SCAP与胆固醇水平的调节机制

SREBP裂解激活蛋白（SREBP cleavage-activating protein,SCAP）是哺乳动物脂质合成和摄入的中心调节因素。在胆固醇代谢的反馈调节系统中,SCAP与膜结合转录因子胆固醇调节元件结合蛋白（sterol regulatory element binding proteins,SREBPs）等调节因子,共同控制一系列酶编码基因的转录过程,包括胆固醇和脂肪酸生物合成过程中所需的酶。作者介绍了SREBP的结合、分类和功能及其二步蛋白水解释放;SCAP的结合和作用机制及其在胆固醇水平调节中的作用;SCAP基因缺陷型及其胆固醇水平异常,并提出了尚待解决的问题,对SCAP的研究是胆固醇水平调节领域的一个新课题。     

Glucagon regulation of protein phosphorylation. Identification of acetyl coenzyme A carboxylase as a substrate.

A hormonally induced change in the covalent phosphorylation state of several enzymes is generally regarded as an important mechanism for hormonal modulation of enzyme activity. We have previously demonstrated that epinephrine stimulates the phosphorylation of a peptide of Mr = 220,000 in adipocytes. Incubation of 32P-labeled cytosolic proteins from adipocytes and hepatocytes with antisera raised against homogeneous chicken and rat liver acetyl coenzyme A carboxylase results in the specific and complete precipitation of the same phosphopeptide. No other major phosphopeptide is specifically precipitated. In hepatocytes, glucagon stimulates the incorporation of 32P into this peptide associated with an inhibition of enzyme activity. These data, coupled with previous studies in adipocytes, suggest that cyclic AMP-dependent protein phosphorylation plays a major role in the regulation of acetyl-CoA carboxylase activity and of fatty acid biosynthesis in adipose tissue and liver.     

Nitrate activation of cytosolic protein kinases diverts photosynthetic carbon from sucrose to amino Acid biosynthesis: basis for a new concept

The regulation of carbon partitioning between carbohydrates (principally sucrose) and amino acids has been only poorly characterized in higher plants. The hypothesis that the pathway of sucrose and amino acid biosynthesis compete for carbon skeletons and energy is widely accepted. In this review, we suggest a mechanism involving the regulation of cytosolic protein kinases whereby the flow of carbon is regulated at the level of partitioning between the pathways of carbohydrate and nitrogen metabolism via the covalent modulation of component enzymes. The addition of nitrate to wheat seedlings (Triticum aestivum) grown in the absence of exogenous nitrogen has a dramatic, if transient, impact on sucrose formation and on the activities of sucrose phosphate synthase (which is inactivated) and phosphoenolpyruvate carboxylase (which is activated). The activities of these two enzymes are modulated by protein phosphorylation in response to the addition of nitrate, but they respond in an inverse fashion. Sucrose phosphate synthase in inactivated and phosphoenolpyruvate carboxylase is activated. Nitrate functions as a signal metabolite activating the cytosolic protein kinase, thereby modulating the activities of at least two of the key enzymes in assimilate partitioning and redirecting the flow of carbon away from sucrose biosynthesis toward amino acid synthesis.     

基于转录组与翻译组的海洋食烷菌(Alcanivorax)烷烃代谢调控研究

【目的】食烷菌是海洋烃类降解优势菌,其烷烃代谢调控机制有待深入研究。本研究拟从食烷菌转录和翻译水平上认识烷烃降解的调控过程。【方法】分别以乙酸和正十六烷(C16)为唯一碳源与能源,获取柴油食烷菌(Alcanivorax dieselolei) B5菌株的转录组和翻译组数据,并整合数据计算得到该菌在2种碳源培养条件下基因的翻译效率。采用基因本体论(gene ontology, GO)和京都基因和基因组百科全书(Kyoto encyclopedia of genes and genomes, KEGG)对差异翻译和翻译效率基因进行功能和代谢通路注释。【结果】当以C16为唯一碳源与能源时,B5菌株烷烃代谢途径的关键基因在转录与翻译水平均大量提升,包括烷烃单加氧酶、细胞色素P450氧化酶、醇脱氢酶和醛脱氢酶等。KEGG富集结果表明,翻译水平显著上调基因参与了肽聚糖生物合成、脂肪酸降解、氯代烷烃降解、氧化磷酸化和生物膜形成等通路;翻译效率差异基因主要富集在铁载体非核糖体肽的生物合成、氧化磷酸化和不饱和脂肪酸的生物合成等途径。通过转录组和翻译组学的联合分析显示,为了适应烷烃氧化,B5有效地协调了转...     

Regulation of fatty acid composition in Escherichia coli: a proposed common mechanism for changes induced by ethanol, chaotropic agents, and a reduction of growth temperature.

Growth of Escherichia coli in the presence of ethanol and chaotropic salts resulted in the synthesis of lipids containing elevated levels of unsaturated fatty acids analogous to the effect of a reduction in growth temperature. Both ethanol and chaotropic agents acted at the level of fatty acid biosynthesis and altered lipid composition by decreasing the proportion of saturated acyl chains available for the synthesis of phospholipids. A reduction in temperature causes similar effects on fatty acid biosynthesis in vivo and in vitro. Ethanol, chaotropic salts, and a decrease in temperature all weaken hydrophobic interactions. Antichaotropic salts antagonized and effects of these treatments on fatty acid synthesis in vitro. These results are consistent with a common mechanism for the effects of chaotropic agents, temperature, and ethanol on fatty acid synthesis. The biosynthesis of saturated and unsaturated acyl chains may be regulated by the strength of hydrophobic interactions. Changes in the strength of hydrophobic interactions could alter enzyme structure, substrate structure, or the equilibrium between the soluble enzymes of fatty acid synthesis and their respective acyl carrier protein substrates.     

Phospholipase A2 regulation of lipid droplet formation

The classical regard of lipid droplets as mere static energy-storage organelles has evolved dramatically. Nowadays these organelles are known to participate in key processes of cell homeostasis, and their abnormal regulation is linked to several disorders including metabolic diseases (diabetes, obesity, atherosclerosis or hepatic steatosis), inflammatory responses in leukocytes, cancer development and neurodegenerative diseases. Hence, the importance of unraveling the cell mechanisms controlling lipid droplet biosynthesis, homeostasis and degradation seems evident. Phospholipase A₂s, a family of enzymes whose common feature is to hydrolyze the fatty acid present at the sn-2 position of phospholipids, play pivotal roles in cell signaling and inflammation. These enzymes have recently emerged as key regulators of lipid droplet homeostasis, regulating their formation at different levels. This review summarizes recent results on the roles that various phospholipase A₂ forms play in the regulation of lipid droplet biogenesis under different conditions. These roles expand the already wide range of functions that these enzymes play in cell physiology and pathophysiology.     

The Mycobacterium tuberculosis FAS-II condensing enzymes: their role in mycolic acid biosynthesis, acid-fastness, pathogenesis and in future drug development

Mycolic acids are very long-chain fatty acids representing essential components of the mycobacterial cell wall. Considering their importance, characterization of key enzymes participating in mycolic acid biosynthesis not only allows an understanding of their role in the physiology of mycobacteria, but also might lead to the identification of new drug targets. Mycolates are synthesized by at least two discrete elongation systems, the type I and type II fatty acid synthases (FAS-I and FAS-II respectively). Among the FAS-II components, the condensing enzymes that catalyse the formation of carbon-carbon bonds have received considerable interest. Four condensases participate in initiation (mtFabH), elongation (KasA and KasB) and termination (Pks13) steps, leading to full-length mycolates. We present the recent biochemical and structural data for these important enzymes. Special emphasis is given to their role in growth, intracellular survival, biofilm formation, as well as in the physiopathology of tuberculosis. Recent studies demonstrated that phosphorylation of these enzymes by mycobacterial kinases affects their activities. We propose here a model in which kinases that sense environmental changes can phosphorylate the condensing enzymes, thus representing a novel mechanism of regulating mycolic acid biosynthesis. Finally, we discuss the attractiveness of these enzymes as valid targets for future antituberculosis drug development.     

Differential regulation by fatty acids of protein histidine phosphorylation in rat pancreatic islets

Long-chain fatty acids (e.g. arachidonic acid) have been implicated in physiological control of insulin secretion. We previously reported histidine phosphorylation of at least two islet proteins (e.g., NDP kinase and the beta subunit of trimeric G-proteins), and suggested that such a signalling step may have regulatory roles in beta cell signal transduction, specifically at the level of G-protein activation. Since our earlier findings also indicated potential regulation by long-chain fatty acids of islet G-proteins, we undertook the current study to verify putative regulation, by fatty acids, of protein histidine phosphorylation of NDP kinase and Gbeta subunit in normal rat islets. The phosphoenzyme formation of NDP kinase was stimulated by various fatty acids in the following rank order: linoleic acid > arachidonic acid > oleic acid > palmitic acid = stearic acid = control. Furthermore, the catalytic activity of NDP kinase was stimulated by these fatty acids in the rank order of: oleic acid > arachidonic acid > linoleic acid > palmitic acid = stearic acid = control. Arachidonic acid methyl ester, an inactive analog of arachidonic acid, did not significantly affect either the phosphoenzyme formation or the catalytic activity of NDP kinase. Interestingly, arachidonic acid exerted dual effects on the histidine phosphorylation of beta subunit; it significantly stimulated the phosphorylation at 33 microM beyond which it was inhibitory. Together, these findings identify additional loci (e.g., NDP kinase and Gbeta subunit) at which unsaturated, but not saturated, fatty acids could exert their intracellular effects leading to exocytotic secretion of insulin.     

Estrogen regulation of hepatic 3-hydroxy-3-methylglutaryl coenzyme A reductase and acetyl-CoA carboxylase in xenopus laevis

Administration of estradiol-17 beta to male Xenopus laevis evokes the proliferation of the endoplasmic reticulum and the Golgi apparatus and the synthesis and secretion by the liver of massive amounts of the egg yolk precursor phospholipoglycoprotein, vitellogenin. We have investigated the effects of estrogen on three key regulatory enzymes in lipid biosynthesis, 3-hydroxy-3-methylglutaryl (HMG)-CoA reductase, the major regulatory enzyme in cholesterol and isoprenoid synthesis, and acetyl-CoA carboxylase and fatty acid synthetase, which regulate fatty acid biosynthesis. HMG-CoA reductase activity and cholesterol synthesis increase in parallel following estrogen administration. Reductase activity in estrogen stimulated Xenopus liver cells peaks at 40-100 times the activity observed in control liver cells. The increased rate of reduction of HMG-CoA to mevalonic acid is not due to activation of pre-existing HMG-CoA reductase by dephosphorylation, as the fold induction is unchanged when reductase from control and estrogen-stimulated animals is fully activated prior to assay. The estrogen-induced increase of fatty acid synthesis is paralleled by a 16- to 20-fold increase of acetyl-CoA carboxylase activity, indicating that estrogen regulates fatty acid synthesis at the level of acetyl-CoA carboxylase. Fatty acid synthetase activity was unchanged during the induction of fatty acid biosynthesis by estrogen. The induction of HMG-CoA reductase and of acetyl-CoA carboxylase by estradiol-17 beta provides a useful model for regulation of these enzymes by steroid hormones.     

Analysis of proteins with the 'hot dog' fold: Prediction of function and identification of catalytic residues of hypothetical proteins

Background  

The hot dog fold has been found in more than sixty proteins since the first report of its existence about a decade ago. The fold appears to have a strong association with fatty acid biosynthesis, its regulation and metabolism, as the proteins with this fold are predominantly coenzyme A-binding enzymes with a variety of substrates located at their active sites.     

Regulation of Sialyltransferase Activities by Phosphorylation and Dephosphorylation

Abstract: The composition of tissue gangliosides is thought to result mainly from the active regulation and selective expression of specific enzymes responsible for their metabolism. In the last few years, we have purified several rat brain sialyltransferases to homogeneity; the availability of these highly purified enzymes enabled us to investigate their regulation and expression at the molecular level. Thus, we studied the regulation of sialyltransferase activities, in particular, CMP-NeuAc:GM1 and CMP-NeuAc:LacCer sialyltransferases by a phosphorylation/dephosphorylation mechanism. Protein kinase C was added to a standard enzyme assay mixture containing [γ-³²P]ATP, and the activity of the enzyme was measured after various incubation times. We found that treatment of several sialyltransferases by protein kinase C decreased their activities in a time-dependent manner. Analyses of ³²P-labeled amino acids revealed that the major phosphorylation site of CMP-NeuAc:GM1 α2→3 sialyltransferase (ST-IV) was serine and that for CMP-NeuAc:LacCer α2→3 sialyltransferase (ST-I) was primarily threonine. Partial recovery of the enzyme activity could be achieved by treatment of the phosphorylated sialyltransferases with rat brain protein phosphatase. We conclude that the activities of sialyltransferases can be modulated by protein kinase C and protein phosphatase and this may represent a potential regulatory mechanism for ganglioside biosynthesis.     

Phosphorylation of bovine adrenal chromaffin cell tyrosine hydroxylase. Temporal correlation of acetylcholine's effect on site phosphorylation, enzyme activation, and catecholamine synthesis

Tryptic peptide fragments of tyrosine hydroxylase isolated from 32PO4-prelabeled bovine adrenal chromaffin cells are resolved into seven phosphopeptides by reverse phase-high performance liquid chromatography. All seven of the peptides are phosphorylated on serine residues. Three of these putative phosphorylation sites, peptides 3, 5, and 6, are rapidly phosphorylated (5-fold in 15 s) by both acetylcholine stimulation and potassium depolarization of the cells, and this phosphorylation is accompanied by a similarly rapid activation of the enzyme. Both phosphorylation and activation are transient and do not account for the prolonged increase in catecholamine biosynthesis produced by these stimuli. Peptides 4 and 7 show a much slower and sustained increase in phosphorylation (3-fold in 4 min) in response to acetylcholine and potassium. Phosphorylation of these peptides correlates with the sustained increase in catecholamine biosynthesis rather than enzyme activation. Peptides 1 and 2 are not stimulated by any agonist yet employed and thus show no relation to enzyme activation or catecholamine biosynthesis. Phosphorylation of all five peptides by acetylcholine or potassium is calcium-dependent. In contrast to the stimulation of phosphorylation of tyrosine hydroxylase on multiple sites, forskolin stimulates the phosphorylation of only peptide 6, and this is accompanied by a coordinated activation of tyrosine hydroxylase and increased catecholamine biosynthesis. These findings show that the phosphorylation of tyrosine hydroxylase in intact cells is more complex than predicted from in vitro results, that at least two protein kinases are involved in the secretagogue-induced phosphorylation of tyrosine hydroxylase, and that the regulation of catecholamine biosynthesis, in response to phosphorylation, appears to involve both tyrosine hydroxylase activation and other mechanisms.     

Advances in study of ginsenoside biosynthesis pathway in <Emphasis Type="Italic">Panax ginseng</Emphasis> C. A. Meyer

Panax ginseng C. A. Meyer is one of the important nutraceutical and medicinal plants, which is used worldwide. Until now, ginseng has been reported to contain saponins, antioxidants, peptides, polysaccharides, fatty acids, vitamins, alkaloids, lignans, and flavonoids. The saponins, known as ginsenosides, are widely believed to be the major bioactive compounds of ginseng. In this article, ginsenoside biosynthesis pathway and key enzymes regulation are also described. This review provides a reference for improving ginsenoside contents through regulation of ginsenoside biosynthesis pathway.