Fig. 1.

Schematic representation of the membrane-bound formate dehydrogenases and hydrogenases of E. coli. Fdh-H, Fdh-N and Fdh-O, along with Hyd-1, Hyd-2 and Hyd-3, the latter of which forms a key component of the formate-hydrogenlyase (FHL) complex, are shown. With the exception of HycB, the electron-transferring small subunit of Fdh-H, only the catalytic subunits of the enzymes are shown with their respective names. The periplasmic (out) and cytoplasmic (in) sides of the membrane are indicated, while the abbreviations NiFe, MoSe and Q signify the [NiFe] cofactor, the molybdenum cofactor/selenocysteine and quinone pool, respectively. Electron flow within the enzyme complexes and to the quinone pool are indicated by thin arrows in the enzyme complexes, while the thick dashed arrow with the question mark signifies that the Fdh-H also forms a complex with another complex (complex X) or complexes. Complex X could represent the second FHL complex comprising the Hyf components of Hyd-4 [3]; however, whether this complex is membrane-associated remains to be determined.