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University of Illinois at Urbana-Champaign

The Rieske iron sulfur protein (ISP) determines the limiting rate in the cytochrome bc1 complex

Monday November 14, 2011

4:00pm PHYS 203

This seminar will be on Monday November 14th at 4:00pm in Phys 203

http://mcb.illinois.edu/faculty/profile/crofts

The bc1 complex in the mitochondrial respiratory chain, and in many bacterial respiratory and photosynthetic electron transfer chains, oxidizes ubiquinol (QH2) and reduces cytochrome c through a Q-cycle mechanism. The work released is used to pump protons across the membrane and drive ATP synthesis through the proton gradient generated. The critical reaction is that in which QH2 is oxidized, which involves delivery of the two electrons and protons to different pathways, - the so-called bifurcated reaction. The first of these has ISPox as the electron acceptor, and represents the rate limiting process for turnover of the enzyme at saturating substrate. We have explored the dependence on rate of the thermodynamic characteristics of ISP by use of mutants strains in which Em and pK are changed, and used these to establish a mechanism. The reaction is a proton-coupled electron transfer in which the probability of transfer of the proton modulates the frequency of electron transfer, well described by a Marcus mechanism for electron transfer modified by a Brönsted term for the proton distribution. The changes in thermodynamic properties of ISP in mutants have been related to structural features through crystallographic resolution at ~1 Å, and extensive spectroscopic evidence from high-resolution EPR.