Kinetics of the reduction and oxidation of cytochromes b6 and ƒ in isolated chloroplasts

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Abstract

Absorbance changes induced by flash illumination of a chloroplast suspension were monitored kinetically at selected wavelengths between 510 and 575 nm. Digital subtraction of the P518 component from the total transient signal permitted isolation of the responses due to cytochromes ƒ and b6. The half rise time for cytochrome b6 reduction (1.3±0.1 ms) was much greater than a previously reported value (< 10 μs); the half time for cytochrome b6 oxidation was 35±4 ms. Cytochrome ƒ was oxidized with a half time of about 0.22 ms and the subsequent reduction occurred in two phases with half times of about 7.3 and 83 ms. These kinetic data show that cytochrome b6 cannot be a primary electron acceptor in Photosystem 1. The rate of oxidation of cytochrome b6 is consonant with this cytochrome being the source of electrons for the slower phase of cytochrome ƒ reduction.

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