Unusual ribulose 1,5-bisphosphate carboxylase/oxygenase of anoxic Archaea

The predominant pool of organic matter on earth is derived from the biologi cal reduction and assimilation of carbon dioxide gas, catalyzed primarily b y the enzyme ribulose 1,5-bisphosphate carboxylase/oxygenase (RubisCO), By virtue of its capacity to use molecular oxygen as an alternative and compet ing gaseous substrate, the catalytic efficiency of RubisCO and the enzyme's ability to assimilate CO, may be severely limited, with consequent environ mental and agricultural effects. Recent genomic sequencing projects, howeve r, have identified putative RubisCO genes from anoxic Archaea, In the prese nt study, these potential RubisCO sequences, from Methanococcus jannaschii and Archaeoglobus fulgidus, were analyzed in order to ascertain whether suc h sequences might encode functional proteins. We also report the isolation and properties of recombinant RubisCO using sequences obtained from the obl igately anaerobic hyperthermophilic methanogen M. jannaschii, This is the f irst description of an archaeal RubisCO sequence; this study also represent s the initial characterization of a RubisCO molecule that has evolved in th e absence of molecular oxygen. The enzyme was shown to be a homodimer whose deduced sequence, along with other recently obtained archaeal RubisCO sequ ences, differs substantially from those of known RubisCO molecules. The rec ombinant M. jannaschii enzyme has a somewhat low, but reasonable k(cat), ho wever, unlike previously isolated RubisCO molecules, this enzyme is very ox ygen sensitive yet it is stable to hyperthermal temperatures and catalyzes the formation of the expected carboxylation product, Despite inhibition by oxygen, this unusual RubisCO still catalyzes a weak yet demonstrable oxygen ase activity, with perhaps the lowest capacity for CO2/O-2 discrimination e ver encountered for any RubisCO.