The specificity of mitochondrial complex I for ubiquinones.
We report the first detailed study on the ubiquinone (coenzyme Q; abbreviated to Q) analogue specificity of mitochondrial complex I, NADH:Q reductase, in intact submitochondrial particles. The enzymic function of complex I has been investigated using a series of analogues of Q as electron acceptor substrates for both electron transport activity and the associated generation of membrane potential. Q analogues with a saturated substituent of one to three carbons at position 6 of the 2,3-dimethoxy-5-methyl-1,4-benzoquinone ring have the fastest rates of electron transport activity, and analogues with a substituent of seven to nine carbon atoms have the highest values of association constant derived from NADH:Q reductase activity. The rate of NADH:Q reductase activity is potently but incompletely inhibited by rotenone, and the residual rotenone-insensitive rate is stimulated by Q analogues in different ways depending on the hydrophobicity of their substituent. Membrane potential measurements have been undertaken to evaluate the energetic efficiency of complex I with various Q analogues. Only hydrophobic analogues such as nonyl-Q or undecyl-Q show an efficiency of membrane potential generation equivalent to that of endogenous Q. The less hydrophobic analogues as well as the isoprenoid analogue Q-2 are more efficient as substrates for the redox activity of complex I than for membrane potential generation. Thus the hydrophilic Q analogues act also as electron sinks and interact incompletely with the physiological Q site in complex I that pumps protons and generates membrane potential.
Centre for Molecular Biology and Medicine, Monash University, Clayton, Victoria, Australia.
M
M
Degli Esposti
A
A
Ngo
G L
GL
McMullen
A
A
Ghelli
F
F
Sparla
B
B
Benelli
M
M
Ratta
A W
AW
Linnane
D000818:Animals; D016227:Benzoquinones / Q000378:metabolism; D001665:Binding Sites; D002417:Cattle; D008564:Membrane Potentials / Q000502:physiology; D008928:Mitochondria / Q000201:enzymology*; D009243:NAD / Q000378:metabolism; D016660:NAD(P)H Dehydrogenase (Quinone) / Q000378:metabolism*; D010084:Oxidation-Reduction; D012402:Rotenone / Q000494:pharmacology; D012680:Sensitivity and Specificity; D013379:Substrate Specificity; D014451:Ubiquinone / Q000031:analogs & derivatives / Q000378:metabolism*
D016227:Benzoquinones; D012402:Rotenone; D009243:NAD; D014451:Ubiquinone; C004532:quinone; D016660:NAD(P)H Dehydrogenase (Quinone)