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PubMed1986

Regulation of photorespiration in leaves: evidence that the fraction of ribulose bisphosphate oxygenated is conserved and stoichiometry fluctuates.

Under steady-state conditions the combined system of the reductive photosynthetic cycle and the oxidative photorespiratory loop may be defined by two partitioning terms: the fraction of ribulose bisphosphate oxygenated and the fraction of glycolate carbon photorespired (the stoichiometry of photorespiration). A combination of physical and stereochemical methods [K.R. Hanson, and R. B. Peterson, (1985) Arch. Biochem. Biophys. 237,300-310] has been used to estimate these partitionings for tobacco leaf discs. Inverted discs, as compared to normally oriented discs, were found to have greater net photosynthesis; their ratio of photorespiration to net photosynthesis was less, and less of their glycolate carbon was photorespired. An eightfold reduction of irradiance below that of full sunlight for inverted discs in normal air at 32 degrees C reduced both photosynthesis and photorespiration about threefold but had little effect on the partitioning of ribulose bisphosphate and glycolate. Increasing the temperature from 22 to 40 degrees C for inverted discs in normal air and 1000 microE m-2 s-1 irradiance had little effect on net photosynthesis but increased the ratio of photorespiration to net photosynthesis almost threefold; ribulose bisphosphate partitioning was little changed but the fraction of glycolate carbon photorespired more than doubled. If field-grown plants respond to temperature in a similar fashion, genetic intervention to reduce the increase in photorespiration stoichiometry with temperature could increase total daily carbon assimilation and hence improve crop yields.

Carbon Dioxide↗
PubMed1987

Photosynthetic carbon metabolism in isolated pea chloroplasts: metabolite levels and enzyme activities.

We report here that enzyme activation precedes the rise in metabolite levels, which appear to limit photosynthetic CO2 fixation during induction in pea leaf chloroplasts. Therefore light activation may be required for the build-up of photosynthetic intermediates and hence for photosynthesis in isolated chloroplasts. Analysis of metabolite levels and the known kinetic properties of the chloroplast enzymes indicates that the reductive pentose phosphate cycle is subject to control which fluctuates between several points during induction and when CO2 fixation is maximal. The transketolase-aldolase-catalyzed reactions around sedoheptulose-biphosphatase appear to provide a simple and effective primary control for photosynthetic CO2 fixation. When substrate levels and enzyme active site concentrations are taken into account, there is insufficient glyceraldehyde 3-phosphate dehydrogenase, aldolase, and transketolase activity to support photosynthetic CO2 fixation at observed rates. These results suggest that there may be direct transfer of glyceraldehyde 3-phosphate among these enzymes in the pea chloroplast.

Carbon↗
PubMed1987

Photorespiration stoichiometry in leaves estimated by combined physical and stereochemical methods: allowance for isomerase-catalyzed 3H losses in ribulose bisphosphate regeneration.

We showed previously [K.R. Hanson and R.B. Peterson (1986) Arch. Biochem. Biophys. 246, 332-346] that under steady-state photosynthetic conditions the fraction of ribulose bisphosphate oxidized and the fraction of glycolate carbon photorespired (the stoichiometry of photorespiration) may be estimated in leaves by a combination of physical and stereochemical methods. The calculations assumed that when (3R)-D-[3-3H1,3-14C]glyceric acid is supplied to illuminated leaf discs the only loss of 3H from the combined photosynthetic and photorespiratory system is the result of glycolate oxidase action; i.e., the isomerase-catalyzed losses in the regeneration of ribulose bisphosphate are negligible. The present study of tobacco leaf discs under zero-photorespiration conditions (low O2 and high CO2 concentrations), and also of maize leaf discs, shows that some 3H losses occur (between 8 and 13% of the 3H at C-1 of ribulose 5-phosphate). The calculated loss varied moderately with temperature but did not vary when the flux of ribulose bisphosphate formation was altered by changing the irradiance. The calculated loss under zero-photorespiration conditions, therefore, may be used to calculate ribulose bisphosphate and glycolate partitioning under other conditions. Earlier experiments on the influence of O2 and CO2 concentrations of temperature on the partitioning of ribulose bisphosphate and glycolate have been reexamined. The loss corrections decreased all values for the fraction of ribulose bisphosphate oxidized and increased all values for the stoichiometry of photorespiration. Essentially all stoichiometry values were above the theoretical lower limit of 25%. The previous conclusion that the stoichiometry of photorespiration substantially exceeds 25% at higher O2 concentrations and higher temperatures is unchanged. The results with maize leaf discs implied that there is very little oxidation of ribulose 1,5-bisphosphate under normal-air conditions; i.e., photorespiration is indeed suppressed, not merely hidden, by efficient refixation of CO2.

Carbon Dioxide↗
PubMed1975

Regulation of glucose-6-phosphate dehydrogenase in spinach chloroplasts by ribulose 1,5-diphosphate and NADPH/NADP+ ratios.

The activity of glucose-6-phosphate dehydrogenase (EC 1.1.1.49) FROM SPINACH CHLOROPLASTS IS STRONGLY REGULATED BY THE RATIO OF NADPH/NADP+, with the extent of this regulation controlled by the concentration of ribulose 1,5-diphosphate. Other metabolites of the reductive pentose phosphate cycle are far less effective in mediating the regulation of the enzyme activity by NADPH/NADP+ ratio. With a ratio of NADPH/NADP+ of 2, and a concentration of ribulose 1,5-diphosphate of 0.6 mM, the activity of the enzyme is completely inhibited. This level of ribulose 1,5-diphosphate is well within the concentration range which has been reported for unicellular green algae photosynthesizing in vivo. Ratios of NADPH/NADP+ of 2.0 have been measured for isolated spinach chloroplasts in the light and under physiological conditions. Since ribulose 1,5-diphosphate is a metabolite unique to the reductive pentose phosphate cycle and inhibits glucose-6-phosphate dehydrogenase in the presence of NADPH/NADP+ ratios found in chloroplasts in the light, it is proposed that regulation of the oxidative pentose phosphate cycle is accomplished in vivo by the levels of ribulose 1,5-diphosphate, NADPH, and NADP+. It already has been shown that several key reactions of the reductive pentose phosphate cycle in chloroplasts are regulated by levels of NADPH/NADP+ or other electron-carrying cofactors, and at least one key-regulated step, the carboxylation reaction is strongly affected by 6-phosphogluconate, the metabolic unique to the oxidative pentose phosphate cycle. Thus there is an interesting inverse regulation system in chloroplasts, in which reduced/oxidized coenzymes provide a general regulatory mechanism. The reductive cycle is activated at high NADPH/NADP+ ratios where the oxidative cycle is inhibited, and ribulose 1,5-diphosphate and 6-phosphogluconate provide further control of the cycles, each regulating the cycle in which it is not a metabolite.

Adenosine Triphosphate↗
PubMed1979

Purification and properties of D-ribulose-5-phosphate 3-epimerase from calf liver.

D-Ribulose-5-phosphate 3-epimerase (EC 5.1.3.1) was purified 760-fold from calf liver by adsorption on DEAE-cellulose, chromatography on DEAE-Sephadex, chromatography on D-ribose 5-phosphate-Sepharose and gel filtration on Biogel P200. The purified enzyme of specific activity 617 units/mg was obtained in 28% yield and gave a single band on polyacrylamide gel electrophoresis. It had a molecular weight of 45 000 and appeared to contain two identical peptide chains of 22 900 daltons. The Km for D-ribulose 5-phosphate was 0.19 +/- 0.07 mM (S.E.). It was inhibited by reagents reacting with sulphydryl groups, by sulphate ion, and by D-deoxyribose 5-phosphate. The pH-stability and pH-activity curves were determined.

Animals↗
PubMed1980

Inhibition of ribulose-1,5-bisphosphate carboxylase-oxygenase activities by hydroxylamine.

Hydroxylamine directly and reversibly inhibits both activities of homogeneous ribulose-1,5-bisphosphate carboxylase-oxygenase (3-phospho-D-glycerate carboxy-lyase (dimerizing), EC 4.1.1.39) isolated from diverse sources. NH2OH is an uncompetitive inhibitor of carboxylase activity with respect to ribulose-bisphosph ate. This reagent also reacts non-enzymically with ribulosebisphosphate to deplete this substrate. Contrary to previous reports, these results indicate that hydroxylamine directly and indirectly inhibits both activities of this bifunctional enzyme.

Carboxy-Lyases↗
PubMed1981

The preparation of transketolase free from D-ribulose-5-phosphate 3-epimerase.

A procedure for the purification from Candida utilis of transketolase (sedoheptulose-7-phosphate: D-glyceraldehyde-3-phosphate glycolaldehydetransferase, EC 2.2.1.1) free from D-ribulose-5-phosphate 3-epimerase (EC 5.1.3.1) was developed using acetone precipitation, elution from DEAE-cellulose, adsorption of epimerase by thiopropyl-Sepharose, and chromatography on D-ribose 5-phosphate-Sepharose and DEAE--Sephadex. The final product had a specific activity of 43 units/mg, a transketolase/epimerase activity ratio greater than 53 000 to 1, an apparent Km for D-xylulose 5-phosphate and D-ribose 5-phosphate of 77 and 430 microM, respectively, and ran as a single band using electrophoresis on polyacrylamide gel. It was inhibited by D-arabinose 5-phosphate and D-glucose 6-phosphate. During the purification by column chromatography, multiple forms of the enzyme were detected by gel electrophoresis but these gradually disappeared as the enzyme was further purified.

Acetone↗
PubMed1984

Biosynthesis of riboflavin. Enzymatic formation of the xylene moiety from [14C]ribulose 5-phosphate.

We have studied the enzymatic formation of the xylene ring of riboflavin using cell extracts from the flavinogenic yeast Candida guilliermondii. 5-Amino-6-ribitylamino-2,4(1H,3H)-pyrimidinedione or its 5'-phosphate could serve as substrates. In addition, a pentose phosphate or pentulose phosphate was required. Experiments with [14C]ribulose 5-phosphate gave evidence for the incorporation of the ribulose carbon atoms except C-4 into the xylene ring of the vitamin.

Candida↗
PubMed1987

Photoreductive path of carbon fixation in green plant photosynthesis. Reaction pathway of six-carbon ribulose 1,5-bisphosphate carboxylation adduct intermediate.

In this paper we examine the six-carbon intermediate pathway of ribulose 1,5-bisphosphate (RuBP) carboxylation reaction in photosynthesis. Based on the observed reactions of purified RuBP carboxylase, mechanisms are described for carbon dioxide assimilation leading to the hydrolytic splitting of the six-carbon intermediate to two enzyme-bound glycerate-3-P (3-PGA) molecules. It is concluded that, under photosynthetic conditions, the reduction of enzyme-bound NADP+ by the chlorophyll is responsible for the rapid carboxylase turnover rate given by the lifetime, tau L = 0.4 s, which is nearly two orders of magnitude shorter than the corresponding value, tau D = 11 +/- 3 s, for the dark decay of enzyme-bound RuBP. The nocturnal inhibition and photoactivation of RuBP carboxylation are described in terms of the reversible light-dark cycles of the NADP+/NADPH redox couple and endogenous changes that accompany the 2-carboxy-D-arabinitol-1-phosphate binding to the enzyme active site.

Carbon↗
PubMed1988

Non-cyclic photoreductive carbon fixation in photosynthesis. Light and dark transients of the glycerate-3-P special pair.

It is demonstrated that carbon fixation in photosynthesis is regulated in two kinetically coupled pathways involving the specialized pair of non-equivalent, enzyme-bound glycerate-3-P (3-PGA) molecules obtained from ribulose 1,5-bisphosphate (RuBP) carboxylation in the light. A non-cyclic pathway is suggested (reaction 2) for the direct biosynthesis of sucrose from the 3-PGA obtained from C-3, C-4 and C-5 of the six-carbon carboxylation adduct. Concomitant to the appearance of sucrose as the principal product, the Mg2+-bound 3-PGA molecule formed from C-1, C-2 and C-2' of the C6 intermediate is released and subsequently reduced in regenerating the RuBP. It is proposed that the nocturnal inhibitor, 2-carboxyarabinitol-1-phosphate (1-PCA) is obtained from a condensation of 3-PGA and glyceraldehyde.

Carbon↗
PubMed1981

The calculation of the cytoplasmic free [NADP+]/[NADPH] ratio in brain: effect of electroconvulsive seizure.

This study has investigated the feasibility of calculating the cytoplasmic free [NADP+]/[NADPH] ratio in rat brain. The time course of the change in the substrate ratios of the malate dehydrogenase (decarboxylating) [E.C. 1.1.1.40], NADP+-isocitrate dehydrogenase (decarboxylating) [E.C. 1.1.1.42] and 6-phosphogluconate dehydrogenase (decarboxylating) [E.C. 1.1.1.44] reactions was followed for up to 10 min after a single, unmodified electroconvulsive seizure. From the results it has been concluded that during periods of low flux, the direction and magnitude of the change in the cytoplasmic free [NADP+]/[NADPH] ratio can, in fact, be reasonably determined even though there is some uncertainty in the absolute value of the ratio itself. It is recommended that reliance not be placed on a single enzyme system but that one or both of the other systems also be observed under a given experimental condition to increase confidence in the determination. The results also demonstrate that seizure and anoxia have a far lesser effect on the cytoplasmic free [NADP+]/[NADPH] ratio than on the free [NAD+]/[NADH] ratio in the same compartment. These results suggest that the pathways using the nicotinamide-adenine dinucleotide phosphate system are relatively protected from the rapid fluctuations that seizure and anoxia can produce.

Animals↗