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761.
Levels of metabolic intermediates and end products in F. hepatica after 24 and 48 h in Hédon-Fleig salt solution with added glucose were compared with levels obtained immediately on removal from the host. Glycogen levels dropped initially, probably due to the expulsion of eggs; thereafter they remained constant. Internal glucose concentrations increased as the parasites equilibrated with the medium. Other changes in internal pool sizes were consistent with regulation to the in vitro conditions. ATP levels increased; ATP/ADP ratios were maintained. Comparisons of mass action ratios and equilibrium constants suggest that hexokinase, pyruvate kinase and phosphofructokinase are regulatory. Output of excretory products approached linearity; from the calculated regressions the proportions of lactate, acetate and propionate were 1: 2: 4. The implications for metabolic regulation in F. hepatica are briefly discussed, and it is concluded that, for at least 48 h in vitro, energy metabolism is not adversely affected. 相似文献
762.
763.
Intact plastids from cell suspensions of Vitis vinifera L. cv. Muscat de Frontignan, free of detectable contamination by other particles as judged by the distribution of organelle-specific marker enzymes and by electron microscopy, exhibit geranyl-diphosphate synthase activity (EC 2.5.1.1). This synthase activity remains stable after tryptic digestion of unlysed organelles and is enhanced by plastid disruption. We conclude that the enzyme is located within the organelle. The possibility of an isopentenyl diphosphate/dimethylallyl diphosphate translocating system which would play a major role in the regulation of monoterpene metabolism is discussed. 相似文献
764.
765.
Farnesyl diphosphate synthase (FPPS) is a key enzyme in isoprenoid biosynthesis, it catalyzes the head-to-tail condensation of dimethylallyl diphosphate (DMAPP) with two molecules of isopentenyl diphosphate (IPP) to generate farnesyl diphosphate (FPP), a precursor of juvenile hormone (JH). In this study, we functionally characterized an Aedes aegypti FPPS (AaFPPS) expressed in the corpora allata. AaFPPS is the only FPPS gene present in the genome of the yellow fever mosquito, it encodes a 49.6 kDa protein exhibiting all the characteristic conserved sequence domains on prenyltransferases. AaFPPS displays its activity in the presence of metal cofactors; and the product condensation is dependent of the divalent cation. Mg2+ ions lead to the production of FPP, while the presence of Co2+ ions lead to geranyl diphosphate (GPP) production. In the presence of Mg2+ the AaFPPS affinity for allylic substrates is GPP > DMAPP > IPP. These results suggest that AaFPPS displays “catalytic promiscuity”, changing the type and ratio of products released (GPP or FPP) depending on allylic substrate concentrations and the presence of different metal cofactors. This metal ion-dependent regulatory mechanism allows a single enzyme to selectively control the metabolites it produces, thus potentially altering the flow of carbon into separate metabolic pathways. 相似文献
766.
Functional Flexibility of the Transketolase Molecule 总被引:4,自引:0,他引:4
G. A. Kochetov 《Biochemistry. Biokhimii?a》2001,66(10):1077-1085
Transketolase is the simplest representative of the thiamine diphosphate-dependent enzymes. It was the first of these enzymes for which X-ray analysis was performed. Based on the data of X-ray studies and using the mutagenesis technique, the nature of functional groups of the enzyme involved in the interaction with substrates and cofactors and in the coenzyme activation was defined. Thus, considerable achievements have been made in studying the structure of transketolase. However, there is relatively little information on the conformational flexibility of the enzyme molecule while it is functioning, i.e., during its interaction with cofactors and substrates and in the course of intermediate product formation. This review summarizes mainly the results obtained in the author's group, as well as those rare data on this subject that could be found in literature. 相似文献