Quinolinate Synthase: An Example of the Roles of the Second and Outer Coordination Spheres in Enzyme Catalysis

The activation energy barrier of biochemical reactions is normally lowered by an enzyme catalyst, which directly helps the weakening of the bond(s) to be broken. In many metalloenzymes, this is a first coordination sphere effect. Besides having a direct catalytic action, enzymes can fix their reactive groups and substrates so that they are optimally positioned and also modify the water activity in the system. They can either activate substrates prior to their reaction or bind preactivated substrates, thereby drastically reducing local entropic effects. The latter type is well represented by some bisubstrate reactions, where they have been defined as "entropic traps". These can be described as "second coordination sphere" processes, but enzymes can also control the reactivity beyond this point through local conformational changes belonging to an "outer coordinate sphere" that can be modulated by substrate binding. We have chosen the [4Fe-4S] cluster-dependent enzyme quinolinate synthase to illustrate each one of these processes. In addition, this very old metalloenzyme shows low in vitro substrate binding specificity, atypical reactivity that produces dead-end products, and a unique modulation of its active site volume.