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lecture3 - 10.492 Integrated Chemical Engineering(ICE...

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10.492 - Integrated Chemical Engineering (ICE) Topics: Biocatalysis MIT Chemical Engineering Department Instructor: Professor Kristala Prather Fall 2004 Lecture # 3 , 4 – Selecting a Catalyst (Non-Kinetic Parameters), Review of Enzyme Kinetics, Selectivity, pH and Temperature Effects 1. Selecting an enzyme catalyst Now that we’ve discussed the various enzyme classes and talked a bit about specificity, the question becomes how do we determine if an enzyme is appropriate to use as our catalyst? For a given type of reaction, it’s not enough to know that we need a lipase or a protease from the hydrolase family, we need to identify one or two enzymes to carry forward for development. And even if we can narrow down the exact EC number, enzymes with the same function but from different sources will have different reaction characteristics against a particular substrate. For design purposes, we will need to know the kinetic parameters to size a reactor and set the process cycle time. But for the initial selection process, we can choose an enzyme based on non-kinetic characteristics. By looking at several candidate enzymes, we can screen to determine the best choice of enzyme to use. To set up our screen, we can put together a set of reactions that are identical except for the choice of catalyst. (Note that to be fair, the concentration of catalyst in each reaction should be the same.) We then fix the reaction time and at the end of this short batch cycle, we can take a sample and measure the concentrations of substrate and products. From these measurements, we can determine the following: f S R usu i ) (1.1) Conversion = X = S 0 S × % 100 = P ( . , × % 100 S 0 S 0 (1.2) Yield P i = P i × % 100 for each product, and if the products are chiral, S 0 R S (1.3) R EE ) = × % 100 ( R + S We can look at the conversion to get a sense of the kinetics, since enzymes that “work” faster will have converted more substrate in this fixed unit of time. We look at the EE to get a sense of the enantioselectivity/specificity . Given these two parameters, how do you choose an enzyme to take forward? While the kinetics are important for an efficient process, these can often be optimized by adjusting the reaction conditions ( eg , pH, temperature, co-solvents, etc.). Thus, we can use our basic engineering knowledge for optimization to get the fastest reaction. What is much more difficult to control is the Dr. Kristala L. Jones Prather, Copyright 2004. MIT Department of Chemical Engineering
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Lecture #3, p.2 selectivity. This is more likely to require a more complicated biological optimization rather than our standard physico-chemical based engineering optimization. So, all things being equal, we would like an enzyme to deliver the highest EE material possible, while also achieving “good” conversions in the initial phase. The goal of process development is to then make that good conversion better.
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