Wednesday, 5 November 2014
Kinetic evidence against partitioning of the ubiquinone pool and the catalytic relevance of respiratory-chain supercomplexes
James N. Blaza, Riccardo Serreli, Andrew J. Y. Jones, Khairunnisa Mohammed, and Judy Hirst
The electron transport chain (ETC) consists of protein complexes, which pump protons across a membrane to store electrochemical potential energy. This potential energy is then converted by another element of the ETC into an energy currency: ATP. Recent studies have shown that the complexes of the electron transport chain exist in supercomplexes, where multimers of the ETC components combine, which is thought to increase their efficiency.
The authors present evidence disputing the importance of these supercomplexes (but not their existence). Ubiquinone (Q) is a protein which ferries electrons through the lipid membrane of the ETC, and exists in a pool between the leaflets of the membrane. It has been proposed that the Q pool is partitioned such that supercomplexes have a separate pool to the TCA-cycle. However, the authors show that altering the levels of substrate for the ETC and TCA causes global changes in the Q pool, which is evidence against partitioning.
Supercomplexes are also widely cited to exist because of the observation that flux through the ETC is almost entirely controlled by both complex I and III of the ETC, suggesting they exist in a complex together. The authors here, however, question this finding because these studies rely on a particular inhibitor, rotenone, to inhibit complex I. Upon replication of previously published protocols, the authors find flux control coefficients of >100%, which is impossible. Using alternative inhibitors, the authors measured more modest control coefficients, which the authors interpret as evidence against the catalytic importance of supercomplexes.