A single-amino-acid lid renders a gas-tight compartment within a membrane-bound transporter.

Proteins undergo structural fluctuations between nearly isoenergetic substates. Such fluctuations are often intimately linked with the functional properties of proteins. However, in some cases, such as in transmembrane ion transporters, the control of the ion transport requires that the protein is designed to restrict the motions in specific regions. In this study, we have investigated the dynamics of a membrane-bound respiratory oxidase, which acts both as an enzyme catalyzing reduction of O(2) to H(2)O and as a transmembrane proton pump. The segment of the protein where proton translocation is controlled ("gating" region) overlaps with a channel through which O(2) is delivered to the catalytic site. We show that the replacement of an amino acid residue with a small side chain (Gly) by one with a larger side chain (Val), in a narrow part of this channel, completely blocks the O(2) access to the catalytic site and results in formation of a compartment around the site that is impermeable to small gas molecules. Thus, the protein motions cannot counter the blockage introduced by the mutation. These results indicate that the protein motions are restricted in the proton-gating region and that rapid O(2) delivery to the catalytic site requires a gas channel, which is confined within a rigid protein body.