Physiological studies on acid metabolism. 4. Phosphoenolpyruvic carboxylase activity in extracts of Crassulacean plants

Testing the kinetic tradeoff between bicarbonate versus phosphoenolpyruvate affinity and glucose-6 phosphate response of phosphoenolpyruvate carboxylase from two C4 grasses

Phosphoenolpyruvate (PEP) carboxylase (PEPC) has an anaplerotic role in central plant metabolism but also initiates the carbon concentrating mechanism during C4 photosynthesis. The C4 PEPC has different binding affinities (Km) for PEP (K0.5PEP) and HCO3- (K0.5HCO3), and allosteric regulation by glucose-6-phosphate (G6-P) compared to non-photosynthetic isoforms. These differences are linked to specific changes in amino acids within PEPC. For example, region II (residues 302-433, Zea mays numbering) has been identified as important for G6-P regulation and within this region residue 353 may be conserved in C4 PEPC enzymes. Additionally, residue 780 influences the C4 PEPC kinetic properties and may interact with region II as well as residue 353 to influence G6-P regulation. We test the hypothesis that variation within region II, including residue 353, and their interactions with residue 780 influence the kinetic and allosteric regulation by G6-P of two C4 PEPC isozymes from two C4 grasses. The data does not support a kinetic tradeoff between K0.5HCO3 and K0.5PEP in these PEPC isozymes. Additionally, these enzymes had different response to G6-P that was only partially attributed to region II, residue 353 and residue 780. This data provides new insights into factors influencing the kinetic variation of C4 PEPC isozymes.

From Chlorella to chloroplasts: a personal note

An historical and personal reflection on the function of the Benson-Calvin Cycle in isolated chloroplasts, the role of inorganic phosphate and the manner in which this might be best presented to students.

A microtiter plate-based assay for phosphoenolpyruvate carboxylase

A sensitive, quantitative assay for phosphenolpyruvate carboxylase which utilizes microtiter plates is described. The assay depends upon the production of a colored compound in the reaction between oxaloacetate, the product of the phosphoenolpyruvate reaction, and the dye Fast Violet B. The method is particularly appropriate for monitoring chromatographic eluates and its utility for this purpose is demonstrated by the detection of phosphoenolpyruvate carboxylase in fractions of crude maize extract separated by size-exclusion chromatography.

Changes in Metabolite Levels in Kalanchoë daigremontiana and the Regulation of Malic Acid Accumulation in Crassulacean Acid Metabolism

Changes in glucose-6-P, fructose-6-P, fructose-1,6-diP, 6-phospho-gluconate, phosphoenolpyruvate, 3-phosphoglycerate, and pyruvate levels in the leaves of the Crassulacean plant Kalanchoë daigremontiana Hammet et Perrier were measured enzymically during transitions from CO(2)-free air to air, air to CO(2)-free air, and throughout the course of acid accumulation in darkness. The data are discussed in terms of the involvement of phosphoenolpyruvate carboxylase in malic acid synthesis and in terms of the regulation of the commencement of malic acid synthesis and accumulation through the effects of CO(2) on storage carbohydrate mobilization and its termination through the effects of malic acid on phosphoenolpyruvate carboxylase activity.

Photoperiodism and enzyme activity: towards a model for the control of circadian metabolic rhythms in the crassulacean Acid metabolism

Metabolic readjustments after a change from long days to short days appear, in Kalanchoe blossfeldiana, to be achieved through the operation of two main mechanisms: variation in enzyme capacity, and circadian rhythmicity. After a lag time, capacity in phosphoenolpyruvate carboxylase and capacity in aspartate aminotransferase increase exponentially and appear to be allometrically linked during 50 to 60 short days; then a sudden fall takes place in the activity of the former. Malic enzyme and alanine aminotransferase behave differently. Thus, the operation of the two sections of the pathway (before and after the malate step) give rise to a continuously changing functional compartmentation in the pathway. Circadian rhythmicity, on the other hand, produces time compartmentation through phase shifts and variation in amplitude, independently for each enzyme. These characteristics suggest that the operation of a so-called biological clock would be involved. We propose the hypothesis that feedback regulation would be more accurate and efficient when applied to an already oscillating, clock-controlled enzyme system.

Multiple forms of plant phosphoenolpyruvate carboxylase associated with different metabolic pathways

The physical and kinetic properties of multiple forms of phosphoenolpyruvate carboxylase were studied in leaves of C(4) and C(3) species, their F(1) and F(3) hybrids, in greening maize leaves, in Crassulacean acid metabolism plants, and in nongreen root tissues. Four different forms are suggested: a C(4) photosynthetic phosphoenolpyruvate carboxylase with high Km for phosphoenolpyruvate ( approximately 0.59 mm), Km Mg ( approximately 0.5 mm), and V(max) ( approximately 29 micromoles per minute per milligram of chlorophyll); a C(3) photosynthetic phosphoenolpyruvate carboxylase with low Km for phosphoenolpyruvate ( approximately 0.14 mm), Km for Mg ( approximately 0.097 mm), and V(max) (1.5); a Crassulacean acid metabolism type with low Km for phosphoenolpyruvate (0.14 mm), and high V(max) (14 micromoles per minute per milligram of chlorophyll); and a nongreen or nonautotrophic type with low Km for phosphoenolpyruvate, Km for Mg, and low V(max). In closely related species or within species, the types can be differentiated by anion exchange column chromatography. Each of the four forms is associated with a different metabolic pathway: the phosphoenolpyruvate carboxylase of C(4) species for malate generation as a photosynthetic intermediate, the phosphoenolpyruvate carboxylase of C(3) species in malate generation as a photosynthetic product, the phosphoenolpyruvate carboxylase of Crassulacean acid metabolism species in malate generation as a CO(2) donor for photosynthesis during the subsequent light period, and a nongreen or root type producing malate for ionic balance and reduced nicotinamide adenine dinucleotide phosphate generation. The data in this paper in conjunction with published information support the notion of different molecular forms of a protein functioning in different metabolic pathways which have common enzymic steps.

The Relation between Photosynthesis, Respiration, and Crassulacean Acid Metabolism in Leaf Slices of Aloe arborescens Mill

Leaves and leaf slices from Aloe arborescens Mill. were used to study the interrelations between Crassulacean acid metabolism, photosynthesis, and respiration. Oxygen exchange of leaf slices was measured polarographically. It was found that the photosynthetic utilization of stored malic acid resulted in a net evolution of oxygen. This oxygen production, and the decrease in acid content of the leaf tissue, were completely inhibited by amytal, although the rate of respiratory oxygen uptake was hardly affected by the presence of this inhibitor of mitochondrial electron transport. Other poisons of respiration (cyanide) and of the tricarboxylic acid cycle (trifluoroacetate, 2-diethyl malonate) also were effective in preventing acid-dependent oxygen evolution. It is concluded that the mobilization of stored acids during light-dependent deacidification of the leaves depends on the operation of the tricarboxylic acid cycle and of the electron transport of the mitochondria.A comparison of enzyme activities in extracts from Aloe leaves and from other plants and studies of leaf anatomy and chloroplast morphology revealed typical characteristics of C(3)-, as well as C(4)-, plants in Aloe.

Variation in levels of some leaf enzymes

Several procedures were compared for efficiency in the extraction of certain leaf enzymes (phosphoenolpyruvate carboxylase, ribulose 1,5-diphosphate carboxylase and malate dehydrogenase) in Atriplex hastata (a "C3" species exhibiting conventional photosynthetic metabolism), and in A. spongiosa (a "C4" species in which the initial photosynthetic products are C4 dicarboxylic acids). Glycolate oxidase was also assayed in some cases, and Atriplex nummularia and Sorghum bicolor were also used as test material. A simple procedure, involving a mortar and pestle grind with carborundum added to the grinding mixture, was found to be as effective as glass bead grind procedures. In addition, it was more rapid and showed less variability with different operations.Using the carborundum grind procedure, sources of variability in enzyme activity in apparently uniform leaves were compared, as were effects of time of day, leaf age and storage procedure. In general, if apparently uniform leaves could be selected, variability in levels of enzyme activity appeared to be relatively small, not exceeding about 12%. Time of day also appeared to be relatively unimportant for the enzymes examined. However, the ontogentic status of the plant was found to be an important source of variability. Leaf age was also a major source of variability where the activity was expressed on a fresh weight basis, but specific activity (i.e. activity expressed on a protein basis) was relatively constant, at least with the range of species and leaf ages examined here.Storage of fresh samples in liquid nitrogen for 24 h, prior to extraction and assay, led to only a small reduction in activity, but substantial changes occurred if storage was in dry ice or in ice and also where extracts were stored in a deep freeze.

Compartmentation of malate in relation to ion absorption in beet

Malate in beet discs treated in different salt solutions was labeled by a 30 min pulse of (14)CO(2), and subsequent changes in specific activity were followed for several hr. In treatments which resulted in net acid synthesis in response to excess cation absorption, malate specific activity fell slowly after removal of (14)CO(2). In solutions where no net acid synthesis occurred, and from which cation and anion were absorbed equally, malate specific activity fell rapidly when (14)CO(2) was removed. The foregoing suggests that the net synthesis of organic acids in response to excess cation absorption leads to the removal of organic anions from cytoplasmic metabolic pools as counter-ions in salt transport to the vacuole.The latter hypothesis was further examined by direct measurement of the distribution of labeled malate between cytoplasm and vacuole using the wash-exchange method of compartmental analysis, previously described for inorganic ions. The method satisfied the criterion of exchange specificity necessary for this purpose. Much higher retention of label in the cytoplasm was observed in KCl solutions (no net synthesis) than in K(2)SO(4) solutions (net synthesis) after 3 hr (14)CO(2) fixation and subsequent wash-exchange. The observed distribution is consistent with the rapid removal of organic anions to the vacuole during net acid synthesis. The significance of organic acid transport in relation to metabolism is discussed.

Temperature features of enzymes affecting crassulacean Acid metabolism

Enzymes involved in malic acid production via a pathway with 2 carboxylation reactions and in malic acid conversion via total oxidation have been demonstrated in mitochondria of Bryophyllum tubiflorum Harv. Activation of the mitochondria by Tween 40 was necessary to reveal part of the enzyme activities. The temperature behavior of the enzymes has been investigated, revealing optimal activity of acid-producing enzymes at 35 degrees . Even at 53 degrees the optimum for acid-converting enzymes was not yet reached. From the simultaneous action of acid-producing and acid-converting enzyme systems the overall result at different temperatures was established. Up to 15 degrees the net result was a malic acid production. Moderate temperatures brought about a decrease in this accumulation, which was partly accompanied by a shift to isocitrate production, while at higher temperatures total oxidation of the acids exceeded the production.

Physiological Studies on Acid Metabolism in Green Plants. VI. Transaminases in Cell-Free Extracts from Kalanchoë Leaves

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