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 (L₂). However, further assembly into pentamers of L₂ (L₁₀) occurs when expressed in tobacco chloroplasts or E. coli producing RuBP. In vitro analyses indicate that the sequential assembly of L₂ into L₁₀ (via detectable L₄ and L₆ 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 L₂ and L₁₀ M. burtonii Rubisco (MbR) were indistinguishable. At 25 °C they both shared a low specificity for CO₂ over O₂ (1.1 mol·mol^-1) and RuBP carboxylation rates that were distinctively enhanced at lowpH(∼4 s^-1 at pH 6, relative to 0.8 s^-1 at pH 8) with a temperature optimum of 55 °C. Like other archaeal Rubiscos, MbR also has a high O₂ affinity (K_m(O₂)=∼2.5 μM). The catalytic and structural similarities of MbR to other archaeal Rubiscos contrast with its closer sequence homology to bacterial L₂ Rubisco, complicating its classification within the Rubisco superfamily.