The rates of the spontaneous hydration of CO2 and the reciprocal reaction in neutral aqueous solutions between 0° and 38°

Magid, Erik; Turbeck, Børge Ove

doi:10.1016/0304-4165(68)90232-8

Cited by 110 publications

(56 citation statements)

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“…Whether these species reach equilibrium with each other, however, depends on how the rate of uncatalyzed interconversion compares to the rates of other processes (e.g., transport and enzymatic catalysis) that produce and consume specific species. The spontaneous dehydration of H total to CO 2 [T 1=2 > 10 s (13)] is much slower than diffusion and transport, and so the CCM can maintain H total out of equilibrium with CO 2 in the cytosol (10). Equilibration between HCO 3 − , H 2 CO 3 , and CO 3 2− within the H total pool is, however, extremely fast [T 1=2 < 1 μs (30)].…”

Section: Resultsmentioning

confidence: 99%

“…The carboxysome lumen is densely packed (>400 mg protein/mL) with about 2,000 RuBisCO and 100 CA active sites (12). Because CA activity is absent from the cytosol and the spontaneous dehydration of HCO 3 − to CO 2 is relatively slow (13), HCO 3 − does not equilibrate with CO 2 in the cytosol (10). Rather, HCO 3 − enters the carboxysome where CA activity readily equilibrates it with CO 2 (10).…”

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confidence: 99%

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pH determines the energetic efficiency of the cyanobacterial CO ₂ concentrating mechanism

Mangan

Flamholz

Hood

et al. 2016

Proc. Natl. Acad. Sci. U.S.A.

177

181

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Many carbon-fixing bacteria rely on a CO 2 concentrating mechanism (CCM) to elevate the CO 2 concentration around the carboxylating enzyme ribulose bisphosphate carboxylase/oxygenase (RuBisCO). The CCM is postulated to simultaneously enhance the rate of carboxylation and minimize oxygenation, a competitive reaction with O 2 also catalyzed by RuBisCO. To achieve this effect, the CCM combines two features: active transport of inorganic carbon into the cell and colocalization of carbonic anhydrase and RuBisCO inside proteinaceous microcompartments called carboxysomes. Understanding the significance of the various CCM components requires reconciling biochemical intuition with a quantitative description of the system. To this end, we have developed a mathematical model of the CCM to analyze its energetic costs and the inherent intertwining of physiology and pH. We find that intracellular pH greatly affects the cost of inorganic carbon accumulation. At low pH the inorganic carbon pool contains more of the highly cellpermeable H 2 CO 3 , necessitating a substantial expenditure of energy on transport to maintain internal inorganic carbon levels. An intracellular pH ≈8 reduces leakage, making the CCM significantly more energetically efficient. This pH prediction coincides well with our measurement of intracellular pH in a model cyanobacterium. We also demonstrate that CO 2 retention in the carboxysome is necessary, whereas selective uptake of HCO 3 − into the carboxysome would not appreciably enhance energetic efficiency. Altogether, integration of pH produces a model that is quantitatively consistent with cyanobacterial physiology, emphasizing that pH cannot be neglected when describing biological systems interacting with inorganic carbon pools.carbon fixation | RuBisCO | cyanobacteria | inorganic carbon | systems biology C yanobacteria and many other autotrophs use a CO 2 concentrating mechanism (CCM) to increase the cellular pool of inorganic carbon and facilitate the Calvin-Benson-Bassham (CBB) cycle (1). Specifically, the CCM functions to supply CO 2 to ribulose bisphosphate carboxylase/oxygenase (RuBisCO), the primary carboxylating enzyme of the CBB cycle. High levels of CO 2 are essential to cyanobacterial metabolism because RuBisCO has relatively slow carboxylation kinetics and is promiscuous, catalyzing an off-pathway reaction with O 2 called oxygenation (2-4).RuBisCO oxygenation produces 2-phosphoglycolate (2PG), which is not part of the CBB cycle and must be recycled. Recycling 2PG through photorespiratory pathways is costly, consuming reduced carbon and energy resources (5, 6). The problem of RuBisCO's limited specificity is all the more pronounced because there is ≈ 20 times more O 2 than CO 2 in aqueous solutions equilibrated with present-day atmosphere (SI Appendix, Fig. S1). CCMs overcome these problems by concentrating CO 2 near RuBisCO, favorably increasing the ratio of CO 2 to O 2 . High concentrations of CO 2 maximize the rate of carboxylation and competitively inhibit oxygenation. Indeed, it is widel...

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Section: Resultsmentioning

confidence: 99%

mentioning

confidence: 99%

pH determines the energetic efficiency of the cyanobacterial CO ₂ concentrating mechanism

Mangan

Flamholz

Hood

et al. 2016

Proc. Natl. Acad. Sci. U.S.A.

177

181

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“…Two things must be demonstrated before this suggestion can be considered valid: (a) the uncatalyzed rate of CO2 formation from the HCO2-pool must be less than the rate of photosynthesis and (b) the amount of bicarbonate in the pool must be known. The operational rate constant for the dehydration of bicarbonate is about I0 sec'1 at a pH close to 8.0 (16). For chloroplasts at this pH and in equilibrium with the atmospheric pressure of C02, the HCO3-concentration is about 1 mM.…”

Section: Resultsmentioning

confidence: 92%

“…-Present address: The Biological Laboratories, 16 Divinity Avenue, Harvard University, Cambridge, Mass. 02138.…”

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confidence: 99%

Carbonic Anhydrase of Spinach

Jacobson¹,

Fong²,

Heath³

1975

Plant Physiol.

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Carbonic anhydrase activity was determined in spinach (Spinacia oleracea) leaf organelles isolated on sucrose density gradients and was found to be predominantly in the intact chloroplast fraction. The small amount of activity associated with the mitochondrial fractions was probably due to intact chloroplast contamination. No activity could be associated with the broken chloroplast or microbody fractions. Based upon inhibitor studies, carbonic anhydrase was found to be around 2 mM in the chloroplast. Ethoxzolamide, an inhibitor of carbonic anhydrase, reduced C02 fixation in intact chloroplasts. The concentration required to inhibit CO2 fixation 20 to 40% was in excess of that required to inhibit the purified enzyme. The inhibition was partially reversed by C02. Ethoxzolamide had no effect on photosynthetic NADP reduction or photophosphorylation measured by methyl viologen reduction. The In addition, the finding that acetazolamide inhibited photosynthetic electron transport as well as carbonic anhydrase seriously questions whether or not the acetazolamide inhibition of photosynthetic CO2 fixation was a function of carbon anhydrase activity. This paper is further concerned with the study of another inhibitor or carbonic anhydrase-ethoxzolamide-with respect to its inhibitory kinetics and its effect upon isolated intact chloroplasts. MATERIALS AND METHODSThe leaves of washed, commercial spinach (Spinacia oleracea), after deribbing, were chopped gently with razor blades in a grinding medium of 400 mm sorbitol, 20 mm HEPES buffer (pH 8.0), 10 mm MgNa2-EDTA, 1 mM dithiothreitol, and 1% bovine serum albumin at 0 C (1.5 ml of grinding medium per g fresh weight). The leaf homogenate was then gently ground in a mortar and pestle at 0 C which contained about 0.5 ml of partially frozen grinding medium per g fresh weight of leaves.The slurry was then filtered once through a 30-Mtm diameter nylon mesh to remove cell wall debris and unbroken cells. The supernatant of this solution obtained from a low speed centrifugation (either 1,000g or 3,000g) was then layered onto sucrose density gradients, as previously described (24), with the following modifications. Pellets were resuspended in grinding medium without MgNa2-EDTA, and 1 mM dithiothreitol was substituted for mercaptoethanol. The gradients were centrifuged at 25,000 rpm in a Beckman SW27 swinging bucket rotor in a Beckman L2-50 ultracentrifuge.Isolated intact spinach chloroplasts, used for the inhibitor studies, were prepared as previously described (12). Published methods were used to assay for glyceraldehyde-3-P dehydrogenase, malate dehydrogenase (24), and chlorophyll concentrations (1). The carbonic anhydrase assay consisted of a reaction mixture of 4.0 ml of barbituric acid buffer (22.5 mM at pH 9.0), 0.1 ml of enzyme suspension, and 0.9 ml of distilled water. Blanks were either 0.1 ml of boiled enzyme suspension or distilled water. The reaction was followed by recording the pH changes on a Radiometer pH meter (type TTTlc) and a Bausch and Lomb recorder. The ...

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“…The rate constant for exchange, kex, is a lumped parameter that depends on the number of copies of AE1 per cell, the cell surface-to-volume ratio, the substrate ion concentrations, substrate binding affinities, and the translocation rate constants, which are in general asymmetric (14,37 Step 2: protonation of intracellular base B Ϫ (HCO 3 Ϫ or HS Ϫ ) to acid HB (CO2 or H2S). (43). In erythrocyte cytosol, which has very high carbonic anhydrase activity, the forward and reverse rate constants are assumed to be 20,000-fold higher than the rate constants without catalyst (13,44,45,73).…”

Section: Modelingmentioning

confidence: 99%

Transport of H₂S and HS⁻ across the human red blood cell membrane: rapid H₂S diffusion and AE1-mediated Cl⁻/HS⁻ exchange

Jennings

2013

American Journal of Physiology-Cell Physiology

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(1,25,41,54,68,75). However, there are very wide variations in estimates of circulating H 2 S levels in mammals (56, 71), and much remains to be learned about the conditions under which endogenously produced H 2 S has significant physiological effects (50).An understanding of the cellular physiology of H 2 S requires quantitative information on the transport of H 2 S, as well as HS Ϫ , which, at neutral or alkaline pH, is present at a higher concentration than H 2 S [pK a ϭ 6.76 at 37°C (26)]. Two recent studies have provided strong evidence for rapid H 2 S diffusion through lipid bilayers. Mathai et al. (46) measured pH changes associated with H 2 S diffusion across planar lipid bilayers and found that the permeability coefficient for H 2 S is Ն0.5 cm/s. Cuevasanta et al. (7) showed that the lipid-water partition coefficient of H 2 S is ϳ2, which is consistent with a bilayer permeability coefficient of 3 cm/s. The time course of H 2 S transport in liposomes is too fast to measure with stopped flow, indicating that the permeability coefficient is Ͼ1 cm/s (7). The permeability of lipid bilayers to HS Ϫ has not been measured but is undoubtedly orders of magnitude lower than that of H 2 S, as is true of other weak acids and their respective anions (58).Although it is clear that the H 2 S permeability of lipid bilayers is quite high, there have been relatively few measurements of H 2 S and/or HS Ϫ transport in biological systems; these are summarized briefly below.In 1936, Jacques (28) measured the influx of H 2 S in the alga Valonia macrophysa and found that the half time for influx was ϳ5 min. If V. macrophysa is modeled as a sphere of 1-cm radius, the influx half time corresponds to a permeability coefficient of 8 ϫ 10 Ϫ4 cm/s. This permeability is orders of magnitude lower than the above estimates for lipid bilayers, but influx in these very large cells could be limited in part by unstirred layers.Julian and Arp (33) determined the H 2 S and HS Ϫ permeabilities of the body wall and hindgut of the marine worm Urechis caupo and found that the H 2 S permeability is only moderately higher than that of HS Ϫ . The highest permeability coefficient was for H 2 S permeation of stretched hindgut (5 ϫ 10 Ϫ4 cm/s). Again, this permeability coefficient is orders of magnitude lower than that of lipid bilayers; as in Valonia, diffusional barriers other than cell membranes may contribute to the low permeability.The hydrothermal vent tubeworm Riftia pachyptila takes up H 2 S/HS Ϫ from the environment for the purpose of supplying H 2 S for oxidation by symbiotic intracellular bacteria. Goffredi et al. (17) found that, surprisingly, H 2 S diffusion into R. pachyptila is slower than expected and that HS Ϫ influx is the more important mechanism of H 2 S acquisition. Another tubeworm, Lammelibrachia, acquires H 2 S through its root; the permeability coefficient for H 2 S/HS Ϫ transport through the root tube wall of this species is ϳ10 Ϫ3 cm/s for the thinnest walls (34). There is no significant effect of pH on influx, indicating t...

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The rates of the spontaneous hydration of CO2 and the reciprocal reaction in neutral aqueous solutions between 0° and 38°

Cited by 110 publications

References 20 publications

pH determines the energetic efficiency of the cyanobacterial CO ₂ concentrating mechanism

pH determines the energetic efficiency of the cyanobacterial CO ₂ concentrating mechanism

Carbonic Anhydrase of Spinach

Transport of H₂S and HS⁻ across the human red blood cell membrane: rapid H₂S diffusion and AE1-mediated Cl⁻/HS⁻ exchange

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The rates of the spontaneous hydration of CO2 and the reciprocal reaction in neutral aqueous solutions between 0° and 38°

Cited by 110 publications

References 20 publications

pH determines the energetic efficiency of the cyanobacterial CO 2 concentrating mechanism

pH determines the energetic efficiency of the cyanobacterial CO 2 concentrating mechanism

Carbonic Anhydrase of Spinach

Transport of H2S and HS− across the human red blood cell membrane: rapid H2S diffusion and AE1-mediated Cl−/HS− exchange

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pH determines the energetic efficiency of the cyanobacterial CO ₂ concentrating mechanism

pH determines the energetic efficiency of the cyanobacterial CO ₂ concentrating mechanism

Transport of H₂S and HS⁻ across the human red blood cell membrane: rapid H₂S diffusion and AE1-mediated Cl⁻/HS⁻ exchange