Understanding and Harnessing Hydrogenases, Biological Dihydrogen Catalysts - The Metal-Driven Biogeochemistry of Gaseous Compounds in the Environment - page 107

Figure 5 The possible routes for achieving photosynthetic hydrogenase (Hase) catalyzed
dihydrogen production. Whereas some cyanobacterial species produce [NiFe] hydrogenases
which can be reduced by both NADPH and reduced ferredoxin (Fd), the [FeFe] hydrogenases
produced by some algal species only receive electrons from ferredoxin. In all cases the rate of
biohydrogen production will not be limited by the Calvin cycle, unlike photosynthetic production
of carbon fuels. Within the thylakoid membrane the light-driven oxygen-producing reaction which
induces hydrogenase inhibition and down regulation is catalyzed by photosystem II (PS II), which
is wired to the second solar excitation center, photosystem I (PS I), via the mobile redox carriers
plastoquinone (PQ) and plastocyanin (PC) and the proton pumping protein complex cytochrome
b 6 f (Cytb 6 f). Other abbreviations: 'ADP' is adenosine 5'-diphosphate; 'FNR' is ferredoxin-
NADP + reductase; 'G3P' is glyceraldehyde 3-phosphate; 'ATPase' is adenylpyrophosphatase.
For further details see [ 41 ], [ 40 ] and [ 39 ].
to boost solar H 2 activity internally induced anaerobicity must occur. When cell
cultures are deprived of sulfur the rate of photosynthesis is reduced to the level of
cellular respiration and the algae consume the O 2 produced by photosystem II at the
rate it is made [ 41 , 44 ]. Low O 2 conditions are therefore established leading to
expression of the hydA1 hydrogenase gene and sustained H 2 evolution at the expense
of CO 2 assimilation. Alternatively, rather than using sulfur-restricted growth condi-
tions, the same metabolic outcome has been induced by creating mutant cells with
diminished sulfate transport activities [ 41 ]. A second approach is to re-engineer algal
[FeFe] hydrogenases to make them O 2 -insensitive, and this requires developments in
screening methodologies as well as genetic experimentation.
Engineering cyanobacteria or algae to act as catalytic solar H 2 units requires a
'systems biology' approach, e.g., the biology of the whole cell must be considered.
We need an in depth understanding of the genetic and biochemical control of
hydrogenase assembly, a chemical understanding of the reactivity and redox