Differences in protonation of ubiquinone and menaquinone in fumarate reductase from Escherichia coli.

Escherichia coli quinol-fumarate reductase operates with both natural quinones, ubiquinone (UQ) and menaquinone (MQ), at a single quinone binding site. We have utilized a combination of mutagenesis, kinetic, EPR, and Fourier transform infrared methods to study the role of two residues, Lys-B228 and Glu-C29, at the quinol-fumarate reductase quinone binding site in reactions with MQ and UQ. The data demonstrate that Lys-B228 provides a strong hydrogen bond to MQ and is essential for reactions with both quinone types. Substitution of Glu-C29 with Leu and Phe caused a dramatic decrease in enzymatic reactions with MQ in agreement with previous studies, however, the succinate-UQ reductase reaction remains unaffected. Elimination of a negative charge in Glu-C29 mutant enzymes resulted in significantly increased stabilization of both UQ-* and MQ-* semiquinones. The data presented here suggest similar hydrogen bonding of the C1 carbonyl of both MQ and UQ, whereas there is different hydrogen bonding for their C4 carbonyls. The differences are shown by a single point mutation of Glu-C29, which transforms the enzyme from one that is predominantly a menaquinol-fumarate reductase to one that is essentially only functional as a succinate-ubiquinone reductase. These findings represent an example of how enzymes that are designed to accommodate either UQ or MQ at a single Q binding site may nevertheless develop sufficient plasticity at the binding pocket to react differently with MQ and UQ.

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