ABSTRACT
M. thermoautotrophicum cells grown in the presence of an 80%:20% H2/C02 mixture produce an MCR that can exist in different states, depending on different harvesting and isolation conditions [74]. Anaerobic isolation of MCR leads to the catalytically inactive EPR-silent form named MCRsiient, containing Ni(II). Treating the cells with 100% H2 prior to harvesting produces a catalytically competent and EPR-active form named MCRredl, which displays an EPR spectrum characteristic of Ni(I)-F430 [141]. On the other hand, treating the cells with an 80%:20% N2/C 0 2 mixture prior to collection produces the catalytically inactive, EPR-active, form named MCR0Xl [142,143]. It has been proposed that the MCRoxl form contains Ni(III) bound to F43o [144]. MCRoxl is also obtained by incubating the cells prior to harvesting in the presence of Na2S, while incubation with Na2S03 yields MCRox2, a different EPR-active and catalytically competent form [144]. Reduction of MCR0Xi with a strong reductant (Ti(III) citrate at pH 10) generates MCR^n [141]. The M CR^i, MCRoxl, and MCRox2 forms are transformed into EPR-silent and catalytically inactive forms named, respectively, MCR^n-siient, MCRoxl-snent, and MCRox2.siient, upon expo sure to oxygen [74]. M C R ^i is particularly sensitive to oxygen, and its activity dis appears rapidly even in strictly anaerobic conditions due to its conversion to the
Ni(II)-containing MCR^i-siient form [141]. On the other hand, MCRoxl is relatively stable in the presence of oxygen [145].
