Finally, one of the most characteristic features of the
green algal [Fe]-hydrogenase type is confirmed by the hydA
sequence of C. fusca. There is no indication of any accessory
[Fe–S]-cluster (F-cluster), which usually functions as a
mediator of electron transport between the natural electron
donor and the catalytic center of the [Fe]-hydrogenase [7,9].
The crystal structure of the [Fe]-hydrogenase from C.
pasteurianum [21] was used as a template to develop a
model on the presumable spatial structure of C. fusca HydA,
using the protein modeling server Swiss Model (Fig. 5). In
this structural model, the position of the active center and
especially the sites of the cysteinyl residues coordinating the
H-cluster are conserved. The analysis of the modeled
structure showed that other cysteinyl residues within the
amino acid sequence do not compensate the lack of cysteines,
which usually bind the F-clusters. Since the PetF
functions as an effective electron donor for the [Fe]-hydrogenase
of green algae [13], it is likely that PetF interacts
directly with the catalytic center of HydA. By comparing the
modeled structure of HydA from C. fusca with the crystallographically
determined structure of the C. fusca ferredoxin
[22], it became obvious that structural and electrostatic
interactions between both molecules are possible.