Rubisco oxygenation

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... 

The enzyme responsible for the fixation of CO2, ribulose bisphosphate carboxylase/oxygenase (RUBISCO), also catalyses the oxygenation of ribulose bisphosphate to form one molecule of phosphoglycerate and one molecule of phosphoglycolate. Under normal C02 and 02 concentrations six ribulose bisphosphate molecules react with oxygen for every 15 that react with carbon dioxide. 

When rubisco uses 02 rather than C02 as substrate, the 2-phosphoglycolate so formed is disposed of in an oxygen-dependent pathway. The result is increased consumption of... 

N. E. Tolbert4 has argued that the dual specificity of rubisco for C02 and 02 is not simply a leftover from evolution in a low-oxygen environment. He suggests that the relative activities of the carboxylase and oxygenase activities of rubisco actually have set, and now maintain, the ratio of C02 to 02 in the earth s atmosphere. Discuss the pros and cons of this hypothesis, in molecular terms and in global terms. How does the existence of C4 organisms bear on the hypothesis ... 

In most plants photosynthesis is also strongly inhibited by 02. This observation led to the discovery that 02 competes directly for C02 at the active site of rubisco in a process called photorespiration. Chloroplasts inhibited by oxygen produce glycolate in large amounts2823 as a result of the reaction of the intermediate enediolate ion formed in step b of Eq. 13-48 with 02... 

Molecular oxygen usually reacts rapidly with only those organic substrates, such as dihydroflavins, that are able to form stable free radicals. However, the endiolate anion of Eq. 13-50 may be able to donate a single electron to 02 to form a superoxide-organic radical pair prior to formation of the peroxide (see also Eq. 15-30). Similar oxygenase side reactions have been observed for a variety of other enzymes that utilize carbanion mechanisms.283 The reaction of rubisco with 02 is of both theoretical and practical interest, the latter because of its significance in lowering the yield in photosynthesis (Chapter 23). 

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 CBB cycle enzyme, RubisCO, is the most abundant protein in the world [8], as it can comprise up to 50% of the total soluble protein in the chloro-plasts or in bacteria using this cycle. This fact is a consequence of the notorious catalytic inefficiency of RubisCO, that is, a low affinity for C02, a slow catalytic turnover rate, and a wasteful oxygenase side reaction responsible for photorespiration, resulting in a futile cleavage of the substrate to form phosphoglycolate as a side product. However, the CBB cycle enzymes are oxygen-insensitive and can easily be controlled, because the whole pathway is separated from... 

This set of reactions is very detrimental to the efficiency of photosynthesis. Oxygen is added to carbon, CO2 is lost, energy is consumed, and ribulose bisphosphate is destroyed. For a plant to be able to increase the discrimination of Rubisco for CO2 would obviously be advantageous, but that hasn t happened, either naturally or through the efforts of scientists. An increased concentration of CO2 in the atmosphere may lead to increased photosynthesis and decreased photorespiration, but high CO2 concentrations would also contribute to global warming (and the increased photosynthetic carbon fixation would not likely reduce the amount of CO2 in any event). 

PEP carboxylase has a lower Km for C02 than does Rubisco. Further, 02 is a very poor substrate for this enzyme. This means that, at relatively low concentrations of C02, the delivery of carbon into photosynthesis products is more efficient than in C3 plants and oxygenation doesn t occur. 

Rubisco and the Composition of the Atmosphere N. E. Tolbert has argued that the dual specificity of rubisco for C02 and 02 is not simply a leftover from evolution in a low-oxygen environment. 

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. 

Figure 20.6. A Wasteful Side Reaction. The reactive enediolate intermediate on rubisco also reacts with molecular oxygen to form a hydroperoxide intermediate, which then proceeds to form one molecule of 3-phosphogly cerate and one molecule of phosphoglycolate.

V. Irihimovitch and M. Shapira. 2000. Glutathione redox potential modulated by reactive oxygen species regulates translation of rubisco large subunit in the chloroplast J. Biol. Chem. 275 16289-16295. (PubMed)... 

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