Comparison of the Structural Features of Ubiquinone Reduction Sites Between Glucose Dehydrogenase in Escherichia coli and Bovine Heart Mitochondrial Complex I

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Abstract
To characterize the structural features of the ubiquinone reduction site of glucose dehydrogenase (GlcDH) in Escherichia coli, we performed structure/activity studies of a systematic set of synthetic ubiquinone analogues and specific inhibitors (synthetic capsaicins) of this site. Considering the proposed similarity of the quinone binding domain motif between GlcDH and one subunit of mitochondrial complex I [Friedrich, T., Strohdeicher, M., Hofhaus, G., Preis, D., Sahm, H. & Weiss, H. (1990) FEBS Lett. 265, 37–40], we compared the structure/activity profiles of the substrates and inhibitors for GlcDH with those for bovine heart mitochondrial complex I. With respect to GlcDH, replacement of one or both methoxy groups in the 2 and 3 positions of benzoquinone ring by ethoxy group(s) resulted in a drastic decrease in the electron accepting activity. The presence of a 5‐methyl group and the conformational property of the 6‐alkyl side chain did not significantly contribute to the activity. These results suggested that only half of the benzoquinone ring (the moiety corresponding to the 2 and 3 positions) is recognized by the quinone reduction site in a strict sense. In contrast, quinone analogues with structural modifications at all positions in the benzoquinone ring retained the activity with mitochondrial complex I. This finding indicated that the catalytic site of complex I is spacious enough to accommodate a variety of structurally different quinone derivatives. The correlation of the inhibitory potencies of a series of synthetic capsaicins between the two enzymes was very poor. These findings indicated that the binding environment of ubiquinone in GlcDH is very specific and differs from that in mitochondrial complex I.