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 [Zhang00c]. 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.
Superpathways: superpathway of photosynthetic 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
Zhang00c: Zhang L, Paakkarinen V, van Wijk KJ, Aro EM (2000). "Biogenesis of the chloroplast-encoded D1 protein: regulation of translation elongation, insertion, and assembly into photosystem II." Plant Cell 12(9);1769-82. PMID: 11006346
Dubini09: Dubini A, Mus F, Seibert M, Grossman AR, Posewitz MC (2009). "Flexibility in anaerobic metabolism as revealed in a mutant of Chlamydomonas reinhardtii lacking hydrogenase activity." J Biol Chem 284(11);7201-13. PMID: 19117946
Florin01: Florin L, Tsokoglou A, Happe T (2001). "A novel type of iron hydrogenase in the green alga Scenedesmus obliquus is linked to the photosynthetic electron transport chain." J Biol Chem 276(9);6125-32. PMID: 11096090
Forestier03: Forestier M, King P, Zhang L, Posewitz M, Schwarzer S, Happe T, Ghirardi ML, Seibert M (2003). "Expression of two [Fe]-hydrogenases in Chlamydomonas reinhardtii under anaerobic conditions." Eur J Biochem 270(13);2750-8. PMID: 12823545
Godman10: Godman JE, Molnar A, Baulcombe DC, Balk J (2010). "RNA silencing of hydrogenase(-like) genes and investigation of their physiological roles in the green alga Chlamydomonas reinhardtii." Biochem J 431(3);345-51. PMID: 20726841
Happe93: Happe T, Naber JD (1993). "Isolation, characterization and N-terminal amino acid sequence of hydrogenase from the green alga Chlamydomonas reinhardtii." Eur J Biochem 214(2);475-81. PMID: 8513797
Meuer99: Meuer J, Bartoschek S, Koch J, Kunkel A, Hedderich R (1999). "Purification and catalytic properties of Ech hydrogenase from Methanosarcina barkeri." Eur J Biochem 265(1);325-35. PMID: 10491189
Park06: Park YJ, Yoo CB, Choi SY, Lee HB (2006). "Purifications and characterizations of a ferredoxin and its related 2-oxoacid:ferredoxin oxidoreductase from the hyperthermophilic archaeon, Sulfolobus solfataricus P1." J Biochem Mol Biol 39(1);46-54. PMID: 16466637
Soboh04: Soboh B, Linder D, Hedderich R (2004). "A multisubunit membrane-bound [NiFe] hydrogenase and an NADH-dependent Fe-only hydrogenase in the fermenting bacterium Thermoanaerobacter tengcongensis." Microbiology 150(Pt 7);2451-63. PMID: 15256587
Sun10: Sun J, Hopkins RC, Jenney FE, McTernan PM, Adams MW (2010). "Heterologous expression and maturation of an NADP-dependent [NiFe]-hydrogenase: a key enzyme in biofuel production." PLoS One 5(5);e10526. PMID: 20463892
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