RESPIRATORY ENZYMES OF THIOBACILLUS-FERROOXIDANS - KINETIC-PROPERTIESOF AN ACID-STABLE IRON-RUSTICYANIN OXIDOREDUCTASE

Rusticyanin is an acid-stable, soluble blue copper protein found in ab undance in the periplasmic space of Thiobacillus ferrooxidans, an acid ophilic bacterium capable of growing autotrophically on soluble ferrou s sulfate. An acid-stable iron:rusticyanin oxidoreductase activity was partially purified from cell-free extracts of T. ferrooxidans. The en zyme-catalyzed, iron-dependent reduction of the rusticyanin exhibited three kinetic properties characteristic of aerobic iron oxidation by w hole cells. (i) A survey of 14 different anions indicated that catalys is by the oxidoreductase occurred only in the presence of sulfate or s elenate, an anion specificity identical to that of whole cells. (ii) S aturation with both sulfatoiron(II) and the catalyst produced a concen tration-independent rate constant of 3 s(-1) for the reduction of the rusticyanin, which is an electron transfer reaction sufficiently rapid to account for the flux of electrons through the iron respiratory cha in. (iii) Values for the enzyme-catalyzed pseudo-first-order rate cons tants for the reduction of the rusticyanin showed a hyperbolic depende nce on the concentration of sulfatoiron(II) with a half-maximal effect at 300 mu M, a value similar to the apparent KM for iron shown by who le cells. On the basis of these favorable comparisons between the beha vior patterns of isolated biomolecules and those of whole cells, this iron:rusticyanin oxidoreductase is postulated to be the primary cellul ar oxidant of ferrous ions in the iron respiratory electron transport chain of T. ferrooxidans.