Chem763FinalSpring 2007answers1

Chem763FinalSpring 2007answers1 - Spring 2007 Chemistry 763...

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Spring 2007 Chemistry 763 Membranes FINAL EXAMINATION 1. (6 pts) Describe three mechanisms to alter energy transfer to PSII involving the ChlZ cycle, LHC migration, and zeaxanthin cycle. Chlz- The Chlz cycle operates once every 1000 times one electron goes on to the electron transfer branch of PSII. The reason that this occurs is to prevent the formation of a long- lived P680+ cation. An electron coming down the ChlZ cycle is capable of re-reducing the cation to prevent destruction of the D1 protein of the reaction center. LHC- when phosphorylated, an LHC-II-P migrates to stroma-exposed thylakoids and associates with the PSI in order to funnel energy to the PSI reaction center. This occurs when light incident on the plant is closer to 680 nm and is controlled by a regulated enzyme- LHCII kinase (activated by QH 2 ). When 700 nm light is incident, LHC-II is incident, LHC-II are dephosphorylated by phospho LHC-II (phosphatase always active) and the LHC-II dissocates from PSI and migrates toward the granal thylakoids and associates with PSII. Zea cycle- Under high light illumination, the pH of the thylakoid lumen becomes more acidic (pH = 5.0). Under these conditions, the enzyme de-epoxidase is activated that converts xanthophyll violaxanthin to zeaxanthin. Zeaxanthin has the capacity of dispersing excess light energy as heat and hence does not allow the formation of free radicals and reactive oxygen species. At low light, the pH of the thylakoid lumen is once again made more alkaline, now an energy dependent epoxidase will convert zeaxanthin to violaxanthin, this xanthophylls is more efficient at light harvesting for transfer to chlorophyll for photochemical purposes. 2. (6 pts) Compare and contrast the structural organization of cofactors of the PSII and PSI reaction center complexes. Speculate on why there is parallel electron transfer using both pathways in PSI but not in PSII. Important points: C-2 symmetry PSI : show Chlsp Two sets of Chl monomers Two Phylloquinones Fx center PSII : show Chlsp Two Chl monomers Two Pheophytins Qa and Qb and non-heme Fe Two Chlxs and Cyt b559 heme
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In PS1 the two pathways converge on a common final electron acceptor, Fx and PSII the two pathways do not converge on a common electron acceptor and Qa and Qb have very different chemistries in terms of electron and proton acceptance.
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Chem763FinalSpring 2007answers1 - Spring 2007 Chemistry 763...

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