ABSTRACT
In earlier chapters, we have seen the increased regulation and complexity that is possible as a result of protein oligomerization. We saw that many enzymes have evolved from domains that bind the individual substrates, which are then put together and tinkered with. In Chapter 8 we saw how combining domains in different ways can produce signaling of remarkable complexity and control. And in Chapter 9 we saw that actually most proteins function as part of complexes rather than as individuals. However, sometimes this is not enough, and in a rather small number of cases evolution has had to come up with something special. These cases are discussed in this chapter on the grounds that by comparing ‘special’ enzymes with normal ones, it gives a better idea of what normal ones can and cannot do. They are special because of the high degree of communication between subunits, and the reactions catalyzed fall into two main categories. There are some reactions in which the product of a reaction is so reactive and/or expensive that it cannot be allowed to be released into the cell, and it has to be held on to and converted in a second reaction to something less dangerous or reactive. And then there is a larger group of reactions in which the product of one reaction forms the substrate for a second reaction, and for which the gain in linking together the separate reactions outweighs the evolutionary cost. These are typically biosynthetic pathways, in particular cyclic ones.
