Oxidation of cytochrome 605 is the rate limiting step when Ferrimicrobium ferriphilum respires aerobically on soluble iron

Proteins that oxidize extracellular substrates in Gram-positive bacteria are poorly understood. Ferrimicrobium acidiphilum is an actinobacterium that respires aerobically on extracellular ferrous ions at pH 1.5. In situ absorbance measurements were conducted on turbid suspensions of intact Fm. acidiphilum using an integrating cavity absorption meter designed for that purpose. Initial velocity kinetic studies monitored the appearance of product ferric ions in the presence of catalytic quantities of cells. Cell-catalyzed iron oxidation obeyed the Michaelis-Menten equation with values for KM and Vmax of 71 µM and 0.29 fmol/min/cell, respectively. Cell-monitored turnover kinetic studies conducted with higher concentrations of cells revealed that only a single cytochrome with reduced absorbance peaks at 448 and 605 nm was visible as the cells respired aerobically on iron. The reduced cytochrome 605 exhibited mathematical and correlational properties that were consistent with the hypothesis that oxidation of the cytochrome constituted the rate-limiting step in the aerobic respiratory process with a turnover number of 35 ± 2 s-1. Genomic and proteomic analyses showed that Fm. acidiphilum expressed two a-type heme copper terminal oxidases. Cytochrome 605 was associated with the terminal oxidase genes that occur earlier in the annotated circular genome of this bacterium.