Reaction Intermediates of Quinol Oxidation in a Photoactivatable System that Mimics Electron Transfer in the Cytochrome bc1 Complex

Journal Article · Wed Mar 30 04:00:00 EST 2005 · Journal of the American Chemical Society

DOI:https://doi.org/10.1021/ja043955g· OSTI ID:15015964

Cape, Jonathan L; Bowman, Michael K; Kramer, David M

Current competing models for the two-electron oxidation of quinol (QH{sub 2}) at the cytochrome bc{sub 1} complex and related complexes have different requirements for the reaction intermediate. At present, the intermediate species of the enzymatic oxidation process have not been observed or characterized, probably due to their transient nature. Here, we use a biomimetic oxidant, Ru(bpy){sub 2}(pbim)(PF6)2 (bpy = 2,2'-dipyridyl, pbim = 2-(2-benzimidazolate)pyridine) in an aprotic medium to probe the oxidation of the ubiquinol analogue, 2,3-dimethoxy-5-methyl-1,4-benzoquinol (UQH{sub 2}-0), an the plastoquinol analogue, trimethyl-1,4-benzoquinol (TMQH{sub 2}-0), using time-resolved and steady state spectroscopic techniques. This system qualitatively reproduces key features observed during ubiquinol oxidation by the mitochondrial cytochrome bc1 complex. Comparison of isotope dependent activation properties in the native and synthetic systems, as well as, analysis of the time-resolved direct-detection electron para magnetic resonance signals in the synthetic system allows us to conclude that: (1) the initial and rate-limiting step in quinol oxidation, both in the biological and biomimetic systems, involves electron and proton transfer, probably via a proton coupled electron transfer mechanism; (2) a neutral semiquinone intermediate is formed in the biomimetic system; and (3) oxidation of the QH*/QH{sub 2} couple for UQH{sub 2}-0, but not TMQH{sub 2}-0, exhibits a non-classical primary deuterium kinetic isotope effect on its Arrhenius activation energy ({Delta}G{sup TS}), where {Delta}G{sup TS} for the protiated form is larger than for the deuterated form. The same behavior is observed during steady state turnover of the cyt bc{sub 1} complex using ubiquinol, but not plastoquinol, as a substrate, leading to the conclusion that similar chemical pathways are involved in both systems. The synthetic system is an unambiguous n=1 electron acceptor and it is thus inferred that sequential oxidation of ubiquinol (by two sequential n=1 processes) is more rapid than a truly concerted (n=2) oxidation in the cyt bc{sub 1} complex.

• Research Organization: Pacific Northwest National Lab., Richland, WA (US), Environmental Molecular Sciences Laboratory (US)
• Sponsoring Organization: US Department of Energy (US)
• DOE Contract Number: AC05-76RL01830
• OSTI ID: 15015964
• Report Number(s): PNWD-SA-6728; 2358
• Journal Information: Journal of the American Chemical Society, Journal Name: Journal of the American Chemical Society Journal Issue: 12 Vol. 127; ISSN JACSAT; ISSN 0002-7863
• Country of Publication: United States
• Language: English

Related Subjects

59 BASIC BIOLOGICAL SCIENCES
72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS
ACTIVATION ENERGY
BINDING ENERGY
CONCERTED ELECTRON TRANSFER
CYTOCHROME BC1 COMPLEX
CYTOCHROMES
DEUTERIUM
ELECTRON TRANSFER
ELECTRONS
ENVIRONMENTAL MOLECULAR SCIENCES LABORATORY
ISOTOPE EFFECTS
KINETIC ISOTOPE EFFECT
KINETICS
MAGNETIC RESONANCE
OXIDATION
PROBES
PROTON COUPLED ELECTRON TRANSFER
PROTONS
QO SITE
REACTION INTERMEDIATES
RUTHENIUM
SEMIQUINONE
TIME-RESOLVED ELECTRON PARAMAGNETIC RESONANCE
TRANSIENTS
TRANSITION STATE
UBIQUINOL
VALENCE