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Fig 6 - Metastable radical state, nonreactive with oxygen, is inherent to catalysis by respiratory and photosynthetic cytochromes bc 1 / b 6 f

Fig. 6. Simplified diagram of relative energy levels at different stages of reaction catalyzed by cyt b 6 f (A) and cyt bc 1 (B). Black dots represent electrons on the respective cofactor or quinone molecule. Yellow squares, Q p empty or occupied by substrate; purple squares, Q p with bound DBMIB semiquinone. Green and red arrows, downhill and uphill transitions between the states, respectively. Red frame indicates the states that are inaccessible in antimycin-inhibited cyt bc 1 . State 1 represents the most populated initial state for QH 2 oxidation under steady-state turnover (in antimycin-inhibited cyt bc 1 heme b n is already reduced after the first turnover that takes place within experimental dead time). This reaction results in reduction of FeS and heme b p (transition from 1 to 3 involving unstable SQ in 2). From 3 downhill reactions to 4 or 5 are possible and 5 undergoes further downhill transition to 6 (oxidation of heme b n ) allowing next turnover. State 4 is metastable state containing SQ spin-coupled to reduced FeS. Population of 4 increases when transition to 5 is blocked (antimycin) or transition from 5 to 6 is slow with respect to the transition from 1 to 3. The stationary level of 4 depends on energetic gap between 4 and 5 (gray double arrow), which differs within bc family. State 4 lays below energetic level of 8 in which O 2 − is generated. State 7 involves stable DBMIB semiquinone spincoupled to FeS. The gray square with a gradient depicts uncertainty in E m values for hemes b in cyt b 6 f.  
Simplified diagram of relative energy levels at different stages of reaction catalyzed by cyt b 6 f (A) and cyt bc 1 (B). Black dots represent electrons on the respective cofactor or quinone molecule. Yellow squares, Q p empty or occupied by substrate; purple squares, Q p with bound DBMIB semiquinone. Green and red arrows, downhill and uphill transitions between the states, respectively. Red frame indicates the states that are inaccessible in antimycin-inhibited cyt bc 1 . State 1 represents the most populated initial state for QH 2 oxidation under steady-state turnover (in antimycin-inhibited cyt bc 1 heme b n is already reduced after the first turnover that takes place within experimental dead time). This reaction results in reduction of FeS and heme b p (transition from 1 to 3 involving unstable SQ in 2). From 3 downhill reactions to 4 or 5 are possible and 5 undergoes further downhill transition to 6 (oxidation of heme b n ) allowing next turnover. State 4 is metastable state containing SQ spin-coupled to reduced FeS. Population of 4 increases when transition to 5 is blocked (antimycin) or transition from 5 to 6 is slow with respect to the transition from 1 to 3. The stationary level of 4 depends on energetic gap between 4 and 5 (gray double arrow), which differs within bc family. State 4 lays below energetic level of 8 in which O 2 − is generated. State 7 involves stable DBMIB semiquinone spincoupled to FeS. The gray square with a gradient depicts uncertainty in E m values for hemes b in cyt b 6 f.  
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