Substrate-induced Assembly of Methanococcoides burtonii D-Ribulose-1,5-bisphosphate Carboxylase/Oxygenase Dimers into Decamers

Like many enzymes, the biogenesis of the multi-subunit CO2-fixing enzyme ribulose-1,5-bisphosphate (RuBP) carboxylase/oxygenase (Rubisco) in different organisms requires molecular chaperones. When expressed in Escherichia coli, the large (L) subunits of the Rubisco from the archaeabacterium Methanococcoides burtonii assemble into functional dimers (L-2). However, further assembly into pentamers of L-2 (L-10) occurs when expressed in tobacco chloroplasts or E. coli producing RuBP. In vitro analyses indicate that the sequential assembly of L-2 into L-10 (via detectable L-4 and L-6 intermediates) occurs without chaperone involvement and is stimulated by protein rearrangements associated with either the binding of substrate RuBP, the tight binding transition state analog carboxyarabinitol-1,5-bisphosphate, or inhibitory divalent metal ions within the active site. The catalytic properties of L-2 and L10 M. burtonii Rubisco (MbR) were indistinguishable. At 25 C they both shared a low specificity for CO2 over O-2 (1.1 mol.mol(-1)) and RuBP carboxylation rates that were distinctively enhanced at low pH (similar to 4 s(-1) at pH6, relative to 0.8 s(-1) at pH 8) with a temperature optimum of 55 degrees C. Like other archaeal Rubiscos, MbR also has a high O-2 affinity (K-m(O-2) = similar to 2.5 mu M). The catalytic and structural similarities of MbR to other archaeal Rubiscos contrast with its closer sequence homology to bacterial L-2 Rubisco, complicating its classification within the Rubisco superfamily.