High-Resolution Crystal Structure of Chloroplastic Ribose-5-Phosphate Isomerase from Chlamydomonas reinhardtii-An Enzyme Involved in the Photosynthetic Calvin-Benson Cycle.

Théo Le Moigne,Pierre Crozet,Julien Henri,Stéphane D Lemaire

doi:10.3390/ijms21207787

Abstract

The Calvin–Benson cycle is the key metabolic pathway of photosynthesis responsible for carbon fixation and relies on eleven conserved enzymes. Ribose-5-phosphate isomerase (RPI) isomerizes ribose-5-phosphate into ribulose-5-phosphate and contributes to the regeneration of the Rubisco substrate. Plant RPI is the target of diverse post-translational modifications including phosphorylation and thiol-based modifications to presumably adjust its activity to the photosynthetic electron flow. Here, we describe the first experimental structure of a photosynthetic RPI at 1.4 Å resolution. Our structure confirms the composition of the catalytic pocket of the enzyme. We describe the homo-dimeric state of the protein that we observed in the crystal and in solution. We also map the positions of previously reported post-translational modifications and propose mechanisms by which they may impact the catalytic parameters. The structural data will inform the biochemical modeling of photosynthesis.

Highlights

Photosynthesis is the biological process allowing the conversion of light energy into chemical energy through fixation of atmospheric carbon [1]
We describe the first structure of a ribose-5-phosphate isomerase from a Viridiplanta, the model microalga Chlamydomonas reinhardtii
CrRPI1 catalytic site is described free from substrate and product, representing the apo state of the enzyme

Summary

Introduction

Photosynthesis is the biological process allowing the conversion of light energy into chemical energy through fixation of atmospheric carbon [1]. Fixation of carbon dioxide occurs through the Calvin–Benson cycle (CBC) by carboxylation of the acceptor pentose ribulose-1,5-bisphosphate (RubP) by Rubisco [2]. RubP is generated by phosphorylation of ribulose-5-phosphate (Ru5P) by phosphoribulokinase (PRK). Prior to this phosphorylation step, ribose-5-phosphate (R5P) carbonyl group is transferred from carbon 1 to carbon 2. This reaction is catalyzed by the metabolic enzyme ribose-5-phosphate isomerase (RPI) [3]. Analysis of RPI enzymatic kinetics has been principally performed on RPIB in several species, and research on RPIA enzymology is comparatively scarcer. The catalytic mechanism of RPIA has been established in several species and summarized in a recent review [4]

Methods

Results

Conclusion