Carbon Isotope Fractionation by Ribulose-1,5-Bisophosphate Carboxylase from Various Organisms

Abstract

Carbon Isotope fractionaton by structurally and catalytically distinct rbulose-1,5-blspbosphate carboxylases from one eucaryotic and four procaryotic organisms has been measured under itrogen. The Spinach leaf RuBP carboxylase is a large protein having a mol wt of 557,000 and is composed of eight large subunits, mol wt 55,800, and eight small subunits, mol wt 12,000 (21, 22). RuBP carboxylases have been isolated from procaryotic systems which vary in mol wt, structural composition, and regulatory properties. For example, when isolated and purified from the purple, nonsulfur, bacterium Rhodospirillum rubrum, the carboxylase has a mol wt of 114,000, is composed of only two large subunits, and is not regulated by the effector 6-P-gluconate (12, 27). Furthermore, two structurally and catalytically distinct RuBP carboxylases have been isolated from the photosynthetic bacterium Rhodopseudomonas sphaeroides (9). Peak I carboxylase is structurally and catalytically similar to the enzyme from higher plants. Peak II carboxylase has a mol wt of 360,000 and is composed of only six large subunits. It is also apparent that a diversity of structure may exist in the blue-green algae (28,29), similar to the bacteria (9, 27); i.e. Hydrogenomonas eutropha, a chemosynthetic bacterium, has a high mol wt carboxylase similar in structure to that of the spinach enzyme (12).In this paper we have examined carbon isotope fractionation by RuBP carboxylases from spinach, R rubrum, R sphaeroides, Agmenellum quadruplicatum, and H. eutropha, for several reasons. First, we have produced a reproducible fractionation value, A dissolved C02, by carefully controlling reaction conditions and purification procedures of both the enzyme and the product phosphoglyceric acid. Second, certain reaction conditions were purposely modified to test their effects on the fractionation: NaHCO concentration, 02 level, temperature, buffer, substrate, and metal cofactors. In addition, several purification states ofeach enzyme were used in order to determine the effect on the A dissolved CO2. Moreover, in an attempt to see if enzymes of different mol wt, purity, quaternary structure, and catalytic prop-

Diffusional Contribution to Carbon Isotope Fractionation during Dark CO2 Fixation in CAM Plants

“…It is generally assumed that this discrimination is due to isotope fractionation by the pertinent carboxylase (2,15,32). Experiments conducted with isolated carboxylases are qualitatively consistent with this assumption, in that both C3 plants and the associated RuP22 carboxylase show comparatively large isotope discriminations (3,9,31), whereas C4 plants and PEP carboxylase show small discriminations (28,31,34 (10,24). Under these conditions the malic acid accounted for 93% of the total CO2 fixed in the dark (Table I).…”

“…For C4 plants, E3 is 0.994. For C3 plants, the value of E3 is somewhat uncertain, but it is probably near 1.03 (3,9,31). If postcarboxylation events are assumed not to contribute significantly to the total isotope composition of the plant, then this analysis suggests that the rate of diffusion plays an important role in both C3 and C4 plants.…”

Carbon Isotope Ratios Demonstrate Carbon Flux from C4 Host to C3 Parasite

“…1). Although Farquhar et al (9) suggest that b lies close to -27, estimates have ranged from this value to -39 (1,7,24,28). In order to account for the measured 6"3C in terms of the gas exchange data, b would have to equal -21.1 and -19.7 for S. hermonthica and S. asiatica, respectively.…”

13C/12C ratios in marine invertebrates from reducing sediments: confirmation of nutricional importance of chemoautotrophic endosymbiotic bacteria

“…Benedict 1978, Estep et al 1978, Galimov 1985. Photoautotrophic plankton from middle latitudes usually shows 6I3C Y&o values of -18 to -23 compared to -2.7 to -6.7 for dissolved bicarbonate, though individual algae in culture show a slightly wider range; marine angiosperms such as Zostera are usually less depleted, 613C %O from -7 to -1 , owing to their active uptake of bicarbonate (Sackett et al 1965, Degens et al 1968, Benedict 1978, Fry & Sherr 1984, Descolas-Gros & Fontugne 1985, Galimov 1985, Zieman et al 1984).…”