Plasticity of the quinone-binding site of the complex II homolog quinol: Fumarate reductase

Background: Different quinone substrates are used by complex II. Results: Structural and kinetic analyses show that two arginine residues modulate the enzyme interaction with different quinones. Conclusion: Specific arginines compensate for each other in proton transfer during quinone oxidoreduction in the complex II homolog fumarate reductase. Significance: Plasticity in quinone binding may be important for bioenergetic transformations. Respiratory processes often use quinone oxidoreduction to generate a transmembrane proton gradient, making the 2H⁺/2e^- quinone chemistry important for ATP synthesis. There are a variety of quinones used as electron carriers between bioenergetic proteins, and some respiratory proteins can functionally interact with more than one quinone type. In the case of complex II homologs, which couple quinone chemistry to the interconversion of succinate and fumarate, the redox potentials of the biologically available ubiquinone and menaquinone aid in driving the chemical reaction in one direction. In the complex II homolog quinol:fumarate reductase, it has been demonstrated that menaquinol oxidation requires at least one proton shuttle, but many of the remaining mechanistic details of menaquinol oxidation are not fully understood, and little is known about ubiquinone reduction. In the current study, structural and computational studies suggest that the sequential removal of the two menaquinol protons may be accompanied by a rotation of the naphthoquinone ring to optimize the interaction with a second proton shuttling pathway. However, kinetic measurements of site-specific mutations of quinol: fumarate reductase variants show that ubiquinone reduction does not use the same pathway. Computational docking of ubiquinone followed by mutagenesis instead suggested redundant proton shuttles lining the ubiquinone-binding site or from direct transfer from solvent. These data show that the quinone-binding site provides an environment that allows multiple amino acid residues to participate in quinone oxidoreduction. This suggests that the quinone-binding site in complex II is inherently plastic and can robustly interact with different types of quinones.