Structure of D-ribulose-l,5-bisphosphate carboxylase/oxygenase from Alcaligenes eutrophyus H16

Abstract

RuBPCase, D-ribulose-l,5-bisphosphate carboxylase/oxygenase (EC4.1.1.39) is the key enzyme of the reductive pentose phosphate cycle. Because of its biological significance, many structural studies on a number of plant and bacterial RuBPCases¹ have been undertaken, including the enzyme isolated from the autotrophic hydrogen-oxidizing bacterium Alcaligenes eutrophus H16 (refs 2–6). Although both the higher plant enzyme and the A. eutrophus enzyme consist of eight large and eight small subunits (L₈S₈), no model describing the quaternary structure is generally accepted. Here we present a model for the A. eutrophus RuBPCase derived from X-ray crystallography of three-dimensional (3D) crystals, and electron microscopy and image analysis of two-dimensional (2D) crystals of the enzyme. The X-ray electron density of RuBPCase in the presence of HCO⁻₃, Mg²⁺, and the transition state analogue 2-carboxyarabinitol-l,5-bisphosphate (CABP) shows an L₈S₈ molecule in which the L₄S₄ half molecules have local 4-fold symmetry (C4). The local 4-fold axes of the two L₄S₄ halves do not coincide but are shifted by 36 Å and are related by a crystallographic 2-fold axis perpendicular to and between the local 4-fold axes. Electron microscope data of the enzyme without CABP, which can be perfectly modelled using the X-ray densities, do not show this shift and the low-resolution point group of the molecules in the 2D crystals is D4. Both structures are presented.