Dissimilatory Fe (III) reduction by cytochrome c-552 in a thermophilic, obligately chemolithoautotrophic bacterium, Hydrogenobacter thermophilus TK-6

A thermophilic, obligately chemolithoauto- trophic H2-oxidizing bacterium, Hydrogenobacter thermophilus TK-6 can utilize O2 or NO3- as an electron acceptor. Here we show that this bacterium can grow autotrophically by the reduction of Fe(III)-EDTA or Fe(III)-DTPA (diethylene-triamine-pentaacetic acid) as an electron acceptor, through oxidation of H2 as an electron donor. A 7.6 kDa c-type cytochrome was purified from soluble fraction as the iron reducing protein and the protein was identified as cytochrome c-552. A rate constant for the reduction of Fe(III)-EDTA by the cytochrome was kinetically determined to be 1.73 x 107 s-1 by stopped-flow spectrophotometry, which is more than 20-fold higher than the constants of other iron reductases. The redox potential was measured as E = +0.27 V (vs NHE [Normal Hydrogen Electrode]), which is high enough to reduce Fe(III)-EDTA. These results suggest that cytochrome c-552 works as a terminal electron donor for Fe(III) in the periplasmic space.