Hemopexin: structure,function, and regulation

Hemopexin (HPX) is the plasma protein with the highest binding affinity to heme among known proteins. It is mainly expressed in liver, and belongs to acute phase reactants, the synthesis of which is induced after inflammation. Heme is potentially highly toxic because of its ability to intercalate into lipid membrane and to produce hydroxyl radicals. The binding strength between heme and HPX, and the presence of a specific heme-HPX receptor able to catabolize the complex and to induce intracellular antioxidant activities, suggest that hemopexin is the major vehicle for the transportation of heme in the plasma, thus preventing heme-mediated oxidative stress and heme-bound iron loss. In this review, we discuss the experimental data that support this view and show that the most important physiological role of HPX is to act as an antioxidant after blood heme overload, rather than to participate in iron metabolism. Particular attention is also put on the structure of the protein and on its regulation during the acute phase reaction.     

Glucose transporters: structure, function, and regulation

Glucose is transported into the cell by facilitated diffusion via a family of structurally related proteins, whose expression is tissue-specific. One of these transporters, GLUT4, is expressed specifically in insulin-sensitive tissues. A possible change in the synthesis and/or in the amount of GLUT4 has therefore been studied in situations associated with an increase or a decrease in the effect of insulin on glucose transport. Chronic hyperinsulinemia in rats produces a hyper-response of white adipose tissue to insulin and resistance in skeletal muscle. The hyper-response of white adipose tissue is associated with an increase in GLUT4 mRNA and protein. In contrast, in skeletal muscle, a decrease in GLUT4 mRNA and a decrease (tibialis) or no change (diaphragm) in GLUT4 protein are measured, suggesting a divergent regulation by insulin of glucose transport and transporters in the 2 tissues. In rodents, brown adipose tissue is very sensitive to insulin. The response of this tissue to insulin is decreased in obese insulin-resistant fa/fa rats. Treatment with a beta-adrenergic agonist increases insulin-stimulated glucose transport, GLUT4 protein and mRNA. The data suggest that transporter synthesis can be modulated in vivo by insulin (muscle, white adipose tissue) or by catecholamines (brown adipose tissue).     

Granule-bound starch synthase: structure, function, and phylogenetic utility

Interest in the use of low-copy nuclear genes for phylogenetic analyses of plants has grown rapidly, because highly repetitive genes such as those commonly used are limited in number. Furthermore, because low- copy genes are subject to different evolutionary processes than are plastid genes or highly repetitive nuclear markers, they provide a valuable source of independent phylogenetic evidence. The gene for granule-bound starch synthase (GBSSI or waxy) exists in a single copy in nearly all plants examined so far. Our study of GBSSI had three parts: (1) Amino acid sequences were compared across a broad taxonomic range, including grasses, four dicotyledons, and the microbial homologs of GBSSI. Inferred structural information was used to aid in the alignment of these very divergent sequences. The informed alignments highlight amino acids that are conserved across all sequences, and demonstrate that structural motifs can be highly conserved in spite of marked divergence in amino acid sequence. (2) Maximum-likelihood (ML) analyses were used to examine exon sequence evolution throughout grasses. Differences in probabilities among substitution types and marked among-site rate variation contributed to the observed pattern of variation. Of the parameters examined in our set of likelihood models, the inclusion of among-site rate variation following a gamma distribution caused the greatest improvement in likelihood score. (3) We performed cladistic parsimony analyses of GBSSI sequences throughout grasses, within tribes, and within genera to examine the phylogenetic utility of the gene. Introns provide useful information among very closely related species, but quickly become difficult to align among more divergent taxa. Exons are variable enough to provide extensive resolution within the family, but with low bootstrap support. The combined results of amino acid sequence comparisons, maximum-likelihood analyses, and phylogenetic studies underscore factors that might affect phylogenetic reconstruction. In this case, accommodation of the variable rate of evolution among sites might be the first step in maximizing the phylogenetic utility of GBSSI.      

Biology of amyloid: structure, function, and regulation

Amyloids are highly ordered cross-β sheet protein aggregates associated with many diseases including Alzheimer's disease, but also with biological functions such as hormone storage. The cross-β sheet entity comprising an indefinitely repeating intermolecular β sheet motif is unique among protein folds. It grows by recruitment of the corresponding amyloid protein, while its repetitiveness can translate what would be a nonspecific activity as monomer into a potent one through cooperativity. Furthermore, the one-dimensional crystal-like repeat in the amyloid provides a structural framework for polymorphisms. This review summarizes the recent high-resolution structural studies of amyloid fibrils in light of their biological activities. We discuss how the unique properties of amyloids gives rise to many activities and further speculate about currently undocumented biological roles for the amyloid entity. In particular, we propose that amyloids could have existed in a prebiotic world, and may have been the first functional protein fold in living cells.     

Guanylyl cyclase structure, function and regulation

Nitric oxide, bicarbonate, natriuretic peptides (ANP, BNP and CNP), guanylins, uroguanylins and guanylyl cyclase activating proteins (GCAPs) activate a family of enzymes variously called guanyl, guanylyl or guanylate cyclases that catalyze the conversion of guanosine triphosphate to cyclic guanosine monophosphate (cGMP) and pyrophosphate. Intracellular cyclic GMP is a second messenger that modulates: platelet aggregation, neurotransmission, sexual arousal, gut peristalsis, blood pressure, long bone growth, intestinal fluid secretion, lipolysis, phototransduction, cardiac hypertrophy and oocyte maturation. This review briefly discusses the discovery of cGMP and guanylyl cyclases, then nitric oxide, nitric oxide synthase and soluble guanylyl cyclase are described in slightly greater detail. Finally, the structure, function, and regulation of the individual mammalian single membrane-spanning guanylyl cyclases GC-A, GC-B, GC-C, GC-D, GC-E, GC-F and GC-G are described in greatest detail as determined by biochemical, cell biological and gene-deletion studies.     

Nitrate transporters in plants: structure, function and regulation

Physiological studies have established that plants acquire their NO(-3) from the soil through the combined activities of a set of high- and low-affinity NO(-3) transport systems, with the influx of NO(-3) being driven by the H(+) gradient across the plasma membrane. Some of these NO(-3) transport systems are constitutively expressed, while others are NO(-3)-inducible and subject to negative feedback regulation by the products of NO(-3) assimilation. Here we review recent progress in the characterisation of the two families of NO(-3) transporters that have so far been identified in plants, their structure and their regulation, and consider the evidence for their roles in NO(-3) acquisition. We also discuss what is currently known about the genetic basis of NO(-3) induction and feedback repression of the NO(-3) transport and assimilatory pathway in higher plants.     

Function, structure and regulation of cyanobacterial and chloroplast ATP synthase

The proton-translocating ATP synthase from chloroplasts and cyanobacteria forms ATP upon photosynthetic electron transport by using the proton gradient across the thylakoid membrane. Both enzymes contain nine different subunits and from the similarity in gene organisation and the high degree of amino acid sequence homology of the subunits it appears that these ATP synthases might have a common ancestor. Both enzymes need to be activated by membrane energisation in order to perform catalytic activity but, in contrast to the chloroplast ATP synthase, that from the studied cyanobacteria (with the exception of Spirulina platensis ) shows no effect of the redox state on activation. Functionally, the cyanobacterial enzyme corresponds to the reduced form of the chloroplast ATP synthase. In the chloroplast enzyme a stretch of 9 amino acids, including two cysteines in the γ-subunit, is involved in this redox effect and this stretch is absent in cyanobacteria. With γ-mutants from the cyanobacterium Synechocystis 6803 the role of this stretch is studied. When active, both the cyanobacterial and the reduced chloroplast ATP synthase transport 4 protons per ATP synthesised and hydrolysed. This ratio may depend on the environment of the enzyme such as protein and lipid composition and pH.     

乙酰肝素酶的结构、功能及调控

乙酰肝素酶是目前发现的哺乳动物细胞中唯一能切割细胞外基质中硫酸肝素蛋白多糖侧链--硫酸乙酰肝素--的内源性糖苷酶,是抗肿瘤,抗炎症的理想靶点。对其深入研究将有助于揭示组织修复,血管形成,自身免疫,肿瘤转移等生理及病理过程。本就乙酰肝素酶的发现,分子特性,基因定位,转录,表达调控,细胞内的亚定位及其功能活性调控机制方面的研究进展进行综述。     

Hepatic lipase: structure/function relationship,synthesis, and regulation

Hepatic lipase (HL) is a lipolytic enzyme, synthesized by hepatocytes and found localized at the surface of liver sinusoid capillaries. In humans, the enzyme is mostly bound onto heparan-sulfate proteoglycans at the surface of hepatocytes and also of sinusoid endothelial cells. HL shares a number of functional domains with lipoprotein lipase and with other members of the lipase gene family. It is a secreted glycoprotein, and remodelling of the N-linked oligosaccharides appears to be crucial for the secretion process, rather than for the acquisition of the catalytic activity. HL is also present in adrenals and ovaries, where it might promote delivery of lipoprotein cholesterol for steroidogenesis. However, evidence of a local synthesis is still controversial. HL activity is fairly regulated according to the cell cholesterol content and to the hormonal status. Coordinate regulations have been reported for both HL and the scavenger-receptor B-I, suggesting complementary roles in cholesterol metabolism. However, genetic variants largely contribute to HL variability and their possible impact in the development of a dyslipidemic phenotype, or in a context of insulin-resistance, is discussed.     

Intermediate filaments: structure, regulation of expression and possible function

New data are reviewed on intermediate filaments, i.e. on one of the cytoskeleton components. Structural proteins of intermediate filaments, their enzymatic modification, filament-associated proteins and the peculiarities of filament assembly are dealt with. The regularities of expression of intermediate filament proteins in normal tissues are analysed, as well as during differentiation and cultured cell growth. In the final part of the paper possible functions of intermediate filaments are discussed.     

Microbial isopenicillin N synthase genes: structure, function, diversity and evolution.

Clinically and economically, penicillins and cephalosporins are the most important class of the beta-lactam antibiotics. They are produced by a wide variety of microorganisms including numerous species of Streptomyces, some unicellular bacteria and several filamentous fungi. A key step common to their biosynthetic pathways is the conversion of a linear, cysteine-containing tripeptide to a bicyclic beta-lactam antibiotic by isopenicillin N synthase. Recent successes in the cloning and expression of isopenicillin N synthase genes now permit production of a plentiful supply of this enzyme, which may be used for structural and mechanistic studies, or for biotechnological applications in the creation of novel beta-lactam compounds from peptide analogues. New ideas concerning the evolution and prevalence of the penicillin and cephalosporin biosynthetic genes have emerged from studies of isopenicillin N synthase genes.     

植物类型III聚酮合酶超家族基因结构、功能及代谢产物

植物聚酮类化合物主要包括酚类、芪类及类黄酮化合物等,在植物花色、防止紫外线伤害、预防病原菌、昆虫危害以及作为植物与环境互作信号分子方面行使着重要的生物学功能。该类化合物具有显著多样的生物学活性,对人体保健及疾病治疗有显著意义。植物类型III 聚酮化合物合酶 (PKS) 在该类化合物生物合成起始反应中行使着关键作用,决定该类化合物基本分子骨架建成和代谢途径碳硫走向,为合成途径关键酶和限速酶。以查尔酮合酶为原型酶的植物类型III PKS超家族是研究系统进化和蛋白结构与功能关系的模式分子家族,目前已经分离得到14种植物类型III PKS基因,这些同祖同源基因及其表达产物既有共性,也表现出许多独特个性,这些个性赋予此类次生代谢产物结构上的多样性。以下综述了植物类型III PKS超家族基因结构、功能及代谢产物研究进展。     

Acetylglutamate synthase from Neurospora crassa: structure and regulation of expression

A novel gene shuffle approach has been developed for investigating the functions of genes on the cytoplasmic linear DNA killer plasmids of Kluyveromyces lactis . By transplacing k2ORF5 from the larger plasmid pGKL2(k2) onto pGKL1(k1) we have shown this gene to be essential and functionally interchangeable between plasmids. Once transferred onto k1, k2ORF5 is fully able to complement a k2ORF5⁰ deletion on k2 in trans , giving rise to yeast strains containing only the two recombinant plasmid forms. Additionally, the in vivo product of k2ORF5 has been identified as a 19.5 kDa protein by transplacing an epitope-tagged k2ORF5 allele from k2 to k1. The ease of detection of the tagged ORF5 product in comparison to TRF1, the gene product of k2ORF10, indicates that Orf5p is one of the most abundant k2 products, implying structural rather than regulatory function.