Dynamic Function of the Spacer Region of Acetogenins in the Inhibition of Bovine Mitochondrial NADH-Ubiquinone Oxidoreductase (Complex I)

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Abstract
Studies of the action mechanism of acetogenins, the most potent and structurally unique inhibitors of bovine heart mitochondrial complex I (NADH-ubiquinone oxidoreductase), are valuable in characterizing the inhibitor binding site in this enzyme. Our previous study deepened our understanding of the dynamic function of the spacer region of bis-THF acetogenins [Abe, M., et al. (2005) Biochemistry 44, 14898−14906] but, at the same time, posed new important questions. First, while the two toxophores (i.e., the hydroxylated THF and the γ-lactone rings) span a distance shorter than that of the extended 13 carbon atoms [-(CH2)13-], what is the apparent optimal length of the spacer for the inhibition of 13 carbon atoms? In other words, what is the functional role of the additional methylene groups? Second, why was the inhibitory potency of the mono-THF derivative, but not the bis-THF derivative, drastically reduced by hardening the spacer covering 10 carbon atoms into a rodlike shape [-CH2-(C≡C)4-CH2-]? This study was designed not only to answer these questions but also to further disclose the dynamic functions of the spacer. We here synthesized systematically designed acetogenins, including mono- and bis-THF derivatives, and evaluated their inhibitory effects on bovine complex I. With regard to the first question, we demonstrated that the additional methylenes enhance the hydrophobicity of the spacer region, which may be thermodynamically advantageous for bringing the polar γ-lactone ring into the membrane-embedded segment of complex I. With regard to the second question, we observed that a decrease in the flexibility of the spacer region is more adverse to the action of the mono-THF series than that of the bis-THF series. As a cause of this difference, we suggest that for bis-THF derivatives, one of the two THF rings, being adjacent to the spacer, is capable of working as a pseudospacer to overcome the remarkable decrease in the conformational freedom and/or the length of the spacer. Moreover, using photoresponsive acetogenins that undergo drastic and reversible conformational changes with alternating UV−vis irradiation, we provided further evidence that the spacer region is free from steric congestion arising from the putative binding site probably because there is no receptor wall for the spacer region.