Cellular Workersby Cyrus Ance
Cellular Workersby Cyrus Ance
In chemical engineering most processes depend on catalysts. For example ammonia is made from nitrogen and hydrogen with iron as a catalyst. Without the iron the reaction would proceed too slowly or need lots of expensive heat to be useful for the commercial production of ammonia. This is one of the simplest examples of the action of a catalyst. More complex products require many steps each generating nasty, unwanted waste and/or consuming energy. Another form of catalysis can be based on enzymes, specialized proteins. For example yeast turns sugar into alcohol with the aid of enzymes. Biocatalysis has many advantages over chemical: it operates at room temperature, takes place is benign water, and the unwanted products are usually biodegradable. As a whole biocatalysis is much more efficient than synthetic catalysis. Biocatalysis is today used to produce high-fructose corn syrup, the sweetener in most soft drinks, and aspartame, an artificial sweetener.
Companies are now making plastics via biocatalysis: Degussa and Cargill Dow are leaders. They concentrate on using non-traditional inputs such as vegetable fibers to produce traditional outputs, but with less input in natural resources, mostly fossil fuels.
There are several thousand known enzymes, but only a few are suitable for industrialization. Few are stable and many are difficult to produce or harvest. Stepping into the gap is Diversa, Avecia, and some other companies. They comb the world, concentrating on extreme environments such as deep-sea thermal vents and caustic-soda lakes, looking for microbes that produce enzymes that may be useful. They then extract the DNA that leads to the production of the desired enzyme, put it into a microbe that can be easily grown and maintained, and harvest the enzyme. One headache is distinguishing a useful enzyme from a sort-of useful enzyme. Thus these biocatalysts have to be carefully evaluated by comparing them to a more standard process to actually see if there is a gain.
One way around this is to genetically engineer an enzyme with the desired properties. Willem Stemmer of Maxygen has pioneered a technique of inducing mutations, selecting the best performers, and concentrating the mutations in a single offspring to have the best chance of producing a useful organism. This is called directed evolution with DNA shuffling technology. If that does not provoke a shudder, I am not sure what will. In the realm of pure research is the idea to create a desired catalysis from scratch. We do not really know how an enzyme's structure contributes to its function or how their interaction with other molecules speeds up reactions. At the moment we are moving towards a cellular work force to produce catalysts for chemical reactions.
Another example of cellular workers is occurring in the field of micro-structures. Dirk Volkmer is not actually using cells, but copies the technique of radiolaria to build micron scale structures out of silica. What might these structures be useful for? They can support catalysts. Their surface area is very high compared to their volume due to the large number of spikes and hollows. This is ideal for a catalyst as one wants the highest possible availability of the catalyst to mediate a reaction.
This column is based on two articles in the Economist. One is archived at Dirk Volkmer's site while the other, called Bugs as Catalysts, is from the 13 March 2003 edition, and is not available on line except to subscribers.