Inhibitor studies on non-photochemical plastoquinone reduction and H2 photoproduction in Chlamydomonas reinhardtii

Abstract

In the absence of PSII, non-photochemical reduction of plastoquinones (PQs) occurs following NADH or NADPH addition in thylakoid membranes of the green alga Chlamydomonas reinhardtii. The nature of the enzyme involved in this reaction has been investigated in vitro by measuring chlorophyll fluorescence increase in anoxia and light-dependent O(2) uptake in the presence of methyl viologen. Based on the insensitivity of these reactions to rotenone, a type-I NADH dehydrogenase (NDH-1) inhibitor, and their sensitivity to flavoenzyme inhibitors and thiol blocking agents, we conclude to the involvement of a type-II NADH dehydrogenase (NDH-2) in PQ reduction. Intact Chlamydomonas cells placed in anoxia have the property to produce H(2) in the light by a Fe-hydrogenase which uses reduced ferredoxin as an electron donor. H(2) production also occurs in the absence of PSII thanks to the existence of a non-photochemical pathway of PQ reduction. From inhibitors effects, we suggest the involvement of a plastidial NDH-2 in PSII-independent H(2) production in Chlamydomonas. These results are discussed in relation to the absence of ndh genes in Chlamydomonas plastid genome and to the existence of 7 ORFs homologous to type-II NDHs in its nuclear genome.

Characterization of Nda2, a Plastoquinone-reducing Type II NAD(P)H Dehydrogenase in Chlamydomonas Chloroplasts

“…The absence of genes encoding subunits of the chloroplast NDH-1 complex was confirmed by the recent sequencing of the C. reinhardtii nuclear genome. Despite the absence of a plastidial NDH-1 complex, significant non-photochemical reduction of PQ has been reported in Chlamydomonas thylakoids (4,5,54). Moreover, both chlororespiration and cyclic electron flow activity around PSI have been reported in the algal species (6,7,55,56).…”

“…Moreover, both chlororespiration and cyclic electron flow activity around PSI have been reported in the algal species (6,7,55,56). Based on a pharmacological approach, Mus et al (5) concluded that a type II NAD(P)H dehydrogenase would be involved in the NAD(P)H-dependent non-photochemical reduction of PQ in C. reinhardtii thylakoids. In bacterial species lacking NDH-1, type II dehydrogenases have been reported to sustain respiratory activity by allowing NADH turnover (12).…”

“…Although significant activity of non-photochemical PQ reduction was reported in Chlamydomonas reinhardtii thylakoids (4,5), it is generally considered that a type I Ndh complex is absent from algal chloroplasts (2). Indeed, most microalgal species, such as C. reinhardtii, and some gymnosperms species, such as Pinus thunbergii, do not harbor plastid ndh genes encoding hydrophobic subunits of the type I Ndh complex and, at least in the case of C. reinhardtii, the corresponding genes are absent from the nuclear genome, therefore, questioning the nature of the enzyme involved in non-photochemical reduction of PQs (2,5).…”

“…Based on pharmacological and physiological studies, we previously proposed that a NDH-2 type enzyme might be responsible for this activity in C. reinhardtii thylakoids (5). Type II NAD(P)H-dehydrogenases are monomeric enzymes that contain a flavin cofactor and, unlike type I NAD(P)H dehydrogenases (complex I), are non-proton pumping enzymes.…”

Chloroplast lipid droplet type II NAD(P)H quinone oxidoreductase is essential for prenylquinone metabolism and vitamin K 1 accumulation

“…Both the PGR5/PGRL1 pathway and the NDH complex have the ability to reduce plastoquinone (PQ) by using ferredoxin as the electron donor (6). A similar activity exists in the green alga Chlamydomonas reinhardtii, but this organism does not encode the subunits of the NDH complex (7)(8)(9). Instead, a monomeric type II NAD(P)H:quinone oxidoreductase, NDA2, takes over the function of the NDH complex in cyclic electron flow and chlororespiration (10,11).…”