Rubisco: Assembly and Mechanism

Abstract

Ribulose-bisphosphate carboxylase/oxygenase (Rubisco, E.C. 4.1.1.39) is unique to photosynthetic metabolism. Two intensively studied aspects of Rubisco physiology are covered in this chapter, its post-translational assembly and its mechanism of action. Bacterial Rubisco can be assembled in vitro and in bacterial hosts but, as yet, assembly in vitro of higher-plant Rubiscos has not been reported. This focuses attention on the assembly pathway for higher plant Rubisco, which has been known for some time to be related to the presence of molecular chaperones in chloroplasts. Analysis of mutants, transformation of plants and bacteria with chloroplast chaperones, and the development of in vitro translation and assembly systems based on chloroplast extracts, have been directed at resolving this problem. It appears from these data that certain bacterial chaperones do not interfere with the assembly of higher plant Rubisco. As in cyanobacterial systems, the absence of S subunits leads to the accumulation of L8-like particles whose subunits can later be recruited to form Rubisco. Subtle differences between the way S subunits assemble with higher-plant and cyanobacterial L8-like particles suggest that this process may be concerted with assembly of L8 in the case of the higher-plant enzyme. The catalytic mechanism of Rubisco depends on two co-factors; a divalent metal ion, usually Mg2+ and a CO2 molecule that carbamylates a specific lysyl residue, K201, in the active site. This carbamate plays a crucial role in initiating catalysis by abstracting the C3 proton of ribulose bisphosphate and it may also act as a general-base catalyst for succeeding steps. Sofar, Rubisco's use of a carbamate as a base appears to be unique among enzymes. The catalytic sequences of both the carboxylation reaction, and the oxygenation reaction that competes with it, proceed through multiple steps, each of a complexity rivaling that of the complete reaction of many other enzymes. The structure of the active site must change subtly between steps. Selectivity between CO2 and O2, of paramount importance to photosynthetic efficiency, is determined by the relative reactivity of the enediol(ate) form of the substrate for the two gases.