Quinone binding in respiratory complex I: Going through the eye of a needle. The squeeze-in mechanism of passing the narrow entrance of the quinone site.

At the joint between the membrane and hydrophilic arms of the enzyme, the structure of the respiratory complex I reveals a tunnel-like Q-chamber for ubiquinone binding and reduction. The narrow entrance of the quinone chamber located in ND1 subunit forms a bottleneck (eye of a needle) which in all resolved structures was shown to be too small for a bulky quinone to pass through, and it was suggested that a conformational change is required to open the channel. The closed bottleneck appears to be a well-established feature of all structures reported so-far, both for the so-called open and closed states of the enzyme, with no indication of a stable open state of the bottleneck. We propose a squeeze-in mechanism of the bottleneck passage, where dynamic thermal conformational fluctuations allow quinone to get in and out. Here, using molecular dynamics simulations of the bacterial enzyme, we have identified collective conformational changes that open the quinone chamber bottleneck. The model predicts a significant reduction-due to a need for a rare opening of the bottleneck-of the effective bi-molecular rate constant, in line with the available kinetic data. We discuss possible reasons for such a tight control of the quinone passage into the binding chamber and mechanistic consequences for the quinone two-electron reduction.