids but the interaction of these amino acids within the 3D shape of the enzyme ensures that catalysis can occur
    • This is achieved by:
      • Binding to the substrate molecule
      • Holding it in position for a chemical reaction to occur
      • Lowering the energy needed for the reaction to occur 

Enzyme specificity diagram

Enzyme Specificity Examples, downloadable IGCSE & GCSE Biology revision notes

An example of enzyme specificity – the enzyme catalase can bind to its substrate hydrogen peroxide as they are complementary in shape, whereas DNA polymerase is not

Formation of enzyme-substrate complex diagram

Enzyme-substrate complex, downloadable AS & A Level Biology revision notes

The temporary formation of an enzyme-substrate complex

Examiner Tip

Don't forget that both enzymes and their substrates are highly specific to each other – this is known as enzyme-substrate specificity.

Induced-fit Binding

The induced-fit hypothesis

  • The original model explaining interactions between enzymes and their substrate molecules was called the lock-and-key model
    • This model proposed that the enzyme active site is precisely complementary to the shape of the substrate molecule
    • The substrate molecule therefore fits into the active site like a key in a lock
  • The modified model of enzyme activity is known as the ‘induced-fit hypothesis
  • Although it is very similar to the lock and key hypothesis, in this model the enzyme and substrate interact with each other:
    • The enzyme and its active site (and sometimes the substrate) can change shape slightly as the substrate molecule enters the enzyme
    • These changes in shape are known as conformational changes
    • This ensures an ideal binding arrangement between the enzyme and substrate is achieved
    • This maximises the ability of the enzyme to catalyse the reaction

Induced-fit model diagram

Induced fit hypothesis, downloadable AS & A Level Biology revision notes

The induced-fit hypothesis of enzyme action

Examiner Tip

Don't forget – our current understanding of enzyme-substrate interactions is based on the induced-fit hypothesis.

Enzyme Catalysis

The role of molecular motion and substrate-active site collisions

  • In order for the substrate molecule to collide with and ultimately bind to the enzyme active site, movement is required
    • This movement is the result of the kinetic energy that molecules have
    • The greater the kinetic energy of the molecules, the faster the movement and the higher the probability of enzyme and substrate colliding
    • This leads to more enzyme-substrate complexes forming and the production of more product molecules
  • In some cases, large substrate molecules are immobilised, while in other cases it is possible to immobilise enzymes by embedding them in membranes
  • These immobilised enzymes can be used in a range of industries such as food processing, environmental management, pharmaceuticals and manufacturing processes
  • There are different methods by which enzymes can be immobilised including:
    • Attachment to an inert substance e.g. glass
    • Entrapment within a matrix e.g. alginate gel
    • Entrapment within a partially permeable membrane

Examples of immobilised enzymes diagram

Different methods of immobilising enzymes 1


Different methods of immobilising enzymes 2

There are many different ways in which enzymes can be immobilised

Advantages of immobilised enzymes

  • There is no enzyme in the product (the product is uncontaminated) and therefore there is no need to further process or filter the end product
  • The immobilised enzyme can be reused multiple times which is both efficient and cost-effective (many enzymes are expensive)
    • Reusing the enzyme also avoids the need to separate the enzyme from the product in downstream processing
  • Immobilised enzymes have a greater tolerance of temperature and pH changes (immobilisation often makes enzymes more stable)
  • Substrates can be exposed to higher enzyme concentrations than when using enzymes in solution, increasing the rate of throughput
  • Conditions can be controlled carefully, allowing immobilised enzymes to function close to their optimum conditions and be more stable

Denaturation: Enzymes

  • Enzymes can be denatured when it is exposed to high temperatures or extremes of pH
  • Bonds (e.g. hydrogen bonds) holding the enzyme molecule in its precise 3D shape start to break
    • Take note that the peptide bonds holding the amino acids together are not broken
  • This causes the 3-dimensional shape of the protein (i.e. the enzyme) to change
  • This permanently changes the shape of the active site, preventing the substrate from binding
  • Denaturation has occurred if the substrate can no longer bind
  • The reaction that was previously catalysed now no longer takes place
  • Denaturation often causes the enzyme to become insoluble and form a precipitate
  • Very few human enzymes can function at temperatures above 50°C
    • This is because humans maintain a body temperature of about 37°C, therefore even temperatures exceeding 40°C will cause the denaturation of enzymes
    • High temperatures cause increased vibrations in the bonds between the R-groups of amino acids so they start to break, changing the conformation of the enzyme

Examiner Tip

Don't forget that enzymes are always proteins and so anything that could denature a protein, rendering it non-operational (extremes of heat, temperature, pH etc.) would also denature an enzyme. Avoid using the term 'destroyed' or saying that the enzyme is 'killed' when describing the disruption to enzyme structure.

You've read 0 of your 10 free revision notes

Unlock more, it's free!

Join the 100,000+ Students that ❤️ Save My Exams

the (exam) results speak for themselves:

Did this page help you?

Marlene

Author: Marlene

Expertise: Biology

Marlene graduated from Stellenbosch University, South Africa, in 2002 with a degree in Biodiversity and Ecology. After completing a PGCE (Postgraduate certificate in education) in 2003 she taught high school Biology for over 10 years at various schools across South Africa before returning to Stellenbosch University in 2014 to obtain an Honours degree in Biological Sciences. With over 16 years of teaching experience, of which the past 3 years were spent teaching IGCSE and A level Biology, Marlene is passionate about Biology and making it more approachable to her students.