THE STRUCTURE OF THE COMPLEX BETWEEN RUBISCO AND ITS NATURAL SUBSTRATE RIBULOSE 1,5-BISPHOSPHATE

The three-dimensional structure of the complex of ribulose 1,5-bisphos phate carboxylase/oxygenase (rubisco; EC 4.1.1.39) from spinach with i ts natural substrate ribulose 1,5-bisphosphate (RuBP) has been determi ned both under activating and non-activating conditions by X-ray cryst allography to a resolution of 2.1 Angstrom and 2.4 Angstrom, respectiv ely. Under activating conditions, the use of calcium instead of magnes ium as the activator metal ion enabled us to trap the substrate in a s table complex for crystallographic analysis. Comparison of the structu re of the activated and the non-activated RuBP complexes shows a tight er binding for the substrate in the non-activated form of the enzyme, in line with previous solution studies. In the non-activated complex, the substrate triggers isolation of the active site by inducing moveme nts of flexible loop regions of the catalytic subunits. In contrast, i n the activated complex the active site remains partly open, probably awaiting the binding of the gaseous substrate. By inspection of the st ructures and by comparison with Other complexes of the enzyme we were able to identify a network of hydrogen bonds that stabilise a closed a ctive site structure during crucial steps in the reaction. The present structure underlines the central role of the carbamylated lysine 201 in both activation and catalysis, and completes available structural i nformation for our proposal on the mechanism of the enzyme. (C) 1997 A cademic Press Limited.