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Query: UNIPROT:P06889 (Mol)
 630,302 document(s) hit in 31,850,051 MEDLINE articles (0.00 seconds)

Phosphoribulokinase (PRK), an enzyme unique to the reductive pentose phosphate pathway of CO2 assimilation, exhibits distinctive contrasting properties when the proteins from eukaryotic and prokaryotic sources are compared. The eukaryotic PRKs are typically dimers of -39 kDa subunits while the prokaryotic PRKs are octamers of -32 kDa subunits. The enzymes from these two classes are regulated by different mechanisms. Thioredoxin of mediated thiol-disulfide exchange interconverts eukaryotic PRKs between reduced (active) and oxidized (inactive) forms. Allosteric effectors, including activator NADH and inhibitors AMP and phosphoenolpyruvate, regulate activity of prokaryotic PRK. The effector binding site has been identified in the high resolution structure recently elucidated for prokaryotic PRK and the7 apparatus for transmission of the allosteric stimulus has been identified. Additional contrasts between PRKs include marked differences in primary structure between eukaryotic and prokaryotic PRKs. Alignment of all available deduced PRK sequences indicates that less than 10% of the amino acid residues are invariant. In contrast to these differences, the mechanism for ribulose 1,5-biphosphate synthesis from ATP and ribulose 5-phosphate (Ru5P) appears to be the same for all PRKs. Consensus sequences associated with M++-ATP binding, identified in all PRK proteins, are closely juxtaposed to the residue proposed to function as general base catalyst. Sequence homology and mutagenesis approaches have suggested several residues that may potentially function in Ru5P binding. Not all of these proposed Ru5P binding residues are closely juxtaposed in the structure of unliganded PRK. Mechanistic approaches have been employed to investigate the amino acids which influence K(m Ru5P) and identify those amino acids most directly involved in Ru5P binding. PRK is one member of a family of phospho or sulfo transferase proteins which exhibit a nucleotide monophosphate kinase fold. Structure/function correlations elucidated for PRK suggest analogous assignments for other members of this family of proteins.
Adv Enzymol Relat Areas Mol Biol 2000
PMID:Phosphoribulokinase: current perspectives on the structure/function basis for regulation and catalysis. 1080 May 94

We have examined adenosine (Ado) suppression of FSH-induced germinal vesicle breakdown (GVB) and its relationship to purine de novo synthesis. Oocyte-cumulus cell complexes (OCC) from PMSG-primed, immature mice were cultured 17-18 hr in medium containing 4 mM hypoxanthine (HX) or 300 microM dibutyryl cAMP (dbcAMP) to maintain meiotic arrest, and FSH was added to stimulate meiotic maturation. In the absence of FSH, Ado (1-250 microM) had no effect in dbcAMP-arrested oocytes but dose-dependently suppressed maturation in HX-treated oocytes. FSH-induced maturation was prevented by Ado, though more effectively in dbcAMP-supplemented cultures. Ado affected the magnitude, but not the kinetics pattern, of the response to FSH. Inosine also blocked meiotic induction, but only in dbcAMP-arrested oocytes. Purine de novo synthesis was nearly doubled in OCC by FSH treatment, and this response was completely prevented by Ado. FSH had no effect on HX salvage, although Ado reduced this activity by 98%. Inosine effects on metabolism were intermediate between the control and Ado groups. Experiments with radiolabeled energy substrates showed that Ado suppressed FSH activation of the pentose phosphate pathway but did not prevent significant activation of glycolysis or oxidation of pyruvate. Finally, in cultured follicles from primed mice, hCG-induced maturation was blocked by Ado as effectively as by the purine de novo synthesis inhibitor, azaserine. It is concluded that Ado has an inhibitory action on hormone-induced maturation that is due, at least in part, to suppression of glucose metabolism, leading to compromised purine de novo synthesis.
Mol Reprod Dev 2000 Jun
PMID:Adenosine blocks hormone-induced meiotic maturation by suppressing purine de novo synthesis. 1081 49

In response to overfeeding, the Landes goose develops a fatty liver that is twice as large as that of the Poland goose, despite similar food intake. The role of hepatic lipogenesis in the genetic susceptibility to fatty liver was assessed in male overfed geese of the two breeds. For a similar hepatic protein content, total activities of malic enzyme, glucose-6-phosphate dehydrogenase, acetyl-Coa-carboxylase and fatty acid synthase, and specific activity and mRNA level of malic enzyme were about two-fold higher in the Landes goose. In the Poland goose, the weight of the fatty liver was correlated positively with the specific activity of ME and the VLDL concentration, which was not the case in the Landes breed. These results show that: (1) hepatic lipogenesis remains very active until the end of the overfeeding period; (2) the pentose-phosphate pathway may function in birds, contrary to what is assumed usually; (3) the level of hepatic lipogenesis is a major factor in the susceptibility to hepatic steatosis in different breeds of geese; and (4) ME activity may be a limiting factor of lipid synthesis in the less susceptible Poland breed.
Comp Biochem Physiol B Biochem Mol Biol 2000 May
PMID:Role of hepatic lipogenesis in the susceptibility to fatty liver in the goose (Anser anser). 1082 67

The cbb(I) and cbb(II) operons encode structural genes which are important for carbon dioxide fixation via the Calvin-Benson-Bassham reductive pentose phosphate pathway in Rhodobacter capsulatus. Each operon is regulated by cognate LysR-type transcriptional activators, CbbR(I) and CbbR(II), with the product of the cbbR(I) gene, CbbR(I), able to control its own transcription under some growth conditions. Furthermore, CbbR(I) may at least partially regulate the cbb(II) operon, with significant, yet regulated transcription of the cbb(II) operon occurring in the absence of any CbbR. These results suggested the importance of additional regulators. Thus, in addition to the rather specific control exerted by CbbR, a more globally significant regulatory system, the RegA-RegB (PrrA-PrrB) two-component system, was found to contribute to transcriptional regulation of each cbb operon. The regA and regB mutant strains were found to contain constitutive levels of form I and form II RubisCO, the major proteins encoded by the cbb(I) and cbb(II) operons, respectively. In addition, DNaseI footprint analyses indicated that RegA*, a constitutively active mutant form of RegA, binds specifically to cbb(I) and cbb(II) promoter-operator regions. CbbR(I), CbbR(II), and RegA binding loci were localized relative to transcription start sites, leading to a coherent picture of how each of these regulators interacts with specific promoter-operator sequences of the cbb operons.
J Mol Biol 2000 Jul 28
PMID:Multiple regulators and their interactions in vivo and in vitro with the cbb regulons of Rhodobacter capsulatus. 1090 56

Unlike its predecessors B. subtilis rosR and 41, riboflavin producing B. subtilis 24 strain does not utilize pentose and gluconate and poorly assimilates glucose. Simultaneous addition of glutamic and shikimic acid restored its capacity to grow and produce riboflavin in medium with pentose and gluconate. This strain lacks the activity of transketolase, the key enzyme of the pentose phosphate cycle, and possesses normal ribulose-5-phosphate-epimerase and glucose phosphate isomerase activities. Like enterobacteria, B. subtilis has two different transport systems for glucose and mannose. The data are discussed from the viewpoint of increasing riboflavin production by transketolase mutants. Probable consequences of cell wall and cytoplasmatic membrane damage in B. subtilis with this mutation are discussed.
Mol Gen Mikrobiol Virusol 2000
PMID:[Transketolase mutation in riboflavin-synthesizing strains of Bacillus subtilis]. 1097 72

We perform a comprehensive genome analysis on two spirochetes, T. pallidum and B. burgdorferi. First, we focus on the occurrence of protein structures in these organisms. We find that there are only a few spirochete-specific folds, relative to those in other types of bacteria. The most common fold, by far, in the spirochetes is the P-loop NTP hydrolase, followed by the TIM barrel. These folds also happen to be amongst the most multifunctional of the known folds. We also survey the membrane-protein structures in T. pallidum and find a notable large family with twelve transmembrane (TM) helices, reflecting the prevalence of 12-TM transporters in bacteria. Then we move to analysis of the metabolic pathways and overall metabolism in the spirochetes, using the metabolic-flux-balancing method. We find that the lipid biosynthesis pathway is absent from the spirochetes. This strongly limits the degree to which these organisms can metabolize NADPH. In turn, we find that the spirochetes distribute flux disproportionately through the glycolytic pathway instead of the NADPH-providing pentose phosphate pathway. Further information is available at http://bioinfo.mbb.yale.edu
J Mol Microbiol Biotechnol 2000 Oct
PMID:Genome analyses of spirochetes: a study of the protein structures, functions and metabolic pathways in Treponema pallidum and Borrelia burgdorferi. 1107 10

Insulin or agents that can mimic its action (insulin-mimetics) are necessary to promote the entry of glucose into tissues where the glucose can either be converted into energy or stored for later use. In recent years, selenium has been shown to mediate a number of insulin-like actions both in vivo and in vitro. These insulin-like actions include stimulating glucose uptake and regulating metabolic processes such as glycolysis, gluconeogenesis, fatty acid synthesis and the pentose phosphate pathway. The mechanism by which selenium is capable of mimicking insulin is not clear; however, reports indicate that selenium does activate key proteins involved in the insulin-signal cascade. Various proteins in the insulin-signal cascade have been shown to be necessary for different insulin-regulated events, and presumably data will be forthcoming soon that illustrate this similarly for selenium. This review compares the action of selenium to that of insulin and discusses the available evidence in support of selenium as an insulin-mimetic.
Cell Mol Life Sci 2000 Dec
PMID:Selenium: an insulin-mimetic. 1121 14

The structure and function of the pseudobranch has long interested scientists, but its overall role has remained a mystery. Previous studies have attributed respiratory, endocrine, osmoregulatory and sensory roles to the pseudobranch, and the present review concentrates on new findings. Perfusion experiments on the pseudobranch of the rainbow trout (Oncorhynchus mykiss) using both erythrocyte suspensions and Ringer solution have shown that this organ is able to generate values for the respiratory quotient (RQ) greater than 1.0. The release of carbon dioxide into the perfusate was found to be largely independent of flow between perfusion rates of 120-190 microl/min and could be inhibited by acetazolamide (10(-5) M), indicating a role for carbonic anhydrase. Noradrenaline (10(-5) M) had no effect on oxygen consumption or carbon dioxide release of the pseudobranch. The rate of carbon dioxide release was also dependent on the pH of the pre-pseudobranch perfusate, carbon dioxide release being reduced at lower perfusate pH values. Based on the glucose balance of the isolated saline-perfused rainbow trout pseudobranch and on the enzyme profiles for the rainbow trout, cod, swordfish and deep-water grenadier pseudobranch, it is suggested that the pentose phosphate shunt might be a source of carbon dioxide, yielding the high RQ values found for this organ. Most evidence now available indicates that the pseudobranch is integrally linked with the choroid rete and the supply of oxygen to the retina of the fish eye.
Comp Biochem Physiol A Mol Integr Physiol 1998 Jan
PMID:Physiology and biochemistry of the pseudobranch: an unanswered question? 1125 20

Bacterial cytidine monophosphate (CMP) kinases are characterised by an insert enlarging their CMP binding domain, and by their particular substrate specificity. Thus, both CMP and 2'-deoxy-CMP (dCMP) are good phosphate acceptors for the CMP kinase from Escherichia coli (E. coli CMPK), whereas eukaryotic UMP/CMP kinases phosphorylate the deoxynucleotides with very low efficiency. Four crystal structures of E. coli CMPK complexed with nucleoside monophosphates differing in their sugar moiety were solved. Both structures with CMP or dCMP show interactions with the pentose that were not described so far. These interactions are lost with the poorer substrates AraCMP and 2',3'-dideoxy-CMP. Comparison of all four structures shows that the pentose hydroxyls are involved in ligand-induced movements of enzyme domains. It also gives a structural basis of the mechanism by which either ribose or deoxyribose can be accommodated. In parallel, for the four nucleotides the kinetic results of the wild-type enzyme and of three structure-based variants are presented. The phosphorylation rate is significantly decreased when either of the two pentose interacting residues is mutated. One of these is an arginine that is highly conserved in all known nucleoside monophosphate kinases. In contrast, the other residue, Asp185, is typical of bacterial CMP kinases. It interacts with Ser101, the only residue conserved in all CMP binding domain inserts. Mutating Ser101 reduces CMP phosphorylation only moderately, but dramatically reduces dCMP phosphorylation. This is the first experimental evidence of a catalytic role involving the characteristic insert of bacterial CMP kinases. Furthermore, this role concerns only dCMP phosphorylation, a feature of this family of enzymes.
J Mol Biol 2002 Feb 01
PMID:Sugar specificity of bacterial CMP kinases as revealed by crystal structures and mutagenesis of Escherichia coli enzyme. 1182 79

Studies of nectar sugar composition in the Proteaceae, an ancient southern hemisphere plant family, have demonstrated that xylose comprises up to 39% of nectar sugar in two genera, Protea and Faurea, and may therefore represent a substantial fraction of the energy available to pollinators of these plants. Although insect and bird pollinators of Protea species are averse to xylose, mice (Aethomys namaquensis) will drink pure xylose, which is metabolized either by gut bacteria or by the mouse tissues. In the form of xylan polymers, the pentose sugar D-xylose is a structural component of plant cell walls, and there is considerable biotechnological interest in xylose fermentation. Bacteria and yeasts convert D-xylose to D-xylulose and thence via the pentose phosphate pathway to fructose-6-phosphate, which is either oxidized or fermented to ethanol. Gut symbionts of rodent pollinators may be analogous to ruminal xylose-metabolizing bacteria. The presence of xylose in Protea and Faurea nectar remains puzzling in view of pollinator aversions: even for rodent pollinators, it is the least preferred nectar sugar. In the generalized pollination systems of the Proteaceae, a coevolutionary explanation for nectar xylose as an attractant for mammalian pollinators is probably less likely than one involving plant physiology, with xylose in phloem sap being secreted passively into the nectar.
Comp Biochem Physiol B Biochem Mol Biol 2002 Apr
PMID:Xylose as a nectar sugar: from biochemistry to ecology. 1192 77


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