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21.
The Calvin cycle of carbon dioxide fixation constitutes a biosynthetic pathway for the generation of (multi-carbon) intermediates
of central metabolism from the one-carbon compound carbon dioxide. The product of this cycle can be used as a precursor for
the synthesis of all components of cell material. Autotrophic carbon dioxide fixation is energetically expensive and it is
therefore not surprising that in the various groups of autotrophic bacteria the operation of the cycle is under strict metabolic
control. Synthesis of phosphoribulokinase and ribulose-1,5-bisphosphate carboxylase, the two enzymes specifically involved
in the Calvin cycle, is regulated via end-product repression. In this control phosphoenolpyruvate most likely has an alarmone
function. Studies of the enzymes isolated from various sources have indicated that phosphoribulokinase is the target enzyme
for the control of the rate of carbon dioxide fixation via the Calvin cycle through modulation of existing enzyme activity.
In general, this enzyme is strongly activated by NADH, whereas AMP and phosphoenol-pyruvate are effective inhibitors. Recent
studies of phosphoribulokinase inAlcaligenes eutrophus suggest that this enzyme may also be regulated via covalent modification. 相似文献
22.
A dialysis membrane technique was developed that enabled ultrastructural investigations of the interaction of nematode-trapping fungi and their nematode prey. It allowed the sectioning of individual traps that had been selected by light microscopy and was used in kinetic studies on trap formation, nematode capture, and subsequent nematode digestion. The method can also be used for enzyme cytochemical experiments. 相似文献
23.
Abstract The kinetic parameters of NH+ 4 -uptake in yeast cells were determined by a method that is based on the following changes in the external NH+ 4 concentration in cell suspensions by using NADH-dependent glutamate formation from NH+ 4 and 2-oxoglutarate. The kinetics of the observed NADH oxidation were analyzed by computer and enabled an estimation of V max and K m of the NH+ 4 -uptake system of the cells. 相似文献
24.
During growth of the facultative methylotroph Arthrobacter P1 on methylamine or ethylamine both substrates are metabolized initially in an identical fashion, via the respective aldehydes. The regulatory mechanisms governing the synthesis and activities of enzymes involved in amine and aldehyde utilization were studied in substrate transition experiments. Transfer of ethylamine-grown cells into a medium with methylamine resulted in immediate exeretion of low levels of formaldehyde (max. 0.5 mM) and formate. In the reverse experiment, transfer of methylaminegrown cells into a medium with ethylamine, excretion of much higher levels of acetaldehyde (max. 3.5 mM) occurred. These different levels of aldehyde accumulation were also observed in studies with mutants of Arthrobacter P1 blocked in the synthesis of hexulose phosphate synthase or acetaldehyde dehydrogenase. In wild type Arthrobacter P1, aldehyde production resulted in rapid induction of the synthesis of enzymes involved in their degradation but also in temporary inhibition of further amine utilization and growth. The latter aetivities only resumed at normal rates after the disappearance of the aldehydes from the cultures. Acetaldehyde utilization resulted in intermittent excretion of ethanol and acetate, whereas formaldehyde utilization resulted in further accumulation of formate.During growth of Arthrobacter P1 in the presence of methylamine accumulation of toxic levels of formaldehyde is prevented because of the rapid synthesis of hexulose phosphate synthase to high activities and, in transient state situations, by feedback inhibition of formaldehyde on the activities of the methylamine transport system and amine oxidase.Abbreviations DTNB
5,5-dithiobis-(2-nitrobenzoate)
- HPS
hexulosephosphate synthase
- MS
mineral salts
- RuMP
ribulose monophosphate 相似文献
25.
Development of crystalline peroxisomes in methanol-grown cells of the yeast Hansenula polymorpha and its relation to environmental conditions 总被引:13,自引:0,他引:13
The development of peroxisomes has been studied in cells of the yeast Hansenula polymorpha during growth on methanol in batch and chemostat cultures. During bud formation, new peroxisomes were generated by the separation of small peroxisomes from mature organelles in the mother cells. The number of peroxisomes migrating to the buds was dependent upon environmental conditions. Aging of cells was accompanied by an increase in size of the peroxisomes and a subsequent increase in their numbers per cell. Their ultimate shape and substructure as well as their number per cell was dependent upon the physiological state of the culture. The change in number and volume density of peroxisomes was related to the level of alcohol oxidase in the cells. Development of peroxisomes in cells of batch cultures was accompanied by an increase in size of the crystalline inclusions in the organelles; they had become completely crystalline when the cells were in the stationary phase. Peroxisomes in cells from methanol-limited chemostat cultures were completely crystalline, irrespective of growth rate. Results of biochemical and cytochemical experiments suggested that alcohol oxidase is a major component of the crystalline inclusions in the peroxisomes of methanol-grown Hansenula polymorpha. Possible mechanisms involved in the ultrastructural changes in peroxisomes during their development have been discussed.Abbreviations DAB
3,3-diaminobenzidine
- OD
optical density (663 nm) 相似文献
26.
Summary The numerical relationship between radiation-induced chromosome aberrations in humanG
0 lymphocytes and reproductive lethality of human bone-marrow lymphocytes is analysed within a large LET interval. The comparison is based upon the evaluation of the coefficient of the linear component of the corresponding dose-effect-relation for the frequency of cells without aberrations and for the frequency of surviving cells respectively. The good correlation between these coefficients over a large LET interval supports the hypothesis that structural chromosome aberrations and reproductive cell death both result from the same gross chromatin damage.This work was supported by the Bundesministerium des Innern, Bonn 相似文献
27.
A facultative methylotrophic bacterium was isolated from enrichment cultures containing methylamine as the sole carbon source.
It was tentatively identified as an Arthrobacter species. Extracts of cells grown on methylamine or ethylamine contained high levels of amine oxidase (E.C. 1.4.3.) activity.
Glucose- or choline-grown cells lacked this enzyme. Oxidation of primary amines by the enzyme resulted in the formation of
H2O2; as a consequence high levels of catalase were present in methylamine-and ethylamine-grown cells. The significance of catalase
in vivo was demonstrated by addition of 20 mM aminotriazole (a catalase inhibitor) to exponentially growing cells. This completely
blocked growth on methylamine whereas growth on glucose was hardly affected. Cytochemical studies showed that methylamine-dependent
H2O2 production mainly occurred on invaginations of the cytoplasmic membrane. Assimilation of formaldehyde which is generated
during methylamine oxidation was by the FBP variant of the RuMP cycle of formaldehyde fixation. The absence of NAD-dependent
formaldehyde and formate dehydrogenases indicated the operation of a non-linear oxidation sequence for formal-dehyde via hexulose
phosphate synthase. Enzyme profiles of the organism grown on various substrates suggested that the synthesis of amine oxidase,
catalase and the enzymes of the RuMP cycle is not under coordinate control. 相似文献
28.
Modification of flavin adenine dinucleotide in alcohol oxidase of the yeast Hansenula polymorpha. 总被引:1,自引:0,他引:1
Alcohol oxidase, a major peroxisomal protein of methanol-utilizing yeasts, may possess two different forms of flavin adenine dinucleotide, classical FAD and so-called modified FAD (mFAD). Conversion of FAD into mFAD was observed both in purified preparations of the enzyme and in cells grown in batch and continuous culture. The relative amount of mFAD in the enzyme varied from 5 to 95%, depending on the growth or storage conditions. The presence of mFAD led to a slight decrease in Vmax and a significant (about one order) decrease in the Km of alcohol oxidase with respect to methanol. The kinetics of modification measured in purified preparations of the enzyme obeyed first-order kinetics (k = 0.78 h-1). The modification process was strongly inhibited by methanol, formaldehyde or hydroxylamine. Modification observed in continuous culture under steady state conditions depended on the dilution rate and could also be described as a spontaneous first-order reaction (kapp = 0.27 h-1). FAD modification could only be detected in alcohol oxidase and not in other yeast peroxisomal flavoenzymes, such as D-amino acid oxidase from Candida boidinii. 相似文献
29.
Development of multipurpose peroxisomes in Candida boidinii grown in oleic acid-methanol limited continuous cultures.
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H R Waterham I Keizer-Gunnink J M Goodman W Harder M Veenhuis 《Journal of bacteriology》1992,174(12):4057-4063
We have studied the development and metabolic significance of peroxisomes in the yeast Candida boidinii following adaptation of the organism to cultivation conditions which require the simultaneous presence and activity of two independent peroxisome-mediated pathways for growth. After the addition of methanol to oleic acid-grown cells at late exponentional growth, a number of new small peroxisomes developed which, apart from the presence of beta-oxidation enzymes, were characterized by the presence of enzymes involved in methanol metabolism (alcohol oxidase and dihydroxyacetone synthase). The latter proteins, however, were absent in the larger organelles which were originally present in the oleic acid-grown cells prior to the addition of methanol and which contained only enzymes of the beta-oxidation pathway. Subsequent experiments on cells from continuous cultures grown on a mixture of oleic acid and methanol at steady-state conditions revealed that both the enzymes of the beta-oxidation pathway and those involved in methanol metabolism were found in one and the same compartment. Thus, under these conditions the cells contained peroxisomes which were concurrently involved in the metabolism of two different carbon sources simultaneously used for growth. Our results indicated that the heterogeneity in the peroxisomal population of a single cell, observed in the transient state following the addition of methanol, is only temporary and due to heterogeneity among these organelles with respect to their capacity to incorporate newly synthesized matrix proteins. 相似文献
30.
We have studied the use of yeast peroxisomal alcohol oxidase (AO) as a model protein for in vitro binding by GroEL. Dilution of denatured AO in neutral buffer leads to aggregation of the protein, which is prevented by the addition of GroEL. Formation of complexes between GroEL and denatured AO was demonstrated by a gel-shift assay using non-denaturing polyacrylamide gel electrophoresis, and quantified by laser-densitometry of the gels. In the presence of MgAMP-PNP or MgADP the affinity of GroEL for AO was enhanced. Under these conditions up to 70% of the purified GroEL formed a complex with this protein. Release was stimulated at room temperature by MgATP, and was further enhanced by addition of GroES. 相似文献