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Tham khảo tài liệu 'materials for the hydrogen economy (2007) episode 6', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | 126 Materials for the Hydrogen Economy 5.2.1 Oxygen-Tolerant Hydrogenase Systems The development of an O2-tolerant hydrogenase by either mutagenesis of existing hydrogenases or transforming O2-tolerant hydrogenase genes from bacteria into cyanobacteria is only one of the biological requirements needed to obtain a photosynthetic system that produces H2 efficiently and at high rates under aerobic conditions. As indicated above the genes encoding for an O2-tolerant enzyme and its accompanying assembly enzymes must also be expressed under aerobic conditions and the assembly process must occur in the presence of O2 as well. These are major issues that we have just recently started to investigate in green algae 13 27 28 and the factors required for optimal expression of the hydrogenase genes in algae and cyanobacteria are not completely understood at present. However even if the hydrogenase-O2-sensitivity problem is solved other major issues remain to be addressed before a biological system might become a commercial reality. The first which is common to both green algae and cyanobacteria is the existence of competing pathways for photosynthetic reductants. In both organisms the major competing pathway under aerobic conditions is the CO2 fixation pathway. In green algae photosynthetically reduced ferredoxin donates electrons to FNR ferredoxin-NADPH-oxidoreductase FTR ferredoxin-thioredoxin reductase nitrite reductase sulfite reductase glutamate synthase or hydrogenase.29 The binding affinity of ferredoxin to each of these enzymes varies from 2.6 pM in the case of FNR30 to 20 pM in the case of nitrite reductase31 and has been estimated to be 10 to 35 pM in the case of the hydrogenase.32-34 These estimates suggest that the interaction between ferredoxin and hydrogenase will have to be genetically manipulated in order to ensure that most of the photosynthetically generated reductants will be utilized for H2 production. In the case of cyanobacteria two additional competing .
