Crystal structure of the MgtE Mg2+ transporter

The magnesium ion Mg2+ is a vital element involved in numerous physiological processes. Mg2+ has the largest hydrated radius among all cations, whereas its ionic radius is the smallest. It remains obscure how Mg2+ transporters selectively recognize and dehydrate the large, fully hydrated Mg2+ cation for transport. Recently the crystal structures of the CorA Mg2+ transporter were reported. The MgtE family of Mg2+ transporters is ubiquitously distributed in all phylogenetic domains, and human homologues have been functionally characterized and suggested to be involved in magnesium homeostasis. However, the MgtE transporters have not been thoroughly characterized. Here we determine the crystal structures of the full-length Thermus thermophilus MgtE at 3.5 A resolution, and of the cytosolic domain in the presence and absence of Mg2+ at 2.3 A and 3.9 A resolutions, respectively. The transporter adopts a homodimeric architecture, consisting of the carboxy-terminal five transmembrane domains and the amino-terminal cytosolic domains, which are composed of the superhelical N domain and tandemly repeated cystathionine-beta-synthase domains. A solvent-accessible pore nearly traverses the transmembrane domains, with one potential Mg2+ bound to the conserved Asp 432 within the pore. The transmembrane (TM)5 helices from both subunits close the pore through interactions with the 'connecting helices', which connect the cystathionine-beta-synthase and transmembrane domains. Four putative Mg2+ ions are bound at the interface between the connecting helices and the other domains, and this may lock the closed conformation of the pore. A structural comparison of the two states of the cytosolic domains showed the Mg2+-dependent movement of the connecting helices, which might reorganize the transmembrane helices to open the pore. These findings suggest a homeostasis mechanism, in which Mg2+ bound between cytosolic domains regulates Mg2+ flux by sensing the intracellular Mg2+ concentration. Whether this presumed regulation controls gating of an ion channel or opening of a secondary active transporter remains to be determined.     

镁离子转运蛋白CorA间质结构域删除分析

为了研究CorA的镁离子转运机理,对大肠杆菌CorA的间质结构域区进行了删除突变分析,并利用酵母突变体互补技术进行了突变体活性测定.以酿酒酵母质膜镁离子转运系统敲除突变体菌株CM66及其基因型对照菌株CM52为宿主菌,建立了大肠杆菌CorA转运活性测定体系.结果显示:大肠杆菌CorA间质结构域N-端起始的24个残基对于CorA在酵母质膜中的表达或维持其本身正确构象发挥重要作用, CorA M124到D154段的部分残基在CorA介导的镁离子转运过程中起到重要作用.     

Crystal structure of a phosphorylation-coupled saccharide transporter

Saccharides have a central role in the nutrition of all living organisms. Whereas several saccharide uptake systems are shared between the different phylogenetic kingdoms, the phosphoenolpyruvate-dependent phosphotransferase system exists almost exclusively in bacteria. This multi-component system includes an integral membrane protein EIIC that transports saccharides and assists in their phosphorylation. Here we present the crystal structure of an EIIC from Bacillus cereus that transports diacetylchitobiose. The EIIC is a homodimer, with an expansive interface formed between the amino-terminal halves of the two protomers. The carboxy-terminal half of each protomer has a large binding pocket that contains a diacetylchitobiose, which is occluded from both sides of the membrane with its site of phosphorylation near the conserved His250 and Glu334 residues. The structure shows the architecture of this important class of transporters, identifies the determinants of substrate binding and phosphorylation, and provides a framework for understanding the mechanism of sugar translocation.     

Crystal structure of bacterial multidrug efflux transporter AcrB

AcrB is a major multidrug exporter in Escherichia coli. It cooperates with a membrane fusion protein, AcrA, and an outer membrane channel, TolC. We have determined the crystal structure of AcrB at 3.5 A resolution. Three AcrB protomers are organized as a homotrimer in the shape of a jellyfish. Each protomer is composed of a transmembrane region 50 A thick and a 70 A protruding headpiece. The top of the headpiece opens like a funnel, where TolC might directly dock into AcrB. A pore formed by three alpha-helices connects the funnel with a central cavity located at the bottom of the headpiece. The cavity has three vestibules at the side of the headpiece which lead into the periplasm. In the transmembrane region, each protomer has twelve transmembrane alpha-helices. The structure implies that substrates translocated from the cell interior through the transmembrane region and from the periplasm through the vestibules are collected in the central cavity and then actively transported through the pore into the TolC tunnel.     

Crystal structure of a catalytic intermediate of the maltose transporter

The maltose uptake system of Escherichia coli is a well-characterized member of the ATP-binding cassette transporter superfamily. Here we present the 2.8-A crystal structure of the intact maltose transporter in complex with the maltose-binding protein, maltose and ATP. This structure, stabilized by a mutation that prevents ATP hydrolysis, captures the ATP-binding cassette dimer in a closed, ATP-bound conformation. Maltose is occluded within a solvent-filled cavity at the interface of the two transmembrane subunits, about halfway into the lipid bilayer. The binding protein docks onto the entrance of the cavity in an open conformation and serves as a cap to ensure unidirectional translocation of the sugar molecule. These results provide direct evidence for a concerted mechanism of transport in which solute is transferred from the binding protein to the transmembrane subunits when the cassette dimer closes to hydrolyse ATP.     

Crystal structure of a potassium ion transporter, TrkH

The TrkH/TrkG/KtrB proteins mediate K(+) uptake in bacteria and probably evolved from simple K(+) channels by multiple gene duplications or fusions. Here we present the crystal structure of a TrkH from Vibrio parahaemolyticus. TrkH is a homodimer, and each protomer contains an ion permeation pathway. A selectivity filter, similar in architecture to those of K(+) channels but significantly shorter, is lined by backbone and side-chain oxygen atoms. Functional studies showed that TrkH is selective for permeation of K(+) and Rb(+) over smaller ions such as Na(+) or Li(+). Immediately intracellular to the selectivity filter are an intramembrane loop and an arginine residue, both highly conserved, which constrict the permeation pathway. Substituting the arginine with an alanine significantly increases the rate of K(+) flux. These results reveal the molecular basis of K(+) selectivity and suggest a novel gating mechanism for this large and important family of membrane transport proteins.     

ZnF2:Ni2+晶体局域结构的理论研究

从光谱和EPR数据出发,用完全对角化方法确定了ZnF2:Ni2+晶体中的Ni2+-F-键长与键角,证实了ZnF2:Ni2+晶体的局域结构畸变的存在.研究结果表明:ZnF2:Ni2+晶体与ZnF2基晶的结构参数对比,发现掺杂Ni2+离子的ZnF2晶体分别产生了沿C2轴的伸长畸变和在xy平面的压缩畸变;光谱和EPR参量计算的理论值与实验值非常接近.     

ZnF2:Ni2+晶体局域结构的理论研究

从光谱和EPR数据出发,用完全对角化方法确定了ZnF:Ni 晶体中的23(4 515 键长与键角,证实了 ZnF:Ni晶体的局域结构畸变的存在%研究结果表明,ZnF:Ni晶体与/01( 基晶的结构参数对比,发现掺杂23(4 离子的ZnF:Ni晶体分别产生了沿. 轴的伸长畸变和在,平面的压缩畸变.光谱和,-.参量计算的理论值与实验值非常接近.     

Crystal structure of a membrane-embedded H+-translocating pyrophosphatase

H(+)-translocating pyrophosphatases (H(+)-PPases) are active proton transporters that establish a proton gradient across the endomembrane by means of pyrophosphate (PP(i)) hydrolysis. H(+)-PPases are found primarily as homodimers in the vacuolar membrane of plants and the plasma membrane of several protozoa and prokaryotes. The three-dimensional structure and detailed mechanisms underlying the enzymatic and proton translocation reactions of H(+)-PPases are unclear. Here we report the crystal structure of a Vigna radiata H(+)-PPase (VrH(+)-PPase) in complex with a non-hydrolysable substrate analogue, imidodiphosphate (IDP), at 2.35?? resolution. Each VrH(+)-PPase subunit consists of an integral membrane domain formed by 16 transmembrane helices. IDP is bound in the cytosolic region of each subunit and trapped by numerous charged residues and five Mg(2+) ions. A previously undescribed proton translocation pathway is formed by six core transmembrane helices. Proton pumping can be initialized by PP(i) hydrolysis, and H(+) is then transported into the vacuolar lumen through a pathway consisting of Arg?242, Asp?294, Lys?742 and Glu?301. We propose a working model of the mechanism for the coupling between proton pumping and PP(i) hydrolysis by H(+)-PPases.     

Hormonal control of Mg2+ transport in the heart

Magnesium is abundant in the mammalian body and the second most abundant cation in cells. Because the concentration of intracellular free Mg2+ is relatively high (0.2-1 mM), Mg2+ is unlikely to act as a second messenger, like Ca2+, by rapidly changing its cytosolic concentration. But changes in Mg2+ do have profound effects on cellular metabolism, structure and bioenergetics. Key enzymes or metabolic pathways, mitochondrial ion transport, Ca2+ channel activities in the plasma membrane and intracellular organelles, ATP-requiring reactions, and structural properties of cells and nucleic acids are modified by changes in Mg2+ concentration. Yet, although some information is available from giant cells and bacteria, little is known about the regulation of intracellular Mg2+ in mammalian cells. Here we report a new transport mechanism for Mg2+ across the sarcolemma of cardiac cells in both intact hearts and dissociated myocytes. We show that noradrenaline, through beta-adrenergic stimulation and increase of cyclic AMP, stimulates a large efflux of Mg2+ from cardiac cells. This transport is of major dimensions and can move up to 20% of total cellular Mg2+ within a few minutes.     

离子色谱法测定饮用水中钠、镁、钙

本文研究了离子色谱法测定饮用水及源水中钠、镁、钙离子的方法.实验表明,该方法灵敏度高,重现性好,操作简便,分析快速.在本方法实验条件下,钠、镁、钙方法检出限,分别为0.013mg/L;0.013mg/L;0.025mg/L.相对标准偏差分布在0.07-0.76%,加标回收率分布在99-102%.     

掺杂Ca^2＋和Mg^2＋对SBN陶瓷介电性能的影响

以分析纯的碳酸锶、碳酸钡、碳酸钙、氧化镁和五氧化二铌为原料,采用固相法制备掺杂Ca(Mg)的Sr0.5Ba0.5Nb2O6陶瓷材料.采用X射线衍射仪、扫描电镜和阻抗分析仪,研究掺杂样品的相组成、微观组织和介电性能.研究结果表明,Ca2+掺杂量≤0.23 mol时,Ca2+取代钨青铜结构中A位的Sr2+和Ba2+,随Ca掺杂量的增加,SBN晶格常数逐渐减小.介电温谱曲线显示:在150℃和330℃处出现介电双峰;当Ca2+掺杂量≥0.28 mol时,出现第二相CaNb2O6,此时对应(Sr,Ba,Ca)Nb2O6的介电峰逐渐消失.Mg掺杂的所有陶瓷样品,XRD谱呈现SBN50与MgNb2O6两相共存的情形.在150℃和310℃处同样出现介电双峰,随着Mg掺杂量增加,介电峰为1个单峰,并移向低温.     

Crystal structure of the human beta2 adrenergic G-protein-coupled receptor

Structural analysis of G-protein-coupled receptors (GPCRs) for hormones and neurotransmitters has been hindered by their low natural abundance, inherent structural flexibility, and instability in detergent solutions. Here we report a structure of the human beta2 adrenoceptor (beta2AR), which was crystallized in a lipid environment when bound to an inverse agonist and in complex with a Fab that binds to the third intracellular loop. Diffraction data were obtained by high-brilliance microcrystallography and the structure determined at 3.4 A/3.7 A resolution. The cytoplasmic ends of the beta2AR transmembrane segments and the connecting loops are well resolved, whereas the extracellular regions of the beta2AR are not seen. The beta2AR structure differs from rhodopsin in having weaker interactions between the cytoplasmic ends of transmembrane (TM)3 and TM6, involving the conserved E/DRY sequences. These differences may be responsible for the relatively high basal activity and structural instability of the beta2AR, and contribute to the challenges in obtaining diffraction-quality crystals of non-rhodopsin GPCRs.     

镁钙离子与π体系相互作用的量子化学研究

运用量子化学HF方法在6—311G^＊＊基组水平上对镁、钙离子与π芳香体系（苯、苯酚、对二硝基苯）相互作用所形成的配合物进行了优化计算，并在此基础上进行了振动分析．得到了配合物的稳定几何构型，电荷密度，能量，前线轨道等信息，从而得到了钙、镁离子-π配合物的一些化学性质，计算表明不仅过渡金属阳离子能与芳环体系发生相互作用，而且碱土金属二价阳离子也能与芳环体系发生相互作用，同时表明HF方法对于研究阳离子-π配合物的可靠性．     

慢性氟中毒对大鼠基础生长的影响及金属离子的拮抗作用

目的 通过动物实验,研究慢性氟中毒对大鼠生长的影响,以及钙、镁和铝离子对慢性氟中毒的拮抗作用,为研究地氟病的分子机制及其治疗提供基础数据.方法 断奶1周的24只雄性SD大鼠,随机分成4个实验组,分别为对照组、高氟组、氟＋镁＋铝拮抗组和氟＋钙拮抗组.全群饲喂3个月,每隔两天称量大鼠体质量、摄食量和饮水量.处死前3d,每天留取24 h尿液,测定尿氟含量.结果 1）与对照组相比,高氟组体质量增长速度明显减缓,加钙拮抗组体质量增长速度比对照组快,也比高氟组快;2）高氟组的尿氟含量高于其他组,加镁铝拮抗组和加钙拮抗组比高氟组低,但仍高于对照组;3）各组摄食量和饮水量无明显差异.结论 慢性氟中毒会减缓动物个体的生长速度,而在饮水中添加镁、铝或钙离子能够起到一定的拮抗作用,钙离子拮抗作用更为明显.     

腐殖质与Ca~(2+)、Mg~(2+)、Co~(2+)、Ni~(2+)的络合作用及其在农业中的应用

用电位滴定法研究腐殖酸FA和HA与金属离子(Ca2+、Mg2+、Co2+、Ni2+)的络合作用,发现它们与金属离子形成的络合物比较稳定,在溶液中的络合形成常数可通过作图法获得 随着溶液中金属离子浓度的增大,络合形成常数减小,同时发现FA与金属离子的络合形成常数比HA的大 腐殖酸是土壤肥力高低的一个重要标志,在农业应用中具有重要作用     

Mg2+-induced LHC II aggregation in thylakoid membrane

Mg²⁺-induced changes in 77K fluorescence spectra of wheat thylakoid membranes were analyzed by a Gaussian deconvolution program. All spectra were fitted well with 7 Gaussian sub-bands. The sub-band areas for the LHC H macroaggregate (F₆₉₉) and PS II (F₆₈₅ and F₆₉₅) were increased and those for the LHC II trimer (F₆₈₁) and PS I (F₇₂₉ and F₇₄₀) were decreased by Mg²⁺. Moreover, it was found that the sub-band area ratio for F₆₉₉ over F₆₈₁ was dramatically enhanced by Mg²⁺ by 1.72 times. It was concluded that the LHC II trimer in thylakoid membranes could be aggregated and transformed into its macroaggregate by Mg²⁺ on a large scale. The possible implication of the Mg²⁺-induced LHC II aggregation was also discussed.