Design of specific inhibitors of quinolinate synthase based on [4Fe-4S] cluster coordination

Iron–Sulfur Clusters toward Stresses: Implication for Understanding and Fighting Tuberculosis

Tuberculosis (TB) remains the leading cause of death due to a single pathogen, accounting for 1.5 million deaths annually on the global level. Mycobacterium tuberculosis, the causative agent of TB, is persistently exposed to stresses such as reactive oxygen species (ROS), reactive nitrogen species (RNS), acidic conditions, starvation, and hypoxic conditions, all contributing toward inhibiting bacterial proliferation and survival. Iron–sulfur (Fe-S) clusters, which are among the most ancient protein prosthetic groups, are good targets for ROS and RNS, and are susceptible to Fe starvation. Mtb holds Fe-S containing proteins involved in essential biological process for Mtb. Fe-S cluster assembly is achieved via complex protein machineries. Many organisms contain several Fe-S assembly systems, while the SUF system is the only one in some pathogens such as Mtb. The essentiality of the SUF machinery and its functionality under the stress conditions encountered by Mtb underlines how it constitutes an attractive target for the development of novel anti-TB.

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.