title: Control of energy dissipation and photochemical activity in photosystem I by NADP-dependent reversible conformational changes.
abstract: Addition of NADP(+) to thylakoid membranes or isolated photosystem I (PSI) submembrane fractions quenched chlorophyll fluorescence by up to 40% at low or room temperature. This quenching was reversed by NADPH. Similar quenching was also observed with the addition of heparin or thenoyltrifluoroacetone (TTFA), inhibitors that bind ferredoxin:NADP(+) reductase (FNR) and prevent reduction of NADP(+). The NADP(+)-induced quenching coincided with a reversible conformational change of the secondary protein structure in the PSI submembrane fractions where 20% of the alpha-helix conformations were transformed mainly into beta-sheet-like structures. Further, P700 photooxidation was retarded due to this conformational change, and about 25% of the centers could not be photooxidized, these changes being also reversible with addition of NADPH. The above modifications in the presence of NADP(+) also increased photodamage processes under strong illumination, and NADPH protected it. Conformational modification of FNR upon binding of NADP(+) or NADPH is proposed to trigger the macromolecular changes in a larger part of the protein complex of PSI. The conformational changes must increase the intermolecular distances and change the mutual orientation between the various cofactors in the PSI complex. This new control mechanism of energy dissipation and photochemical activity by NADP(+)/NADPH is proposed to increase the turnover rate of PSI under conditions when both linear and cyclic electron transport activities must be supported.
authors.affiliation: Groupe de Recherche en Energie et Information Biomoléculaires, Université du Québec à Trois-Rivières, C.P. 500 Trois-Rivières, Québec, Canada G9A 5H7.
authors.forename: Subramanyam
authors.initials: S
authors.lastname: Rajagopal
authors.forename: Nikolai G
authors.initials: NG
authors.lastname: Bukhov
authors.forename: Heidar-Ali
authors.initials: HA
authors.lastname: Tajmir-Riahi
authors.forename: Robert
authors.initials: R
authors.lastname: Carpentier
mesh_terms: D002734:Chlorophyll / Q000737:chemistry; D005288:Ferredoxins / Q000737:chemistry; D008027:Light; D009249:NADP / Q000737:chemistry*; D010084:Oxidation-Reduction; D018532:Pisum sativum; D010777:Photochemistry / Q000379:methods; D045331:Photosystem I Protein Complex / Q000737:chemistry*; D011487:Protein Conformation; D013050:Spectrometry, Fluorescence; D017550:Spectroscopy, Fourier Transform Infrared; D018724:Spinacia oleracea; D013816:Thermodynamics; D020524:Thylakoids / Q000737:chemistry
chemical_list: D005288:Ferredoxins; D045331:Photosystem I Protein Complex; D002734:Chlorophyll; D009249:NADP