chapter 22 p4 Flashcards
Isolated enzymes:
Using isolated enzymes instead of whole organisms has some clear advantages. p1
- Less wasteful - whole microorganisms use up substrate growing and reproducing, producing biomass rather than product. Isolated enzymes do not.
- More efficient - isolated enzymes work at much higher concentrations than is possible when they are part of the whole microorganism.
- More specific - no unwanted enzymes present, so no wasteful side reactions take place.
- Maximise efficiency - isolated enzymes can be given ideal conditions for maximum product formation, which may differ from those needed for the growth of the whole microorganism.
- Less downstream processing - pure product is produced by isolated enzymes. Whole microorganisms give a variety of products in the final broth, making isolation of the desired product more difficult and therefore expensive.
Extracellular enzymes
- Most of the isolated enzymes used in industrial processes are extracellular enzymes produced by microorganisms.
- They are generally easier and therefore cheaper to use than intracellular enzymes.
- Extracellular enzymes are secreted, making them easy to isolate and use.
- Each microorganism produces relatively few extracellular enzymes, making it easy to identify and isolate the required enzyme.
- In comparison, each microorganism produces hundreds of intracellular enzymes which would need extracting from the cell and separating.
- Extracellular enzymes tend to be much more robust than intracellular enzymes.
- Conditions outside a cell are less tightly controlled than conditions in the cytoplasm, so extracellular enzymes are adapted to cope with greater variations in temperature and pH than intracellular enzymes.
However, in spite of the advantages of using extracellular enzymes, intracellular enzymes are still sometimes used as isolated enzymes in manufacturing processes.
- This is because there is a bigger range of intracellular enzymes so in some cases they provide the ideal enzyme for a process.
- In these cases, the benefits of using a very specific intracellular enzyme outweigh the disadvantages of the more expensive extraction and isolation process and the need for more tightly controlled conditions.
- Examples of intracellular enzymes used as isolated enzymes in industry include glucose oxidase for food preservation, asparaginase for cancer treatment, and penicillin acylase for converting natural penicillin into semi-synthetic drugs which are more effective
Immobilised enzymes:
- Increasingly enzymes used in industrial processes are immobilised - attached to an inert support system over which the substrate passes and is converted to product.
- This is a case of technology mimicking nature - enzymes in cells are usually bound to membranes to carry out their repeated cycles of catalysis.
- Because immobilised enzymes are held stationary during the catalytic process, they can be recovered from the reaction mixture and reused time after time.
- The enzymes do not contaminate the end product, so less downstream processing is needed.
- These things all make the process more economical.
Advantages of using immobilised enzymes:
- Immobilised enzymes can be reused - which is cheaper.
- Easily separated from the reactants and products of the reaction they are catalysing so reduced downstream processing - which is cheaper.
- More reliable - there is a high degree of control over the process as the insoluble support provides a stable microenvironment for the immobilised enzymes.
- Greater temperature tolerance - immobilised enzymes are less easily denatured by heat and work at optimum levels over a much wider range of temperatures, making the bioreactor less expensive
- Ease of manipulation - the catalytic properties of immobilised enzymes can be altered to fit a particular process more easily than those of free enzymes - for example, immobilised glucose isomerase can be used continuously for over 1000 hours at temperatures of 60-65°C.
- The ability to keep bioreactors running continuously for long periods without emptying and cleaning helps to keep running costs low.
Disadvantages of using immobilised enzymes:
- Reduced efficiency - the process of immobilising an enzyme may reduce its activity rate.
- Higher initial costs of materials - immobilised enzymes are more expensive than free enzymes or microorganisms.
- However, the immobilised enzymes, unlike free enzymes, do not need to be replaced frequently.
- Higher initial costs of bioreactor - the system needed to use immobilised enzymes is different from traditional fermenters so there is an initial investment cost.
- More technical issues - reactors which use immobilised enzymes are more complex than simple fermenters - they have more things which can go wrong.
How are enzymes immobilised?
Enzymes can be immobilised in a number of ways.
They may be bound to the surface of insoluble supporting materials either by adsorption onto the surface or by covalent or ionic bonds.
They may be entrapped in a matrix, encapsulated in a microcapsule (like proteases for detergent use), or behind a semi-permeable membrane.
Each of these methods has advantages and disadvantages as summarised in Table 1.
four common ways of immobilising enzymes
1st method of immobilising enzymes- surface immobilisation
2nd method of immobilising enzymes- bonding
3rd method of immobilising enzymes - entrapment in matrix
4th method of immobilising enzymes
In some cases whole microorganisms rather than just the enzymes are immobilised.
This has many of the same advantages but avoids the time-consuming and expensive process of extracting the pure enzyme and immobilising it before the process starts.
On the other hand, the organisms need food, oxygen, and a carefully controlled environment to work at their optimum rate.
Using immobilised enzymes:
Immobilised enzymes are very useful when large quantities of product are wanted, because they allow continuous production. Examples include:
Immobilised penicillin acylase used to make semi-synthetic penicillins from naturally produced penicillins.
Immobilised glucose isomerase used to produce fructose from glucose.
Immobilised lactase used to produce lactose-free milk. Some people, and cats, are intolerant of lactose (milk sugar).
Immobilised aminoacylase used to produce pure samples of L-amino acids used in the production of pharmaceuticals, organic chemicals, cosmetics, and food.
Immobilised glucoamylase, which can be used to complete the breakdown of starch to glucose syrup.
Immobilised nitrile hydratase, an enzyme which is playing an increasing role in the plastics industry.
Immobilised penicillin acylase used to make semi-synthetic penicillins from naturally produced penicillins.
Many types of bacteria have developed resistance to naturally occurring penicillins so they are no longer very effective drugs.
Fortunately, many bacteria are still vulnerable to the semi-synthetic penicillins produced by penicillin acylase so they are very important in treating infections caused by bacteria resistant to the original penicillin.
Hundreds of tonnes of these medicines are made every year by immobilised penicillin acylase.
