Generic features of quinone-binding sites.

Introduction: generic features of quinone-binding sites (Q sites) Quinones form a central link between protein complexes in most types of electron-transfer chain. More than 50 distinct types of Q site associated with respiratory and photosynthetic electron-transfer complexes have been identified to date. Most involve rather loose and dynamic binding of quinone, and for these Q sites inhibitory antagonists can bind and disrupt the quinone redox reaction. Several natural and synthetic inhibitors have been exploited commercially [ 11. Q sites are often arranged vectorially so that electron transfer is coupled to proton translocation across the membrane. Some general features of Q sites may be defined. The quinone/quinol binds into the hydrophobic site primarily by interaction of appropriately placed hydrogen bonds with the carbonyllhydroxyl moieties, and with the ring flanked by an aromatic and aliphatic residues. Further interactions with the quinone hydrophobic side chain occur and give steric limitations to possible antagonists. A proton channel(s) to an aqueous interface is usually required and candidates for such can be identified in both the reaction centre (RC) and bc, structures. Binding at the Q site itself may involve transient prebinding steps in an approach channel. Although many of the simpler Q-site inhibitors are likely to bind in place of the quinone in roughly the same manner, the more complex antagonists are likely to interact with the