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91.
Temperature, activating metal ions, and amino-acid substitutions are known to influence the CO2/O2 specificity of the chloroplast enzyme ribulose-1,5-bisphosphate carboxylase/oxygenase. However, an understanding of the physical basis for enzyme specificity has been elusive. We have shown that the temperature dependence of CO2/O2 specificity can be attributed to a difference between the free energies of activation for the carboxylation and oxygenation partial reactions. The reaction between the 2,3-enediolate of ribulose 1,5-bisphosphate and O2 has a higher free energy of activation than the corresponding reaction of this substrate with CO2. Thus, oxygenation is more responsive to temperature than carboxylation. We have proposed possible transition-state structures for the carboxylation and oxygenation partial reactions based upon the chemical natures of these two reactions within the active site. Electrostatic forces that stabilize the transition state of the carboxylation reaction will also inevitably stabilize the transition state of the oxygenation reaction, indicating that oxygenase activity may be unavoidable. Furthermore, the reduction in CO2/O2 specificity that is observed when activator Mg2+ is replaced by Mn2+ may be due to Mg2+ being more effective in neutralizing the negative charge of the carboxylation transition state, whereas Mn2+ is a transition-metal ion that can overcome the triplet character of O2 to promote the oxygenation reaction.Abbreviations CABP
2-carboxyarabinitol 1,5-bisphosphate
- enol-RuBP
2,3-enediolate of ribulose 1,5-bisphosphate
- Kc
Kmfor CO2
- Ko
Kmfor O2
- Rubisco
ribulose-1,5-bisphosphate carboxylase/oxygenase
- RuBP
ribulose 1,5-bisphosphate
- Vc
V
max for carboxylation
- Vo
V
max for oxygenation 相似文献
92.
B. Lubec K. R. Herkner H. Brückner H. Höger J. Gialamas D. Adamiker G. Lubec 《Amino acids》1991,1(2):289-292
Summary A series of alpha-alkyl-amino acids were tested for some biological functions in the mouse (OF-1 Himberg) by adding them to the animal chow (30 mg/kg/day) for a period of six weeks. No differences in fluid or food uptake could be observed during the feeding period, as compared to a control group. Histology of liver and kidney did not show any changes. Testing routine clinical chemical parameters (serum substrates and enzymes) revealed the following changes: Hex-Ala and But-Abu increased the serum glucose levels. But-Abu dramatically lowered the triglycerides. Serum albumin was increased by Pent-Ala, Me-Val, and But-Abu. LDH was inhibited by But-Abu. 相似文献
93.
本文是26篇关于丝状真菌基因表达系统的研究论文的综述,包括两部份内容。前一部分叙述1979年开始建立并迅速发展起来的丝状真菌转化系统,着重介绍丝状真菌中转化系统的构建及转化的一般特点。后一部分叙述在转化系统发展基础上产生的丝状真菌基因工程,文中列出了截至1991年9月为止报道的一些成功的实例,说明它在丝状真菌工业育种和作为外源基因产物的生产和分泌系统中的应用。 相似文献
94.
95.
Wyman's equation for facilitated diffusion of a ligand through a solution slab containing a carrier is recast and solved by means of a regular perturbation expansion in the parameter representing the driving force for facilitation. This new solution is complementary to the previously exploited singular perturbation solution due to Murray and represents facilitation in the low facilitation parameter regime. The most significant physical realization of this regime occurs when there is a large affinity between ligand and carrier, as in the carbon monoxide-hemoglobin system.The validity domains of the regular perturbation solution and the singular perturbation solution of Mitchell and Murray and Rubinow and Dembo are delineated. The equation for facilitated diffusion is solved numerically for parameter values appropriate to the oxygen-myoglobin experiments of Wittenberg, and to the carbon monoxide-hemoglobin experiments of Mochizuki and Forster, and Wittenberg. This solution provides a norm for comparison of the utility of the perturbation solutions.We show how the theory explains the apparent contradiction between the positive observations of Mochizuki and Forster and the negative observations of Wittenberg on facilitation of carbon monoxide transport through a slab of hemoglobin solution.Supported in part by the National Science Foundation under Grant No. PCM77-03344In partial fulfillment of the Ph.D. degree at Cornell UniversityProfessor S. I. Rubinow has passed away on February 22, 1981Computations were performed at the Courant Mathematics and Computing Laboratory, supported by DOE under contract #EY-76-C-02-3077. 相似文献
96.
The thermophilic blue-green alga Synechococcus lividus was grown at 38 and 55°C. The reaction center chlorophyll-protein complexes (CP) of Photosystem (PS I) and PS II, CP aI and CP aII, were isolated by sodium dodecyl sulphate (SDS) polyacrylamide gel electrophoresis at 4°C. SDS solubilization of thylakoids was performed in the temperature range 0–65°C. The low-temperature absorption and fluorescence emission spectral properties of the isolated chlorophyll-protein complexes were analyzed. Only traces of CP aI were solubilized at temperatures below the lipid phase transition temperature. Instead, a minor PS I component, CP a′I, was obtained that had absorption and fluorescence characteristics similar to those of CP aI. CP a′I had a slightly lower mobility than CP aI in SDS-polyacrylamide gel electrophoresis. The amount of CP aI in the gel scan profile increased dramatically when solubilization was carried out above the phase transition temperatures, but started to decrease above 60°C. CP aII, on the other hand, could be efficiently extracted even at 0°C and was stable in the scan profile up to extraction temperatures of 30–40°C. Low-temperature absorption and fluorescence emission spectra were typical for CP aI and CP aII and no specific effects of the two growth temperatures on these properties were observed. The phase transition temperature was considered to be critical for the solubilization of CP aI, either because of the difficulties of SDS (especially as it forms micelles at low temperatures) in penetrating the solidified membrane lipids at temperatures below that of the phase transition or because the CP aI monomers of the PS I antennae are so strongly bound to each other that they cannot be dissociated by SDS before thermal agitation has reached a certain level that is achieved above the phase transition temperature. We consider both the difficulties in solubilizing CP aI at sub-transition temperatures and the heat stability of the two complexes as adaptations which enable Synechococcus to grow under extreme high-temperature regimes. 相似文献
97.
Reaction centres from Rhodopseudomonas sphaeroides, strain R-26, have been solubilised in hexane by the use of phospholipids and cations. Two procedures have been developed: (I) a two-step technique involving the formation of detergent-free proteoliposomes from detergent solubilised reaction centres and phospholipids and mixing these with hexane in the presence of cations; (II) directly sonicating detergent-solubilised reaction centres with phosopholipids before mixing with hexane and cations.The yield of the extracted complex varied with the ratios of protein, phospholipid and cations, species of phospholipid, and sonication time. The spectral characteristics of the complex in the organic phase were similar to those of detergent-solubilised reaction centres. The stability of the reaction centres appeared to be dependent on the presence of phospholipid and water in the hexane. The usefulness of the hexane solution as a model membrane system is discussed and its possible future applications are considered. 相似文献
98.
John M. Olson 《BBA》1981,637(1):185-188
Two reaction center complexes prepared from cytoplasmic membranes of Chlorobium limicola f. thiosulfato-philum were compared by absorption and CD spectrophotometry. Bacteriopheophytin c (670 nm), which is optically active in one complex but not in the other, may serve as a secondary electron acceptor in the reaction center. 相似文献
99.
Rhodospirillum rubrum strain F24.1 is a spontaneous revertant of nonphototrophic mutant F24 derived from wild-type strain S1. Strain F24 shows no detectable photochemical activity and contains, at most, traces of the photoreaction center polypeptides. Strain F24.1 has a phototrophic growth rate close to that of the wild-type strain (Picorel, R., del Valle-Tascón, S. and Ramírez, J.M. (1977) Arch. Biophys. Biochem. 181, 665–670) but shows little photochemical activity. Light-induced absorbance changes in the near-infrared, photoinduced EPR signals and ferricyanide-elicited absorbance changes indicate that strain F24.1 has a photoreaction center content of 7–8% as compared to strain S1. Polyacrylamide gel electrophoresis of isolated F24.1 chromatophores shows the photoreaction center polypeptides to be present in amounts compatible with this value. Photoreaction center was prepared from strain F24.1 and showed no detectable difference with that of strain S1. It is concluded that strain F24.1 photosynthesis is due entirely to its residual 7–8% of typical photoreaction center. 相似文献
100.
Alan Stemler 《BBA》1980,593(1):103-112
In broken chloroplasts the presence of 100 mM sodium formate at pH 8.2 will specifically lengthen the Photosystem II relaxation times of the reactions S′2 → S3 and S′3 → S0. Rates of reactions S′0 → S1 and S′1 → S2 remain unaffected. Evidence is presented which indicates the discrimination among S-states by formate cannot be attributed to a block imposed on the reducing side of Photosystem II. The results are interpreted in context of the known interaction of formate and CO2 which is bound to the Photosystem II reaction center complex. It is proposed that those S-state transitions which show extended relaxation times in the presence of formate must result in the momentary release and rebinding of CO2. Furthermore since formate is acting on the oxygen-evolving side of Photosystem II, it would seem that CO2 is released in reactions that occur there. A chemical model of oxygen evolution is presented. It is based on the hypothesis that hydrated CO2 is the immediate source of photosynthetically evolved oxygen and explains why, under certain conditions formate slows only the S-state transitions S′2 → S3 and S′3 → S0. 相似文献