D-ribulose-1,5-bisphosphate carboxylase/oxygenase: function-dependent structural changes.

The key carboxylating enzyme of the reductive pentose phosphate cycle, D-ribulose-1,5-bisphosphate carboxylase/oxygenase [RuBisCO] isolated from the chemolithoautotrophic, H2-oxidizing bacterium Alcaligenes eutrophus H16 has been analyzed by several different techniques that allow conclusions about structure and function-dependent structural changes. The techniques include a novel approach in which the enzyme was induced to form 2D-crystals suitable for electron microscopy in each of its three stable functional states: as active enzyme [Ea] (in the presence of Mg2+ and HCO3-); as inactivated enzyme [Eia] (in the absence of Mg2+ and HCO3-) and as enzyme locked in an in vitro transition state [CABP-E] (Ea fully saturated with the transition state analogue 2-carboxy-D-arabinitol-1,5-bisphosphate [CABP-E]). In conjunction with X-ray crystallography, X-ray small angle scattering and other biophysical and biochemical data, the results obtained by electron microscopy support the idea that drastic configurational changes occur. Upon transition from Ea to the CABP-E the upper and lower L4S4 halves of the molecule consisting of eight large and eight small subunits (L8S8; MW = 536,000 Da) are assumed to be laterally shifted by as much as 3.6 nm relative to one another while the location of the small subunits on top of the large subunits, and relative to them, remains the same. For the Eia a similar sliding-layer configurational change in the range of 2-2.5 nm is proposed and in addition it is suggested that other configurational/conformational changes take place. The proposed structural changes are discussed with respect to the current model for the tobacco enzyme and correlated with data obtained for various other plant and (cyano) bacterial L8S8 RuBisCOs leading to speculations about structure-function relationships.