天麻素生物合成的研究进展

天麻素是我国名贵中药材天麻中的一种主要活性成分，具有降血压、抗癫痫、抑制肿瘤、保护神经等多方面的药理活性。随着市场对天麻素需求的不断增长以及传统获取方法固有的问题，导致急需新的方法来解决天麻素生产实际中面临的各项困难。生物合成法是一种有别于传统获取法的新方法，已在天麻素获取上取得了较大进展和成果，故现阶段有必要从天麻素生物合成途径、植物转化法和微生物转化法3个方面，对天麻素生物合成进行系统地阐述，从而为进一步改进和完善天麻素生产方法，以满足人们对其不断增长的需求提供有价值的参考。     

虎杖MYB转录因子PcMYB1的表达特性和功能研究

目的对虎杖中1个新的R2R3-MYB转录因子基因PcMYB1进行转录活性鉴定和表达特性分析,并在转基因拟南芥中进行功能研究。方法利用酵母单杂交实验分析PcMYB1的转录活性;荧光定量PCR(RT-PCR)技术检测虎杖中PcMYB1的表达模式;利用Wiesner染色和溴乙酰法检测PcMYB1转基因拟南芥中的木质素含量;RT-PCR技术分析PcMYB1转基因拟南芥木质素合成相关基因的表达。结果酵母单杂实验结果表明,PcMYB1具有转录抑制活性;RT-PCR结果显示PcMYB1在虎杖根、茎、叶中均有表达,在叶中表达量最高,并且紫外照射处理可诱导叶片中PcMYB1的表达;与野生型拟南芥相比,转基因拟南芥的株高降低了24.07%,木质部的细胞染色程度浅,转基因拟南芥木质素含量降低了14.81%,参与木质素合成的AtC4H、AtC3H、AtF5H、AtCOMT、AtCAD基因表达下调。结论 PcMYB1具有转录抑制活性,对植物木质素合成具有负调控作用。     

Purinosome formation as a function of the cell cycle

The de novo purine biosynthetic pathway relies on six enzymes to catalyze the conversion of phosphoribosylpyrophosphate to inosine 5′-monophosphate. Under purine-depleted conditions, these enzymes form a multienzyme complex known as the purinosome. Previous studies have revealed the spatial organization and importance of the purinosome within mammalian cancer cells. In this study, time-lapse fluorescence microscopy was used to investigate the cell cycle dependency on purinosome formation in two cell models. Results in HeLa cells under purine-depleted conditions demonstrated a significantly higher number of cells with purinosomes in the G₁ phase, which was further confirmed by cell synchronization. HGPRT-deficient fibroblast cells also exhibited the greatest purinosome formation in the G₁ phase; however, elevated levels of purinosomes were also observed in the S and G₂/M phases. The observed variation in cell cycle-dependent purinosome formation between the two cell models tested can be attributed to differences in purine biosynthetic mechanisms. Our results demonstrate that purinosome formation is closely related to the cell cycle.Enzymes have been shown to form clusters in a cell to regulate metabolic processes (1–3). More recently, the concept of multienzyme complexes has been expanded to include mesoscale protein assemblies that appear to be substantially larger than a single protein (4). Depending on the metabolic or developmental state of the cells, these types of protein clusters range from transiently associated to very rigid and well defined (4). Examples of such enzyme clusters include the EF-Tu cytoskeleton, RNA degradosome, CTP synthase, carboxysomes, and nucleolus (5–9).In humans, purine nucleotides are synthesized by two different mechanisms. The first mechanism, de novo purine biosynthesis, converts phosphoribosylpyrophosphate (PRPP) to inosine 5′-phosphate (IMP) in 10 highly conserved steps catalyzed by six enzymes. These six enzymes include one trifunctional enzyme (TrifGART: GARS, GART, and AIRS domains), two bifunctional enzymes (PAICS: CAIRS and SAICARS domains; ATIC: AICART and IMPCH domains), and three monofunctional enzymes (PPAT, FGAMS, and ASL). ASL is also necessary for the conversion of IMP to AMP and may be classified as bifunctional (10). The second mechanism uses nucleotide salvage pathways to either phosphorylate a nucleoside (e.g., thymidine kinase) or add a purine base to ribose 5′-phosphate to regenerate the respective monophosphate. For example, hypoxanthine/guanine phosphoribosyl transferase (HGPRT) catalyzes the conversion of hypoxanthine to IMP and the conversion of guanine to guanosine 5′-phosphate (GMP). The de novo pathway is more energy-intensive, with the synthesis of 1 mole of IMP requiring 5 moles of ATP. Therefore, the salvage pathway is the preferred pathway for purine biosynthesis. Cells deficient in HGPRT, such as those that exhibit a Lesch–Nyhan disease (LND) phenotype, rely primarily on the de novo purine biosynthetic pathway to generate purine nucleotides (11–15).Recently, enzymes in the de novo purine biosynthetic pathway were shown to organize and reversibly assemble into punctate cellular bodies known as “purinosomes” under purine-depleted conditions (16). Further investigations into the organization of the purinosome showed that several of the enzymes form a core structure (PPAT, TrifGART, and FGAMS), whereas others appear to interact peripherally (PAICS, ASL, and ATIC) (17). The presence of this protein assembly and its putative function(s) clearly suggest that the spatial organization of pathway enzymes into purinosomes within a cell play an important role in meeting the cellular demand for purines (18). Although many studies have implied up-regulation of the de novo purine biosynthetic pathway during cell cycle progression (14, 19–21), here we used time-lapse microscopy to determine whether a correlation exists between the cell cycle stage and the number of cells with purinosomes or the cells’ purinosome content. Two different cell types, HeLa and LND cells, were used, with the latter deficient in purine salvage, to assess the effect of increased demand on the de novo pathway for purine biosynthesis and the attendant consequences for purinosome assembly.     

X-ray crystallographic and EPR spectroscopic analysis of HydG,a maturase in [FeFe]-hydrogenase H-cluster assembly

Hydrogenases use complex metal cofactors to catalyze the reversible formation of hydrogen. In [FeFe]-hydrogenases, the H-cluster cofactor includes a diiron subcluster containing azadithiolate, three CO, and two CN⁻ ligands. During the assembly of the H cluster, the radical S-adenosyl methionine (SAM) enzyme HydG lyses the substrate tyrosine to yield the diatomic ligands. These diatomic products form an enzyme-bound Fe(CO)_x(CN)_y synthon that serves as a precursor for eventual H-cluster assembly. To further elucidate the mechanism of this complex reaction, we report the crystal structure and EPR analysis of HydG. At one end of the HydG (βα)₈ triosephosphate isomerase (TIM) barrel, a canonical [4Fe-4S] cluster binds SAM in close proximity to the proposed tyrosine binding site. At the opposite end of the active-site cavity, the structure reveals the auxiliary Fe-S cluster in two states: one monomer contains a [4Fe-5S] cluster, and the other monomer contains a [5Fe-5S] cluster consisting of a [4Fe-4S] cubane bridged by a μ₂-sulfide ion to a mononuclear Fe²⁺ center. This fifth iron is held in place by a single highly conserved protein-derived ligand: histidine 265. EPR analysis confirms the presence of the [5Fe-5S] cluster, which on incubation with cyanide, undergoes loss of the labile iron to yield a [4Fe-4S] cluster. We hypothesize that the labile iron of the [5Fe-5S] cluster is the site of Fe(CO)_x(CN)_y synthon formation and that the limited bonding between this iron and HydG may facilitate transfer of the intact synthon to its cognate acceptor for subsequent H-cluster assembly.The assembly of the [FeFe]-hydrogenase diiron subcluster (1, 2) requires three maturase proteins, HydE, HydF, and HydG (3), and in vitro, they can assemble an active hydrogenase (4). The sequence and structure of the maturase HydE (5) indicates that it is a member of the radical S-adenosyl methionine (SAM) superfamily, although the biochemical function of HydE has not been experimentally determined. The GTPase HydF (6, 7) has been shown to transfer synthetic (8) or biologically derived (7, 9) diiron subclusters into apo-hydrogenase, suggesting that HydF functions as a template for diiron subcluster assembly. The tyrosine lyase HydG is also a member of the radical SAM superfamily and uses SAM and a reductant (such as dithionite) to cleave the Cα–Cβ bond of tyrosine, yielding p-cresol as the side chain-derived byproduct (10) and fragmenting the amino acid moiety into cyanide (CN⁻) (11) and carbon monoxide (CO) (12), which are ultimately incorporated as ligands in the H cluster of the [FeFe]-hydrogenase HydA (4). Two site-differentiated [4Fe-4S] clusters in HydG have been identified using a combination of spectroscopy and site-directed mutagenesis (12–16). The cluster bound close to the N terminus ([4Fe-4S]_RS) by the CX₃CX₂C cysteine triad motif (SI Appendix, Fig. S1) is typical of the radical SAM superfamily (17, 18) and has been shown to catalyze the reductive cleavage of SAM (11, 13). The resultant highly reactive 5′-deoxyadenosyl radical is thought to abstract a hydrogen atom from tyrosine, thereby inducing Cα–Cβ-bond homolysis with release of dehydroglycine (DHG) and the spectroscopically characterized 4-oxidobenzyl radical anion (16), which is quenched to yield p-cresol (Fig. 1A, step A). The second (auxiliary) Fe-S cluster is proposed to promote the conversion of DHG into CO and CN⁻ (Fig. 1A, step B) (13, 16). Two intermediates have been observed by stopped-flow IR spectroscopic analysis (19): an enzyme-bound organometallic species (complex A) (Fig. 1A, 4) that converts to a species that features an Fe(CO)₂(CN) moiety (complex B) (Fig. 1A, 5). These results, combined with ⁵⁷Fe electron-nuclear double resonance (ENDOR) studies that showed that iron from HydG is incorporated into mature hydrogenase, led to the proposal that an organometallic synthon with a minimum stoichiometry of [Fe(CO)₂CN] is synthesized at the auxiliary cluster of HydG and eventually transferred to apo-hydrogenase (19).Open in a separate windowFig. 1.Overall [FeFe]-hydrogenase H-cluster assembly and structure of TiHydG. (A) Formation of the Fe(CO)₂CN synthon is proposed to occur at the auxiliary cluster of HydG (square brackets). (B) Overall fold of HydG with an end-on view of the TIM barrel showing the radical SAM core (green), the N-terminal extension (pink), and the C-terminal extension (blue). Monomer A is shown and contains a [4Fe-4S] cluster to catalyze the formation of the 5′-deoxyadenosyl radical from SAM and a [5Fe-5S] auxiliary cluster proposed to promote the conversion of DHG into cyanide and carbon monoxide. (C) The position of the two Fe-S clusters in TiHydG. The strands of the TIM barrel are shown. The orientation is rotated 90° from B.Herein, we report the crystal structure of Thermoanaerobacter italicus HydG (TiHydG) complexed with SAM (the Protein Data Bank ID code for the structure of HydG is 4WCX). The structure, which contains two HydG monomers per asymmetric unit, reveals the auxiliary Fe-S cluster in two states: one monomer contains a [4Fe-5S] cluster, and the other monomer contains a structurally unprecedented [5Fe-5S] cluster consisting of a [4Fe-4S] cubane bridged by a μ₂-sulfide to a mononuclear Fe(II) center (which we term the labile iron). To supplement the crystallographic studies of TiHydG, we also report EPR spectroscopic studies of Shewanella oneidensis HydG (SoHydG) that provide solution-state characterization of the [5Fe-5S] cluster and show its conversion to a [4Fe-4S] cluster in the presences of exogenous cyanide. Taken together, these results support a proposed mechanism for [FeFe]-hydrogenase maturation in which the labile iron of the [5Fe-5S] cluster is the site for Fe(CO)_x(CN)_y synthon assembly.     

Effect of phenytoin and nifedipine on collagen gene expression in human gingival fibroblasts

Phenytoin (PHT), a widely used anticonvulsant, and nifedipine (NF), an anti-anginal drug, cause clinically similar gingival overgrowths in some patients. The aim of this work was to investigate their effects on collagen and protein synthesis and cellular proliferation in normal human gingival fibroblasts in vitro. Gingival fibroblasts were cultured from biopsies taken from three healthy individuals during operations on maxillary canines and incubated with various concentrations of NF (100 and 200 ng/ml) and PHT (5 and 10 micrograms/ml) for up to 7 days. The results showed that NF and PHT have a specific effect in reducing total protein and collagen synthesis but do not influence cell proliferation in healthy gingival fibroblasts in vitro. In addition the level of mRNA for type I collagen was decreased after incubation of the cells with the drugs for 1 or 2 days. The decrease in the level of type I collagen mRNA seemed to be specific since the level of type IV collagenase mRNA used as a reference RNA did not decrease.     

DNA编码化合物库技术新进展及其在生物医药领域的应用与展望

随着医药领域对靶向调节特定生命活动过程的新分子化合物的需求日益增加,高效、低成本地发现亲和配体分子的新型小分子药物筛选技术——DNA编码的化合物库(DNA encoded compound library,DEL)筛选技术应运而生。DEL作为组合化合物库,可以是简单小分子化合物的集合,也可以是具有高级空间结构的复杂小分子库,每个化合物都以共价方式与特异的DNA序列偶联,因而可将库中所有化合物与靶标蛋白进行合并筛选,随后使用高通量测序对DNA编码序列进行测序来鉴定结合的配体分子。近年来,应用DEL技术筛选开发候选药物的例子已越来越多地被报道,本文回顾和总结了DEL技术的实施流程,特别是DEL库构建和亲和筛选方法的最新研究进展,展示了利用DEL技术筛选开发的最新亲和配体分子,并对DEL技术在生命科学领域的应用前景作了展望。     

Production of Active Compounds in Medicinal Plants: From Plant Tissue Culture to Biosynthesis

Over past decades plant tissue culture has emerged as an alternative of whole plant cultivation in the production of valuable secondary metabolites.Adventitious roots culture of Panax ginseng and Echinacea purpure has reached the scale of 1-10 kL.Some molecular biological techniques,such as transgenic technology and genetic stability are increasingly used in the studies on plant tissue cultures.The studies on elicitors have deepened into the induction mechanism,including signal molecules,functional genes,and so on.More and more biological elicitors,such as A.niger and yeast are used to increase the active compounds in plant tissue cultures.We also discussed the application of synthetic biology in the studies on biosynthesis of artemisinin,paclitaxel,and tanshinon.The studies on active ingredients biosynthesis of medicinal plants provide unprecedented possibilities to achieve mass production of active ingredients.Plant tissue cultures can not only produce active ingredients but also as experimental materials for biosynthesis.In order to improve the contents of active compounds in medicinal plants,following aspects could be carried out gene interference or gene silencing,gene overexpression,combination with chemical synthesis,application of elicitors,and site-directed mutagenesis of the key enzymes.     

Desmosterolosis presenting with multiple congenital anomalies

Desmosterolosis is a rare multiple congenital anomaly syndrome caused by a defect in the enzyme 3-beta-hydroxysterol delta-24-reductase (DHCR24) in the cholesterol biosynthesis pathway. Defects in this enzyme cause increased level of the cholesterol precursor desmosterol while disrupting development of cholesterol, impacting embryogenesis. A total of 9 cases of desmosterolosis have been reported to date. We report a 20-month-old male from consanguineous parents with multiple congenital anomalies including corpus callosum hypoplasia, facial dysmorphism, cleft palate, pectus deformity, short and wide neck and distal contractures. On analysis of the regions of homozygosity found by microarray, we identified DHCR24 as a candidate gene. Sterol quantitation showed a desmosterol level of 162 μg/mL (nl: 0.82 ± 0.48). Genetic testing confirmed the diagnosis with a homozygous likely pathogenic mutation (p.Glu191Lys) in the DHCR24 gene. Our case expands the known diagnostic spectrum for Desmosterolosis. We suggest considering Desmosterolosis in the differential diagnosis of patients who present with concurrent agenesis of the corpus callosum with white matter atrophy and ventriculomegaly, retromicrognathia with or without cleft palate, hand contractures, and delay of growth and development. Children of consanguineous mattings may be at higher risk for rare recessive disorders and testing for cholesterol synthesis defect should be a consideration for affected children. Initial evaluation can be performed using sterol quantitation, followed by genetic testing.