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31.
Substituted enzyme (or ping-pong) mechanisms usually involve enzymes that exist in two forms that alternate during the catalytic
reaction. A method is described here for determining the position of the equilibrium of a half reaction in a ping-pong enzyme
mechanism that is based on the kinetics of the burst reaction which occurs upon addition of reactants that recycle the enzyme
from one form to another. The theoretical basis for the analysis is developed, and the method is applied to the half reaction
of the aldimine form of aspartate transaminase with difluoro-oxaloacetate.
Special issue dedicated to Herman Bachelard 相似文献
32.
Gordon C. K. Roberts 《Neurochemical research》1996,21(9):1117-1124
NMR spectroscopy has proved to be a valuable tool in the study of the interactions between enzymes and their substrates. The
kinds of structural and dynamic information which can be obtained are illustrated by studies of three enzymes involved in
drug metabolism. Cytochromes P450 play a crucial role in metabolism of a wide range of exogenous chemicals. NMR has been used to measure distances from the
haem iron of the cytochrome to protons of the bound substrate, leading to detailed structural models for the enzyme-substrate
complexes. The other two enzymes, chloramphenicol acetyltransferase and β-lactamase, are responsible for bacterial resistance
to specific antibiotics. In chloramphenicol acetyltransferase, NMR has been used to determine the conformation of coenzyme
A bound to the enzyme, while in the case of β-lactamase the pK of a specific lysine residue at the active site has been determined,
providing valuable information on the catalytic mechanism.
Special issue dedicated to Dr. Herman Bachelard. 相似文献
33.
This study examined the localized action of neuropeptide Y (NPY) on monoamine transmitter activity in the hypothalamus of
the unrestrained rat as this peptide induced hypothermia, spontaneous feeding or both responses simultaneously. A guide tube
was implanted in the anterior hypothalamic pre-optic area (AH/POA) of Sprague-Dawley rats. Then either control CSF vehicle
or NPY in a dose of either 100 ng/μl or 250 ng/μl was perfused by push-pull cannulae in this structure in the fully sated,
normothermic rat. Successive perfusions were carried out at a rate of 20 μl/min for 6.0 min with an interval of 6.0 min elapsing
between each. Samples of perfusate were assayed by HPLC for their levels of dopamine (DA), norepinephrine (NE), serotonin
(5-HT) and their respective metabolites. Whereas control CSF was without effect on body temperature (Tb) or feeding, repeated perfusions of NPY over 3.0 hr caused dose—dependent eating from 4 to 39 g of food, hypothermia of 0.9
to 2.3°C or both responses concurrently. As the rats consumed 11–39 g of food, the efflux of NE, MHPG, DOPAC and 5-HT was
enhanced significantly, whereas during the fall in Tb the efflux of NE, DOPAC and 5-HIAA from the AH/POA increased. When the Tb of the rat declined simultaneously with eating behavior, the levels in perfusate of DOPAC and HVA increased significantly
while MHPG declined. During perfusion of the AH/POA with NPY the turnover of NE declined while DA and 5-HT turnover increased
during hypothermia alone or when accompanied by feeding. These results demonstrate that the sustained elevation in NPY within
the AH/POA causes a selective alteration in the activity of the neurotransmitters implicated in thermoregulation, satiety
and hunger. These findings suggest that both DA and NE comprise intermediary factors facilitating the action of NPY on neurons
involved in thermoregulatory and ingestive processes. The local activity of NPY on hypothalamic neurons apparently shifts
the functional balance of serotonergic and catecholaminergic neurons now thought to play a primary role in the control of
energy metabolism and caloric intake. 相似文献
34.
Influx and efflux of inorganic carbon in Synechococcus UTEX625 总被引:1,自引:0,他引:1
The CO2 and HCO3? fluxes in air-grown cells of Synechococcus UTEX 625 al pH 8-0 were measured during dark to light and light to dark transitions using a mass spectrometer and sampling of the reaction medium. The kinetic parameters for initial uptake of CO2 and HCO3? were determined during the initial period of illumination. The development of the internal Ci pool was followed up to steady-state photosynthesis, which occurred when the size of the internal inorganic carbon pool remained apparently constant for a limited period. The experimental procedure confirmed that only CO2 transport occurred with 100mmolm?3 Na+ and that both CO2 and HCO?3 transport occurred with 25molm?3 Na+. The K1/2 values of initial CO2 and HCO3 uptake were 0.7 and 17.2 mmolm?3respectively and agreed closely with the K1/2 values of net CO2 and HCO3? transport during steady-state photosynthesis, which were 0.66 and 17.1 mmolm?3 respectively. Maximum rates of CO2and HCO3? transport were 423 and 219mmolh?1 g?1 Chl. Maximum CO2 efflux observed upon darkening was 118mmolh?1 g?1 Chl. A permeability coefficient of the cell for CO2 of 3 × 10?8 m s?1 was determined from the dark CO2 efflux assuming an internal pH of 7.2 in the dark. Following the initial CO2 uptake in the light, the extracellular [CO2] steadily declined when only CO2 transport was allowed, but an increase in the extracellular [CO2] when HCO3? transport was allowed to proceed suggested that an enhanced CO2 efflux occurred as a result of the larger size of the intracellular Ci pool. 相似文献
35.
The leakage of various inorganic carbon species from air-grown cells of Synechococcus UTEX 625 was investigated after a light to dark transition or during a light period using a mass spectrometer under a wide variety of experimental conditions. Total inorganic carbon efflux and CO2 efflux during the initial period of darkness were measured with or without carbonic anhydrase in the reaction medium respectively. The HCO3? efflux after a light to dark transition was estimated by difference. Carbon dioxide efflux in the light was measured by inhibiting CO2 transport with either Na2S or COS3 or quenching the 13C inorganic carbon transport by the addition of 12C inorganic carbon in excess. In cells in which CO2 fixation was inhibited, when only the HCO3? transport system was fully operative, CO2 effluxed continuously during the light period at a rate equal to about 25% of that in darkness. When only the CO2 transport system was operative, HCO3? effluxed during the light period. The difference between the light and dark efflux rates was consistent with a 0.6 unit decrease in the intracellular pH upon darkening the cells. The permeabilities of the cell for CO2 (2.94 ± 0.14 ± 10?8ms?1; mean ± SE, n=137) and HCO3? (1.4–1.7 ± 10?9 ms?1) were calculated. 相似文献
36.
Hydroxynitrile lyases: Functions and properties 总被引:6,自引:0,他引:6
Plant hydroxynitrile lyases (Hnl) have attracted the attention of bioorganic scientists for more than 90 years. However, the most important increase in knowledge of this class of enzymes has only arisen in the recent decade. The industrial application of these enzymes as biocatalysts for the synthesis of enantiomerically pure α-cyanohydrins may be responsible for the growing interest in this area.
The Hnls are involved in the catabolic degradation of cyanogenic glycosides, releasing HCN which serves as defense agent against herbivores and microbial attack, or as a nitrogen source. Hydroxynitrile lyases from various plant families appear to represent a new example of enzymes that originated from the convergent evolution of different precursor proteins. The enzymes have been classified into non-FAD- and FAD-containing proteins. FAD-containing enzymes have been isolated exclusively from the Rosaceae, whereas the FAD-independent Hnls, which are more heterogenous in structure, have been characterized from various plant families (Poaceae, Euphorbiaceae, Linaceae, Olacaceae. Filitaceae). The aim of this review is to present a general survey of the natural function and localization of this class of enzymes and a comprehensive summary of the biochemical and genetic data of the isolated proteins. 相似文献
The Hnls are involved in the catabolic degradation of cyanogenic glycosides, releasing HCN which serves as defense agent against herbivores and microbial attack, or as a nitrogen source. Hydroxynitrile lyases from various plant families appear to represent a new example of enzymes that originated from the convergent evolution of different precursor proteins. The enzymes have been classified into non-FAD- and FAD-containing proteins. FAD-containing enzymes have been isolated exclusively from the Rosaceae, whereas the FAD-independent Hnls, which are more heterogenous in structure, have been characterized from various plant families (Poaceae, Euphorbiaceae, Linaceae, Olacaceae. Filitaceae). The aim of this review is to present a general survey of the natural function and localization of this class of enzymes and a comprehensive summary of the biochemical and genetic data of the isolated proteins. 相似文献
37.
The lactic yeast Kluyveromyces marxianus var.marxianus (formerly K. fragilis) autolyzates at faster rate than Saccharomyces cerevisiae. During K. marxianus autolysis, quite similar release kinetics were observed for intracellular space markers (potassium ions, nucleotides), cell-wall components (polysaccharides, N-acetyl-D-Glucosamine) and non specific products (amino nitrogen). By Scanning Electronic Microscopy examination, no cell burst was observed, but a variation in cell shape (from ellipsoidal to cylindrical), as well as a 43% decrease in the internal volume were observed. The mechanism proposed for S. cerevisiae autolysis appeared also likely for K. marxianus.Abbreviations NacGlc
N-acetyl-D-glucosamine
- x
total biomass (dry cellular weight) concentration 相似文献
38.
Three routes were investigated for the conversion of d-glucose into the title compound. In the first approach, reduction of the 5,6-thürane ring of 5,6-dideoxy-5,6-epithio-1,2-O-isopropylidene α-d-glucofuranose (17) as well as that of its 3-O-allyl derivative (13) with lithium aluminium hydride was investigated; 17 afforded the corresponding 6-deoxy derivative besides di-, tri-, and poly-mers, whereas only polymers were formed from 13. In the second approach, the oxirane ring of was reduced by sodium borohydride and the resulting 6-deoxy derivative was converted into the 5-thiobenzoate; the corresponding hex-4-enofuranose was formed as a byproduct. In the third approach partial mesylation of methyl 5-thio-α-d-glucopyranoside was attempted, but the 6-mesylate 27 could be isolated only in modest yield (28%) together with rearranged 2,5-thioanhydromannofuranoside derivatives. The mechanism of this rearrangement is discussed in detail. The 6-mesylate 27 was converted via the 6-iodo derivative into the title compound. 相似文献
39.
Na2SO3对热-DTT活化的游离CF1及类囊体膜上CF1-ATPase活力均有显著的促进作用,NaHCO3亦有明显的促进作用。Na2SO3和NaHCO3的促进作用与它们解除Mg2+的抑制作用有关。从NaHCO3和Na2SO3及它们与Mg2+之间的竞争性关系,表明三者是结合在酶的同一部位上。Na2SO3可明显降低热-DTT活化的游禹CF1-ATPase催化反应的活化能,这可能与促进产物ADP的释放有关。 相似文献
40.
Kathleen Miller 《Biology & philosophy》1994,9(1):99-103
In Can Abstractions be Causes, David Johnson defends the view that abstractions can have causal force. He offers as his own example of natural kinds ecological niches, arguing that the causal force of these niches in nature is akin to the force of Aristotelian final causes. He concludes that, rooted as it is in seventeenth century mechanism, the currently-accepted model of causality which recognises only efficient causes is inadequate to the needs of contemporary science. In Natural Kinds and Ecological Niches — Response to Johnson's Paper, Melinda Hogan offers a critique of Johnson's paper which, by begging the question in favor of the very sort of causality Johnson seeks to supplement, misses the epistemological implications of his idea. In this paper I will attempt to clarify and defend Johnson's position, pointing out some of its implications for the epistemology of science in general. 相似文献