Summary: wild type showed a significant deuterium kinetic isotope effect on
cytochrome b reduction/oxidation kinetics whereas D2O had little
effect on the mutant. Redox titrations of membranes revealed mid-
point potentials (Em7) of $À30 mV and À120 mV for the two b
hemes in both the mutant and wild type. These functional data
define cytochrome b6 arginine 214 as a key residue for Qi-site func-
tion, cytochrome bf turnover, and photosynthetic growth. In the
bf structures, R214 lies near the Qi pocket and the newly discovered
c0
(or x) heme. Our data suggest involvement of PetB­Arg214
(Chlamydomonas, Mastigocladus R207) in plastoquinone binding,
in midpoint potential determination of the c0
(x) heme, and possibly
in proton transfer reactions at the Qi-site.
Production of reactive oxygen species (ROS) may result from
blockage of electron flow through the low potential chain. To test
this, we measured superoxide production in wild type, PetB­R214H,
and PetC­D2G mutants of Synechococcus (Table 1). Mutations
PetC-D2G in the high potential chain (Yan & Cramer 2003) and
PetB-R214H in the low potential chain (current study) slow overall

Source: Allen, John F. - School of Biological and Chemical Sciences, Queen Mary, University of London

Collections: Renewable Energy; Biology and Medicine