lecture5 - 10.492 Integrated Chemical Engineering(ICE...

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Lecture #5 – Using Whole Cells as Biocatalysts: Why/When, Growth vs Conversion (Screening) Handouts : (1) Economist article, (2) Buckland paper ( Metabolic Engineering ) So far in our discussions of enzymes and enzyme kinetics, we’ve assumed that the biocatalyst to use for a particular conversion has been a purified enzyme. For example, in talking about screening enzymes for a reaction, we discussed setting up a series of identical reactions in which only the identity of the enzyme catalyst was changed. Realize, however, that all enzymes come from some biological source, be it of bacterial, plant, or mammalian origin, to name a few. For simpler organisms like bacteria or fungi, you often have the option of using the entire cell as the biocatalyst without going through the purification process. In this case, you will need to account for both growth of the “catalyst” and conversion of the substrate in your design process. Let’s first discuss the advantages/limitations of using whole cells as catalysts. 1. Why and when to use whole cell catalysts Following are some of the advantages/pros and disadvantages/cons of using whole cells as the catalyst versus using purified enzymes. Note that we are typically interested in using bacteria or yeast ( ie, non-filamentous fungi) because they are easiest to culture. Advantages/Pros Disadvantages/Cons Only limited by the number of organisms that can be cultured, while enzymes are limited by the number that are commercially available. Processes are more complicated (so dev’t takes longer), since you must design for cell growth ( ie, enzyme manufacturing phase) and substrate conversion ( ie, production phase) “Biomass is cheap”, ie , the cost of fermentation is usually not too high, and you’re not paying the purification costs associated with the purchase of an enzyme. Processes are “messier” – the vessel may contain cells and spent growth medium in addition to products and residual substrates (affects downstream purification) You don’t need to provide exogenous co-factor for re-dox reactions since the cells will recycle co-factor with existing machinery (true even if cells aren’t growing). Much more likely to have unwanted by-products since many other enzymes in addition to your desired enzyme will be present in the “catalyst” Also for re-dox reactions, you don’t need to provide a complimentary reaction to recycle the co-factor that is converted (example below).
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lecture5 - 10.492 Integrated Chemical Engineering(ICE...

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