This view shows enzymes only for those organisms listed below, in the list of taxa known to possess the pathway. If an enzyme name is shown in bold, there is experimental evidence for this enzymatic activity.
Synonyms: photosynthetic hydrogen generation
|Superclasses:||Generation of Precursor Metabolites and Energy → Hydrogen Production|
Unicellular green algae can utilize molecular H2 in the dark to drive cellular metabolism, or evolve H2 in the light with electrons derived from the photosynthetic oxidation of water [Melis01]. H2 is produced anaerobically when mitochondrial respiration is inhibited, and this permits electron flow through the electron transport chain with the concommitant generation of ATP [Ghirardi00].
Molecular oxygen suppresses expression of the genes associated with H2 production and also inhibits the hydrogenase enzyme, hence under aerobic conditions, H2 is not evolved [Melis04]. A prior anaerobic incubation of the algae in the dark can remove this inhibition and induces expression of a [FeFe] hydrogenase. A subsequent illumination then drives H2 evolution for a short period of time, till the release of photosynthetic oxygen inhibits the process. Photosystem II (PSII) mediates the light driven reaction of splitting water molecules into H+ and oxygen.
About This Pathway
During eukaryotic photosynthesis, electron transport is driven by photons captured by the antenna proteins LHCII/LHCI bound to PSII and PSI respectively. Linear photosynthetic electron transport originating from water oxidation, proceeds via PSII, cytochrome b6-f and PSI to the chloroplast ferredoxin [Rochaix11]. Ferredoxin can pass electrons to several further acceptors. Under aerobic conditions, most of the electrons are used to reduce NADP+ to NADPH by ferredoxin-NADP+ reductase (FNR). The NADPH is then consumed by reductive CO2 assimilation in the Calvin-Benson-Bassham cycle.
The catalytic site, also known as the "hydrogen cluster" (HC) of [FeFe] hydrogenase accepts electrons from reduced ferredoxin [Melis00]. The HC generates molecular H2 utilizing protons as a sink for these high potential-energy electrons [Melis07].
Under conditions that inhibit photosynthesis and generate anoxia, H2 production naturally occurs. This requires a fully active PSI for electron transfer to ferredoxin. PSII activity is partially inhibited by a block in turnover of the D1 protein causing a reduction in photosynthetic oxygen evolution [Zhang00a]. However, PSII still retains a low level of water oxidation activity, which can feed some electrons into the electron transport chain. When the level of oxygen evolved by PSII drops below respiratory consumption, the cells switch to anaerobic metabolism and express the hydrogenase.
The majority of the electrons required for H2 evolution during anoxia come from the redox equivalents of fermentative metabolism. These are supplied into the electron transport chain by an NAD(P)H-plastoquinone-oxidoreductase through non-photochemical PQ reduction. Prior aerobic PSII activity is crucial for build up of the starch and protein required for this.
During prolonged fermentative metabolism in Chlamydomonas reinhardtii, formate and ethanol are generated. This creates alternate sources for reduced ferredoxin, which feeds directly into H2 production at the level of the [FeFe] hydrogenase.
Variants: hydrogen production I , hydrogen production II , hydrogen production III , hydrogen production IV , hydrogen production V , hydrogen production VI , nitrogen fixation I (ferredoxin) , nitrogen fixation II (flavodoxin) , superpathway of hydrogen production
Hemschemeier08: Hemschemeier A, Jacobs J, Happe T (2008). "Biochemical and physiological characterization of the pyruvate formate-lyase Pfl1 of Chlamydomonas reinhardtii, a typically bacterial enzyme in a eukaryotic alga." Eukaryot Cell 7(3);518-26. PMID: 18245276
Hemschemeier08a: Hemschemeier A, Fouchard S, Cournac L, Peltier G, Happe T (2008). "Hydrogen production by Chlamydomonas reinhardtii: an elaborate interplay of electron sources and sinks." Planta 227(2);397-407. PMID: 17885762
Hemschemeier11: Hemschemeier A, Happe T (2011). "Alternative photosynthetic electron transport pathways during anaerobiosis in the green alga Chlamydomonas reinhardtii." Biochim Biophys Acta. PMID: 21376011
Melis00: Melis A, Zhang L, Forestier M, Ghirardi ML, Seibert M (2000). "Sustained photobiological hydrogen gas production upon reversible inactivation of oxygen evolution in the green alga Chlamydomonas reinhardtii." Plant Physiol 122(1);127-36. PMID: 10631256
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Albus10: Albus CA, Ruf S, Schottler MA, Lein W, Kehr J, Bock R (2010). "Y3IP1, a nucleus-encoded thylakoid protein, cooperates with the plastid-encoded Ycf3 protein in photosystem I assembly of tobacco and Arabidopsis." Plant Cell 22(8);2838-55. PMID: 20807881
Alizadeh94: Alizadeh S, Nechushtai R, Barber J, Nixon P (1994). "Nucleotide sequence of the psbE, psbF and trnM genes from the chloroplast genome of Chlamydomonas reinhardtii." Biochim Biophys Acta 1188(3);439-42. PMID: 7803458
Allahverdiyeva07: Allahverdiyeva Y, Mamedov F, Suorsa M, Styring S, Vass I, Aro EM (2007). "Insights into the function of PsbR protein in Arabidopsis thaliana." Biochim Biophys Acta 1767(6);677-85. PMID: 17320041
Allakhverdiev11: Allakhverdiev SI, Tsuchiya T, Watabe K, Kojima A, Los DA, Tomo T, Klimov VV, Mimuro M (2011). "Redox potentials of primary electron acceptor quinone molecule (QA)- and conserved energetics of photosystem II in cyanobacteria with chlorophyll a and chlorophyll d." Proc Natl Acad Sci U S A 108(19);8054-8. PMID: 21521792
Bingham91: Bingham SE, Xu RH, Webber AN (1991). "Transformation of chloroplasts with the psaB gene encoding a polypeptide of the photosystem I reaction center." FEBS Lett 292(1-2);137-40. PMID: 1959594
Boudreaux01: Boudreaux B, MacMillan F, Teutloff C, Agalarov R, Gu F, Grimaldi S, Bittl R, Brettel K, Redding K (2001). "Mutations in both sides of the photosystem I reaction center identify the phylloquinone observed by electron paramagnetic resonance spectroscopy." J Biol Chem 276(40);37299-306. PMID: 11489879
Buchel99: Buchel C, Barber J, Ananyev G, Eshaghi S, Watt R, Dismukes C (1999). "Photoassembly of the manganese cluster and oxygen evolution from monomeric and dimeric CP47 reaction center photosystem II complexes." Proc Natl Acad Sci U S A 96(25);14288-93. PMID: 10588698
Buschlen91: Buschlen S, Choquet Y, Kuras R, Wollman FA (1991). "Nucleotide sequences of the continuous and separated petA, petB and petD chloroplast genes in Chlamydomonas reinhardtii." FEBS Lett 284(2);257-62. PMID: 2060646
Chi00: Chi YI, Huang LS, Zhang Z, Fernandez-Velasco JG, Berry EA (2000). "X-ray structure of a truncated form of cytochrome f from chlamydomonas reinhardtii." Biochemistry 39(26);7689-701. PMID: 10869174
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