ABSTRACT
Cost contribution of the biocatalyst on the final product, however, is probably most important. It is clear that this cost contribution should be as low as possible, but it is much more difficult to determine HOW low it has to be. This is particularly dependent on the envisaged product: While complex pharmaceutical intermediates (API) that can be sold in the range of hundreds of euros per kilogram can easily afford €10/kg cost contribution of the biocatalyst, the production of bulk chemicals with a market price of less than €2/kg leaves only room for a couple of cents per kilogram (if at all). To determine the cost contribution of an isolated
enzyme it is important to know the key factors: enzyme cost, specific enzyme activity and enzyme stability. • Specific enzyme activity reflects the amount of enzyme needed to reach the envisaged degree of conversion for the selected reaction within a given time frame. It is usually determined at the beginning of the appropriate reaction and given in µmol · g-1 · min-1 (U · g-1). Thus, it allows a rough estimation of the enzyme’s input factor. However, due to the complexity of enzyme kinetics (usually described by the Michaelis-Menten equation) a significant reduction of enzyme activity might be observed at lower concentrations of educts (usually found at later time points of the reaction), leading to a significantly higher demand for enzyme. It is also important to really determine the activity for the desired reaction under the selected reaction conditions. Specific activities given by enzyme suppliers usually refer to certain standardised assays, and activities towards “real” substrates might differ by several orders of magnitude. • Enzyme stability reflects retention of catalytic activity over time and is usually given as half-life time. If the enzyme stability is shorter than the envisaged reaction time, it will be necessary to add additional enzyme during the reaction. If the enzyme is more stable, it might yield the opportunity to recover and reuse it in subsequent reactions, thus maximising total turnover number (TTN) and lowering its cost contribution significantly. Hills has impressively shown this influence (Fig. 15.1).
