Activation of ribulose 1,5-diphosphate carboxylase by nicotinamide adenine dinucleotide phosphate and other chloroplast metabolites

The discovery of rubisco

Rubisco is possibly the most important enzyme on Earth, certainly in terms of amount. This review describes the initial reports of ribulose 1,5-bisphosphate carboxylating activity. Discoveries of core concepts are described, including its quaternary structure, the requirement for post-translational modification, and its role as an oxygenase as well as a carboxylase. Finally, the requirement for numerous chaperonins for assembly of rubisco in plants is described.

Photorespiration: The Futile Cycle?

Photorespiration, or C₂ photosynthesis, is generally considered a futile cycle that potentially decreases photosynthetic carbon fixation by more than 25%. Nonetheless, many essential processes, such as nitrogen assimilation, C₁ metabolism, and sulfur assimilation, depend on photorespiration. Most studies of photosynthetic and photorespiratory reactions are conducted with magnesium as the sole metal cofactor despite many of the enzymes involved in these reactions readily associating with manganese. Indeed, when manganese is present, the energy efficiency of these reactions may improve. This review summarizes some commonly used methods to quantify photorespiration, outlines the influence of metal cofactors on photorespiratory enzymes, and discusses why photorespiration may not be as wasteful as previously believed.

Manganese binding to Rubisco could drive a photorespiratory pathway that increases the energy efficiency of photosynthesis

Most plants, contrary to popular belief, do not waste over 30% of their photosynthate in a futile cycle called photorespiration. Rather, the photorespiratory pathway generates additional malate in the chloroplast that empowers many energy-intensive chemical reactions, such as those involved in nitrate assimilation. Thus, the balance between carbon fixation and photorespiration determines the plant carbon-nitrogen balance and protein concentrations. Plant protein concentrations, in turn, depend not only on the relative concentrations of carbon dioxide and oxygen in the chloroplast but also on the relative activities of magnesium and manganese, which are metals that associate with several key enzymes in the photorespiratory pathway and alter their function. Understanding the regulation of these processes is critical for sustaining food quality under rising CO2 atmospheres.

Photorespiration and nitrate assimilation: a major intersection between plant carbon and nitrogen

C3 carbon fixation has a bad reputation, primarily because it is associated with photorespiration, a biochemical pathway thought to waste a substantial amount of the carbohydrate produced in a plant. This review presents evidence collected over nearly a century that (1) Rubisco when associated with Mn(2+) generates additional reductant during photorespiration, (2) this reductant participates in the assimilation of nitrate into protein, and (3) this nitrate assimilation facilitates the use of a nitrogen source that other organisms tend to avoid. This phenomenon explains the continued dominance of C3 plants during the past 23 million years of low CO2 atmospheres as well as the decline in plant protein concentrations as atmospheric CO2 rises.

Crystal structure of rice Rubisco and implications for activation induced by positive effectors NADPH and 6-phosphogluconate

The key enzyme of plant photosynthesis, D-ribulose 1,5-bisphosphate carboxylase/oxygenase (Rubisco), must be activated to become catalytically competent via the carbamylation of Lys201 of the large subunit and subsequent stabilization by Mg(2+) coordination. Many biochemical studies have reported that reduced nicotinamide adenine dinucleotide phosphate (NADPH) and 6-phosphogluconate (6PG) function as positive effectors to promote activation. However, the structural mechanism remains unknown. Here, we have determined the crystal structures of activated rice Rubisco in complex with NADPH, 6PG, or 2-carboxy-D-arabinitol 1,5-bisphosphate (2CABP). The structures of the NADPH and 6PG complexes adopt open-state conformations, in which loop 6 at the catalytic site and some other loops are disordered. The structure of the 2CABP complex is in a closed state, similar to the previous 2CABP-bound activated structures from other sources. The catalytic sites of the NADPH and 6PG complexes are fully activated, despite the fact that bicarbonate (NaHCO(3)) was not added into the crystallization solution. In the catalytic site, NADPH does not interact with Mg(2+) directly but interacts with Mg(2+)-coordinated water molecules, while 6PG interacts with Mg(2+) directly. These observations suggest that the two effectors promote Rubisco activation by stabilizing the complex of Mg(2+) and the carbamylated Lys201 with unique interactions and preventing its dissociation. The structure also reveals that the relaxed complex of the effectors (NADPH or 6PG), distinct from the tight-binding mode of 2CABP, would allow rapid exchange of the effectors in the catalytic sites by substrate D-ribulose 1,5-bisphosphate for catalysis in physiological conditions.

The regulation of Rubisco activity

The processes of photosynthesis and photorespiration are initiated by Rubisco, but the enzyme must be activated before it will catalyse either the carboxylation or oxygenation of ribulose bisphosphate. Rubisco is activated in vitro by CO 2 and Mg 2+ . The dual roles of CO 2 as both an activator and a substrate led to anomalously high K m (CO 2 ) values until the activation requirement was recognized. During activation, CO 2 forms a carbamate at the ε-amino of a lysine residue on the large subunit of Rubisco. Under conditions thought to exist in the chloroplast during photosynthesis (10 μm CO 2 , 5 mM Mg 2+ and pH 8.0), Rubisco is only partially active since the K act (CO 2 ) is in the range 25-30 μm CO 2 . Thus the mechanism of activation as deduced from in vitro studies is incomplete. Higher activation levels can be obtained by preincubating Rubisco with phosphorylated metabolites, but these occupy ribulose bisphosphate binding sites and thus inhibit catalysis. Recently, a naturally occurring effector, which binds tightly to Rubisco and inhibits activity, has been found. This compound is synthesized in the dark and metabolized upon illumination, but its identity and physiological function are not yet known. In leaves, Rubisco is nearly fully activated at high light intensities. By analysing an Arabidopsis thaliana mutant deficient in the ability to activate Rubisco, we have determined that a soluble protein is required for the in vivo activation process. This enzyme, designated Rubisco activase, reduces the high K act (CO 2 ), observed with the isolated enzyme, to physiological levels in an illuminated reconstituted assay system containing washed thylakoid membranes, Rubisco and ribulose bisphosphate.

Rubisco Activase Mediates ATP-Dependent Activation of Ribulose Bisphosphate Carboxylase

The activation level of ribulosebisphosphate carboxylase following preincubation with ribulose 1,5-bisphosphate was increased by ATP and ribulosebisphosphate carboxylase activase in the absence of thylakoids or illumination. Maximal activation was obtained with 0.5 millimolar ATP in the presence of an ATP regenerating system (phosphoenolpyruvate and pyruvate kinase). Without the ATP regenerating system, activation was lower, linearly dependent on ATP concentration up to 1.0 millimolar, and was strongly inhibited by ADP.

Ribulose 1,5-bisphosphate carboxylase-oxygenase. I. Structural, immunochemical and catalytic properties

Some structural, immunochemical and catalytic properties are examined for ribulose 1,5-bisphosphate carboxylase-oxygenase from various cellular organisms including bacteria, cyanobacteria, algae and higher plants. The native enzyme molecular masses and the subunit polypeptide compositions vary according to enzyme sources. The molecular masses of the large and small subunits from different cellular organisms, on the other hand, show a relatively high homology due to their well-conserved primary amino acid sequence, especially that of the large subunit. In higher plants, the native enzyme and the large subunit are recognized by the antibodies raised against either the native or large subunit, whereas the small subunit apparently cross-reacts only with the antibodies directed against itself. A wide diversity exists, however, in the serological response of the native enzyme and its subunits with antibodies directed against the native enzyme or its subunits from different cellular organisms. According to numerous kinetic studies, the carboxylase and oxygenase reactions of the enzyme with ribulose 1,5-bisphosphate and carbon dioxide or oxygen require activation by carbon dioxide and magnesium prior to catalysis with ribulose 1,5-bisphosphate and carbon dioxide or oxygen. The activation and catalysis are also under the regulation of other metal ions and a number of chloroplastic metabolites. Recent double-labeling experiments using radioactive ribulose 1,5-bisphosphate and 14CO2 have elucidated the carboxylase/oxygenase ratios of the enzymes from different organisms. Another approach, i.e., genetic experiments, has also been used to examine the modification of the carboxylase/oxygenase ratio.

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.

Variation in photosynthetic electron transport capacity in vivo and its effects on the light modulation of ribulose bisphosphate carboxylase

Photosynthetic electron transport capacity was varied in vivo in sugar beets using iron deficiency, and its effects on the light modulation of ribulose bisphosphate carboxylase (RuBPCase) studied. Three treatment groups corresponding to decreasing amounts of thylakoids per leaf area were examined: iron sufficient (control), moderately iron-stressed, and severely iron-stressed. Reduction in electron transport capacity in vivo was correlated with a substantial decrease in the level of RuBPCase activation, even at saturating irradiances. These results indicate a direct relationship between RuBPCase activation and photosynthetic electron transport. In addition, our data suggest that the activation of RuBPCase could not solely account for the increases in the photosynthetic rate at high irradiances; RuBPCase reached maximal activation at irradiances well below light saturation for net photosynthesis.

Limiting Factors in Photosynthesis: V. Photochemical Energy Supply Colimits Photosynthesis at Low Values of Intercellular CO(2) Concentration

Although there is now some agreement with the view that the supply of photochemical energy may influence photosynthetic rate (P) at high CO(2) pressures, it is less clear whether this limitation extends to P at low CO(2). This was investigated by measuring P per area as a function of the intercellular CO(2) concentration (C(i)) at different levels of photochemical energy supply. Changes in the latter were obtained experimentally by varying the level of irradiance to normal (Fe-sufficient) leaves of Beta vulgaris L. cv F58-554H1, and by varying photosynthetic electron transport capacity using leaves from Fe-deficient and Fe-sufficient plants. P and C(i) were determined for attached sugar beet leaves using open flow gas exchange. The results suggest that P/area was colimited by the supply of photochemical energy at very low as well as high values of C(i). Using the procedure developed by Perchorowicz et al. (Plant Physiol 1982 69:1165-1168), we investigated the effect of irradiance on ribulose bisphosphate carboxylase (RuBPCase) activation. The ratio of initial extractable activity to total inducible RuBPCase activity increased from 0.25 to 0.90 as leaf irradiance increased from 100 to 1500 microeinsteins photosynthetically active radiation per square meter per second. These data suggest that colimitation by photochemical energy supply at low C(i) may be mediated via effects on RuBPCase activation.

Effects of Light and Elevated Atmospheric CO(2) on the Ribulose Bisphosphate Carboxylase Activity and Ribulose Bisphosphate Level of Soybean Leaves

Soybean (Glycine max L. Merr. cv Bragg) was grown throughout its life cycle at 330, 450, and 800 microliters CO(2) per liter in outdoor controlled-environment chambers under solar irradiance. Leaf ribulose-1,5-bisphosphate carboxylase (RuBPCase) activities and ribulose-1,5-bisphosphate (RuBP) levels were measured at selected times after planting. Growth under the high CO(2) levels reduced the extractable RuBPCase activity by up to 22%, but increased the daytime RuBP levels by up to 20%.Diurnal measurements of RuBPCase (expressed in micromoles CO(2) per milligram chlorophyll per hour) showed that the enzyme values were low (230) when sampled before sunrise, even when activated in vitro with saturating HCO(3) (-) and Mg(2+), but increased to 590 during the day as the solar quantum irradiance (photosynthetically active radiation or PAR, in micromoles per square meter per second) rose to 600. The nonactivated RuBPCase values, which averaged 20% lower than the corresponding HCO(3) (-) and Mg(2+)-activated values, increased in a similar manner with increasing solar PAR. The per cent RuBPCase activation (the ratio of nonactivated to maximum-activated values) increased from 40% before dawn to 80% during the day. Leaf RuBP levels (expressed in nanomoles per milligram chlorophyll) were close to zero before sunrise but increased to a maximum of 220 as the solar PAR rose beyond 1200. In a chamber kept dark throughout the morning, leaf RuBPCase activities and RuBP levels remained at the predawn values. Upon removal of the cover at noon, the HCO(3) (-) and Mg(2+)-activated RuBPCase values and the RuBP levels rose to 465 and 122, respectively, after only 5 minutes of leaf exposure to solar PAR at 1500.These results indicate that, in soybean leaves, light may exert a regulatory effect on extractable RuBPCase in addition to the well-established activation by CO(2) and Mg(2+).

Effects of Different Inorganic Nitrogen Sources on Photosynthetic Carbon Metabolism in Primary Leaves of Non-nodulated Phaseolus vulgaris L

Young bean plants (Phaseolus vulgaris L. var Saxa) were fed with three different types of inorganic nitrogen, after being grown on nitrogen-free nutrient solution for 8 days. The pattern of (14)CO(2) fixation was investigated in photosynthesizing primary leaf discs of 11-day-old plants (3 days with nitrogen source) and in a pulse-chase experiment in 13-day-old plants (5 days with nitrogen source).Ammonium caused, in contrast to nitrate nutrition, a higher level of (14)C incorporation into sugar phosphates but a lower incorporation of label into malate, glycolate, glycerate, aspartate, and alanine. The labeling kinetics of glycine and serine were little changed by the nitrogen source. Ammonium feeding also produced an increase in the ratio of extractable activities of ribulose-1,5-bisphosphate carboxylase to phosphoenolpyruvate carboxylase and an increase in dark respiration and the CO(2) compensation concentration. Net photosynthesis was higher in plants assimilating nitrate.The results point to stimulated turnover of the photosynthetic carbon reduction cycle metabolites, reduced phosphoenolpyruvate carboxylation, and altered turnover rates within the photosynthetic carbon oxidation cycle in ammonium-fed plants. Mechanisms of the regulation of primary carbon metabolism are proposed and discussed.

Limiting Factors in Photosynthesis: III. Effects of Iron Nutrition on the Activities of Three Regulatory Enzymes of Photosynthetic Carbon Metabolism

When Fe was withheld from sugar beets (Beta vulgaris L. CV F58-44H1), the activities of NADP-glyceraldehyde-3-phosphate dehydrogenase, fructose 1,6-bisphosphatase, and ribulose 5-phosphate kinase were not diminished, while chlorophyll per area was decreased by 75%. On resupplying Fe, chlorophyll per area increased to control levels within 5 days, whereas the activities of the three enzymes remained approximately constant. These results support the view advanced earlier (Terry 1980 Plant Physiol 65: 114-120) that the photosynthetic effects of Fe deprivation are mediated by changes in the lamellar components of chloroplasts and not by changes in stromal enzymes involved in photosynthetic carbon reduction.

Inactivation of crystalline tobacco ribulosebisphosphate carboxylase by modification of arginine residues with 2,3-butanedione and phenylglyoxal

Crystalline tobacco ribulosebisphosphate carboxylase (3-phospho-D-glycerate carboxylase (dimerizing), EC 4.1.1.39) is rapidly and completely inactivated by 2,3-butanedione in borate buffer or phenylglyoxal, reagents which are highly specific for the modification of arginine residues. Inactivation by phenylglyoxal is enhanced in Bicine buffer and partially reversible, whereas inactivation by butanedione is markedly enhanced in borate buffer, irreversible in the presence of borate and partially reversed upon complete removal of borate and excess reagent. When the modification reaction is performed in the presence of various ligands, only the substrate ribulosebisphosphate and the diphosphorylated competitive inhibitor sedoheptulosebisphosphate protect against inactivation. Loss of carboxylase activity is directly proportional to incorporation of [14C]phenylglyoxal until about 15% of the initial activity remains. Extrapolation to zero activity suggests that inactivation by [14C]phenylglyoxal correlates with the modification of three arginine residues per 69 000 dalton protomer. Complete protection by ribulosebisphosphate or sedoheptulosebisphosphate correlates with the shielding of 1-2 (1.27 +/- 0.25) essential arginyl groups per protomer, which are located within the 55 000 dalton catalytic subunits of the protein. Similarly, amino acid analyses of acid hydrolysates of the butanedione- or phenyl-glyoxal-inactivated and substrate-protected enzymes suggest that complete protection by ribulosebisphosphate correlated with the shielding of 1.9-2.4 arginine residues per protomer. However, modification of the control and substrate-protected enzymes are these arginine-selective alpha-dicarbonyls does not alter modulation by anionic effectors.

Presence of two subunit types in ribulose 1,5-bisphosphate carboxylase from Thiobacillus intermedius

Ribulose bisphosphate carboxylase (EC 4.1.1.39) has been purified to homogeneity from glutamate-CO2-thiosulfate-grown Thiobacillus intermedius by pelleting the protein from the 93,000 X g supernatant fluid followed by ammonium sulfate fractionation and sedimentation into a discontinuous sucrose density gradient. The molecular weight of the native protein approximated that of the higher plant enzyme (550,000) based on its relative electrophoretic mobility in polyacrylamide disc gels compared with that of standards of known molecular weight, including crystalline tobacco ribulose bisphosphate carboxylase. Sodium dodecyl sulfate electrophoresis in 12% polyacrylamide disc gels and Sephadex G-100 chromatography in the presence of sodium dodecyl sulfate indicated that the purified Thiobacillus protein, like the tobacco enzyme, consisted of two types of nonidentical subunits. The molecular weights of the large and small subunits were estimated to be about 55,000 and 13,000, respectively, by means of sodium dodecyl sulfate-polyacrylamide gel electrophoresis. The carboxylase activity of the protein purified from spinach leaves and T. intermedius responded similarly to the effectors reduced nicotinamide adenine dinucleotide phosphate and 6-phosphogluconate. Contrary to a previous report (K. Purohit, B. A. McFadden, and A. L. Cohen, J. Bacteriol. 127:505-515, 1976), these results indicate that ribulose bisphosphate carboxylase purified from Thiobacillus intermedius closely resembles the higher plant enzyme with respect to quaternary structure, molecular weight, and regulatory properties.

Carbon dioxide assimilation in cyanobacteria: regulation of ribulose, 1,5-bisphosphate carboxylase

Cyanobacteria assimilate carbon dioxide through the Calvin cycle and therefore must regulate the activity of ribulose 1,5-bisophosphate carboxylase. Using an in situ assay, as well as measuring the activity in crude, partially purified, and homogeneous preparations, we can show that a number of phosphorylated intermediates exert a regulatory role. Three diverse organisms, Agmenellum quadruplicatum, Aphanocapsa 6714, and Anabaena sp. CA, were studied, and it was found that the in situ and cell-free carboxylase activities were particularly affected by low levels of phosphogluconate and reduced nicotinamide adenine dinucleotide phosphate. There was a marked activation by these ligands when the inactive enzyme was assayed in the presence of low levels of bicarbonate, a result significantly different from a previous report. Moreover, the fully activated enzyme was inhibited by phosphogluconate. In situ Anabaena CA carboxylase activity exhibited a particular capacity for activation by phosphogluconate and reduced nicotinamide adenine dinucleotide phosphate. However, activation of the crude, partially purified, or homogeneous Anabaena CA carboxylase by phosphogluconate and reduced nicotinamide adenine dinucleotide phosphate was significantly decreased when compared with enzyme activity in permeabilized cells. It appears that the microenvironment or the conformation of the enzyme within the cell may be significantly different from that of the isolated enzyme.

A kinetic study of ribulose bisphosphate carboxylase from the photosynthetic bacterium Rhodospirillum rubrum

The activation kinetics of purified Rhodospirillum rubrum ribulose bisphosphate carboxylase were analysed. The equilibrium constant for activation by CO(2) was 600 micron and that for activation by Mg2+ was 90 micron, and the second-order activation constant for the reaction of CO(2) with inactive enzyme (k+1) was 0.25 X 10(-3)min-1 . micron-1. The latter value was considerably lower than the k+1 for higher-plant enzyme (7 X 10(-3)-10 X 10(-3)min-1 . micron-1). 6-Phosphogluconate had little effect on the active enzyme, and increased the extent of activation of inactive enzyme. Ribulose bisphosphate also increased the extent of activation and did not inhibit the rate of activation. This effect might have been mediated through a reaction product, 2-phosphoglycolic acid, which also stimulated the extent of activation of the enzyme. The active enzyme had a Km (CO2) of 300 micron-CO2, a Km (ribulose bisphosphate) of 11--18 micron-ribulose bisphosphate and a Vmax. of up to 3 mumol/min per mg of protein. These data are discussed in relation to the proposed model for activation and catalysis of ribulose bisphosphate carboxylase.

Modification of Rhodospirillum rubrum ribulose bisphosphate carboxylase with pyridoxal phosphate. 1. Identification of a lysyl residue at the active site

Ribulose 1,5-bisphosphate carboxylase isolated from Rhodospirillum rubrum was strongly inhibited by low concentrations of pyridoxal 5'-phosphate. Activity was protected by the substrate ribulose bisphosphate and to a lesser extent by other phosphorylated compounds. Pyridoxal phosphate inhibition was enhanced in the presence of magnesium and bicarbonate, but not in the presence of either compound alone. Concomitant with inhibition of enzyme activity, pyridoxal phosphate forms a Schiff base with the enzyme which is reversible upon dialysis and reducible with sodium borohydride. Subsequent to reduction of the Schiff base with tritiated sodium borohydride, tritiated N6-pyridoxyllysine could be identified in the acid hydrolysate of the enzyme. Only small amounts of this compound were present when the reduction was performed in the presence of carboxyribitol bisphosphate, an analogue of the intermediate formed during the carboxylation reaction. Therefore, it is concluded that pyridoxal phosphate modifies a lysyl residue close to or at the active site of ribulose bisphosphate carboxylase.

Studies on the regulatory properties of chloroplast fructose-1,6-bisphosphatase

The regulatory properties of chloroplast fructose-1,6-bisphosphatase (D-fructose-1,6-bisphosphate 1-phosphohydrolase, (EC 3.1.3.11) were examined with a homogeneous enzyme preparation isolated from spinach leaves. The activation of the enzyme, that was earlier shown to occur via reduced thioredoxin, was found to be accompanied by a structural change that took place more slowly than the rate of catalysis. The recently found deactivation of the thioredoxin-activated enzyme by physiological oxidants such as oxidized glutathione and dehydroascorbic acid was also slow relative to catalysis. Under the conditions used, the activated enzyme showed a pH optimum of about 8.0, whereas the corresponding value for the non-activated form was pH 8.8. The importance of the thioredoxin-linked mechanism of enzyme regulation that is effected through photoreduced ferredoxin and ferredoxin-thioredoxin reductase is discussed in relation to other light-controlled regulatory agents in chloroplasts.

Activation of ribulose-1,5-bisphosphate carboxylase by chloroplast metabolites in a reconstituted spinach chloroplast system

In a preparation of soluble components from isolated spinach (Spinecia oleracea L.) chloroplasts, the activity of ribulose-1,5-bisphosphate carboxylase (EC 4.1.1.39) is strongly increased by 6-phosphogluconate or by NADPH at pH 8.0. When the thylakoid system is added to these soluble components (reconstituted chloroplast system) plus ferredoxin, the carboxylase is even more strongly activated in the light. This "light" activation appears to be due to reduction of endogenous NADP(+) by electrons from the light reactions transferred via ferredoxin, since NADPH alone can activate the purified enzyme in the dark while reduced ferredoxin does not. The regulatory properties of the enzyme in the reconstituted chloroplast system are compared with those of the isolated enzyme, and their possible physiologic significance is discussed.

A model for the kinetics of activation and catalysis of ribulose 1,5-bisphosphate carboxylase

Further evidence for time-dependent interconversions between active and inactive states of ribulose 1,5-bisphosphate carboxylase is presented. It was found that ribulose bisphosphate oxygenase and ribulose bisphosphate carboxylase could be totally inactivated by excluding CO2 and Mg2+ during dialysis of the enzyme at 4 degrees C. When initially inactive enzyme was assayed, the rate of reaction continually increased with time, and the rate was inversely related to the ribulose bisphosphare concentration. The initial rate of fully activated enzyme showed normal Michaelis-Menten kinetics with respect to ribulose bisphosphate (Km = 10muM). Activation was shown to depend on both CO2 and Mg2+ concentrations, with equilibrium constants for activation of about 100muM and 1 mM respectively. In contrast with activation, catalysis appeared to be independent of Mg2+ concentration, but dependent on CO2 concentration, with a Km(CO2) of about 10muM. By studying activation and de-activation of ribulose bisphosphate carboxylase as a function of CO2 and Mg2+ concentrations, the values of the kinetic constants for these actions have been determined. We propose a model for activation and catalysis of ribulose bisphosphate carboxylase: (see book) where E represents free inactive enzyme; complex in parentheses, activated enzyme; R, ribulose bisphosphate; M, Mg2+; C, CO2; P, the product. We propose that ribulose bisphosphate can bind to both the active and inactive forms of the enzyme, and slow inter-conversion between the two states occurs.

NADPH/NADP+ ratios in photosynthesizing reconstituted chloroplasts

Levels of reduced and oxidized triphosphopyridine nucleotides have been determined in reconstituted spinach chloroplasts and compared with levels in whole isolated chloroplasts during photosynthesis and darkness. The ratio of NADPH/NADP+ reaches values slightly above 1.0 at the beginning of photosynthesis, less than half the ratio attained with whole chloroplasts. Nonetheless these lower ratios are sufficient to maintain high rates of photosynthetic carbon dioxide fixation and reduction, which are comparable in the reconstituted chloroplasts to the rates found with whole chloroplasts. As with whole chloroplasts there is a decline in the ration of NADPH/NADP+ as a function of time of photosynthesis. The effect of addition of bicarbonate (6 mM) in causing a transient drop in the ratio of NADPH/NADP/ is described and discussed in terms of the reversibility of the reduction of 3-phosphoglycerate to triose phosphate. The ratio NADPH/NADP+ can be improved by the addition of more lamellae either before or during the course of photosynthesis, and this improvement in ratio is accompanied by an improved rate of CO2 fixation or a more sustained rate of CO2 fixation with time of photosynthesis. The importance of NADPH/NADP+ ratio not only to the reduction of 3-phosphoglycerate to triose phosphate but also to the activation of the ribulose-1,5-diphosphate carboxylasemediated step is discussed.