Rubisco carboxylation

Tapia, O., Andres, J. and Safont, V. S. Enzyme catalysis and transition structures in vacuo. Transition structures for the enolization, carboxylation and oxygenation reactions in ribulose- 1,5-bisphosphate carboxylase/oxygenase enzyme (Rubisco), J.Chem.Soc.Faraday Trans., 90 (1994), 2365-2374... 

Tapia, O. and Andres, J. Towards an explanation of carboxylation/oxygenation bifuinctionality in Rubisco. Transition structure for the carboxylation reaction of 2,3,4-pentanetriol., Mo/. Eng., 2 (1992), 37-41... 

FUNCTION RUBISCO CATALYSES TWO REACTIONS THE CARBOXYLATION OF D-RIBULOSE 1,5-BISPHOSPHATE, THE PRIMARY EVENT IN PHOTOSYNTHETIC CARBON DIOXIDE FIXATION, AS WELL AS THE OXIDATIVE FRAGMENTATION OF THE PENTOSE SUBSTRATE IN THE PHOTORESPIRATION PROCESS. BOTH REACTIONS OCCUR SIMULTANEOUSLY AND IN COMPETITION AT THE SAME ACTIVE SITE. 

The simpler dimeric rubisco from Rhodospirillum is very inefficient in carboxylation and catalyzes much more oxygenation than do rubiscos of higher plants.2833 Mutant enzymes that have impaired carboxylase and enhanced oxygenase are also known.284/284a... 

The key reaction of the Calvin-Benson cycle of C02 reduction is the carboxylation of ribulose bisphos-phate to form two molecules of 3-phosphoglycerate (Eq. 13-48). The properties of ribulose bisphosphate carboxylase (rubisco, Figs. 13-10 to 13-12), which catalyzes this reaction, are discussed in Chapter 13. It... 

In this chapter attention will be focused on the chemical transformations of C02 to yield carboxylic acids and related molecules (e.g., cyclic esters), while natural carboxylations reactions such as those dealing with RuBisCO (ribulose-1,5 bis(phosphate) carboxylase/oxidase) and its genetic engineering or the carboxylation of phenol via a phenol phosphate intermediate by the phenol-carboxylase enzyme, will be left aside. 

Carbon dioxide fixation is catalyzed by the enzyme ribulosebisphosphatecar-boxylase oxygenase (RuBisCO), which is the most abundant natural enzyme worldwide, and the least selective. In fact, RuBisCO does not follow the usual selectivity of enzymes, and at the same time promotes the carboxylation of ribulose (a C-5 sugar) to afford a C-6 sugar (with carbon fixation) and the oxidation of the same C-5 sugar, with a selectivity close to 50% [2]. 

Much effort has been spent on the genetic manipulation of RuBisCO, with the intention of increasing its selectivity towards the carboxylation reaction [3]. As a matter of fact, an improvement in carboxylation by 5-10% would solve all problems relevant to the accumulation of C02 in the atmosphere, as all of the anthropogenic C02-which is causing the increases in atmospheric C02... 

Answer In maize, C02 is fixed by the C4 pathway elucidated by Hatch and Slack. Phospho-enolpyruvate is rapidly carboxylated to oxaloacetate, some of which undergoes transamination to aspartate but most of which is reduced to malate in the mesophyll cells. Only after subsequent decarboxylation of labeled malate does 14C02 enter the Calvin cycle for conversion to glucose. The rate of entry into the cycle is limited by the rate of the rubisco-catalyzed reaction. 

Carboxylation of ribulose-1,5-bisphosphate by ribulose 1,5-bisphosphate carboxylase-oxygenase (Rubisco) with the formation of two molecules of glycerate-3-P. 

FIGURE 17. Mechanism for the Rubisco-catalyzed carboxylation and cleavage of ribulose 1,5-diphosphate... 

The crux of the pathway (Fig. 1) is the carboxylation of ribulose 1,5-bisphos-phate (Rbu-1,5-P2) at the C-2 carbon, giving rise to a short-lived six-carbon intermediate which is cleaved to produce two molecules of 3-phosphoglycerate (3-PGA) (Eqn. 2). This reaction is catalysed by ribulose-1,5-bisphosphate carboxylase oxygenase (rubisco), one of the most abundant proteins on earth. 

Rubisco (EC 4.1.1.39) performs a pivotal role in determining biomass yield (for recent reviews, see refs. 1-3). The biosynthetic reaction catalyzed by this enzyme, the carboxylation of RuBP by CO2 to form two equivalents of 3-phospho-D-glycerate (PGA) (Fig. 1, upper pathway), is limited by both slow turnover (fecat = 2-5 s-i) and utilization of O2 in competition with CO2. The... 

Figure 1. Reaction pathways for the carboxylation and oxygenation of RuBP as catalyzed by Rubisco.

The overall carboxylation or oxygenation of RuBP as catalyzed by Rubisco consists of discrete partial reactions illustrated in Fig. 1 (reviewed extensively in 1-3, 19). Because an active-site residue will not necessarily be involved in all catalytic steps, site-directed mutants devoid of overall activity may retain competence in one or more of the partial reactions. Independent of overall carboxylase activity, enolization of RuBP and turnover of the isolated six-carbon reaction intermediate can be assayed as distinct reactions, providing an avenue for discerning the particular step(s) preferentially facilitated by a given active-site residue. 

The most prominent side reaction of Rubisco is its counterproductive oxygenase activity, reflecting competition with CO2 for the enediol(ate) intermediate (1) (23). Partitioning between the two pathways (vjvo) is defined by Vc/Vo = T ([C02]/[02]), where x is VJCJVoK,. (24). Because x can be interpreted in terms of the free energy differential for carboxylated versus oxygenated transition states (25-26), it provides insight into determinants of Rubisco specificity. 

Collectively, these studies illustrate the power of multiple approaches for the analysis of Rubisco mutants. Characterizations of position-329 mutants have not only localized the step of catalysis facilitated by Lys329 but also uncovered its role in intermediate sti)ilization and optimization of carboxylation selectivity. 

As stated earlier, the rate-limiting step in the Calvin cycle is the carboxylation of ribulose 1,5-bisphosphate to form two molecules of 3-phosphogly cerate. The activity of rubisco increases markedly on illumination. The addition of CO2 to... 

---