The functions of the two b-type cytochromes in the plant thylakoid membrane: Cytochrome b6 and cytochrome b559

Paul Nicholas Furbacher, Purdue University

Abstract

Cytochrome b559, an essential component of Photosystem II, remains an enigma. The current consensus view is that the cytochrome functions to provide protection against Photosystem II photoinhibition by providing electrons to quench the highly oxidizing reaction center chlorophyll, P$\sb{680}\sp+.$ A new model is presented which incorporates aspects of the current models and specifies more clearly the conditions in which cyt b559 could be expected to provide protection against photoinhibitory damage. Cytochrome b6 of the cytochrome b6f complex is analogous to cyt b of the cytochrome $bc\sb1$ complex of mitochondria and purple photosynthetic bacteria. By functional analogy, the Q cycle mechanism has been extended to explain the redox role of cyt b6. Interheme electron transfer, an essential aspect of the Q cycle mechanism, was examined. NADPH preferentially reduced heme $b\sb{\rm n},$ but only with ferredoxin present. Whether heme $b\sb{\rm n}$ was reduced or oxidized, flash reduction of cyt b6 showed the same extent of enhancement in the presence of NQNO, an inhibitor thought to prevent oxidation of heme $b\sb{\rm n}.$ This implies that the observed effect of NQNO applies to the kinetics of heme $b\sb{\rm p},$ rather than to heme $b\sb{\rm n}.$ Multiple flashes did not increase the extent of cyt b6 reduction, contrary to what the Q cycle hypothesis would predict. NADPH and ferredoxin induced a four- to five-fold more rapid rate of cyt b6 re-oxidation and an amplitude of re-oxidation which was larger than the initial reduction amplitude. Enhanced re-oxidation suggests that both hemes undergo cooperative oxidation when heme $b\sb{\rm n}$ is pre-reduced, but does not discriminate between the interheme transfer from heme $b\sb{\rm p}$ to heme $b\sb{\rm n},$ or independent transfer of electrons from each heme, to doubly reduce a quinone. The apparent lack of efficient electron transfer between the two hemes and the existence of a pathway from ferredoxin to the quinone pool through heme $b\sb{\rm n}$ suggests that the Q cycle mechanism is not the predominant functional pathway of electron transport in cyt b6, and that the hemes may participate in a number of redox functions, together or separately. (Abstract shortened by UMI.)

Degree

Ph.D.

Advisors

Cramer, Purdue University.

Subject Area

Biophysics|Biochemistry|Botany

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