Figure 29.7.6.

Assay of Cyt b₆f electron transfer activity. The Cyt b₆f complex in cyanobacterial thylakoid membranes catalyzes electron transfer from the electron donor decylplastoquinol (dPQH₂) to the electron acceptor cytochrome c (Cyt c). dPQH₂ is added to the assay buffer and a baseline is recorded. The electron transfer reaction is initiated by addition of cyanobacterial membranes. Reduced Cyt c has an absorbance peak at 551 nm. As one electron is required to reduce one molecule of Cyt c, estimation of Cyt c reduction per unit time yields the catalytic activity of the Cyt b₆f complex (calculations performed from initial electron transfer rate; slope shown as gray dotted line). Since Cyt b₆f concentration cannot be determined in intact membranes, the specific rate is expressed in units of electrons transferred per 10³ chlorophyll per second. The slow background rate of electron transfer from dPQH₂ to Cyt c in the absence of Cyt b₆f (gray dotted line marked background) is subtracted from the slope of b₆f-catalyzed electron transfer.