Phytosterols of marine algae: Insights into the potential health benefits and molecular pharmacology

BackgroundMarine algae are rich in some unique biologically active secondary metabolites having diverse pharmacological benefits. Of these, sterols comprise a group of functional lipid compounds that have attracted much attention to natural product scientists.PurposeThis review was aimed to update information on the health effects of algae-derived phytosterols and their molecular interactions in various aspects of human health and diseases and to address some future perspectives that may open up a new dimension of pharmacological potentials of algal sterols.MethodsA literature-based search was carried out to retrieve published research information on the potential health effects of algal phytosterols with their pharmacological mechanisms from accessible online databases, such as Pubmed, Google Scholar, Web of Science, and Scopus, using the key search terms of ‘marine algae sterol’ and ‘health potentials such as antioxidant or anti-inflammatory or anti-Alzheimer's or anti-obesity or cholesterol homeostasis or hepatoprotective, antiproliferative, etc.’ResultsPhytosterols of marine algae, particularly fucosterol, have been investigated for a plethora of health benefits, including anti-diabetes, anti-obesity, anti-Alzheimer's, antiaging, anticancer, and hepatoprotection, among many others, which are attributed to their antioxidant, anti-inflammatory, immunomodulatory and cholesterol-lowering properties, indicating their potentiality as therapeutic leads. These sterols interact with enzymes and various other proteins that are actively participating in different cellular pathways, including antioxidant defense system, apoptosis and cell survival, metabolism, and homeostasis.ConclusionIn this review, we briefly overview the chemistry, pharmacokinetics, and distribution of algal sterols, and provide critical insights into their potential health effects and the underlying pharmacological mechanisms, beyond the well-known cholesterol-lowering paradigm.     

Mechanisms of lipid-body formation

Most organisms transport or store neutral lipids as lipid bodies - lipid droplets that usually are bounded by specific proteins and (phospho)lipid. Neutral-lipid bodies vary considerably in their morphology and are associated with an extremely diverse range of proteins. However, the mechanisms by which they are generated in plants, animals and microorganisms appear to share many common features: lipid bodies probably arise from microdomains of the endoplasmic reticulum (or the plasma membrane in prokaryotes) that contain lipid-biosynthesis enzymes, and their synthesis and size appear to be controlled by specific protein components.     

海洋细菌CAZymes的研究进展

多糖是大型藻类、浮游植物和微生物的主要成分,多糖降解产物是海洋有机物的主要来源.碳水化合物活性酶(carbohydrate-active enzymes,CAZymes)是负责糖类化合物降解、修饰及生成糖苷键的功能酶系,是糖类物质代谢通路中的基本功能单元.多数异养细菌都具备一套完善的碳水化合物活性酶编码系统,它们是参与...     

Gene replacement of fructose-1,6-bisphosphate aldolase supports the hypothesis of a single photosynthetic ancestor of chromalveolates

Plastids (photosynthetic organelles of plants and algae) are known to have spread between eukaryotic lineages by secondary endosymbiosis, that is, by the uptake of a eukaryotic alga by another eukaryote. But the number of times this has taken place is controversial. This is particularly so in the case of eukaryotes with plastids derived from red algae, which are numerous and diverse. Despite their diversity, it has been suggested that all these eukaryotes share a recent common ancestor and that their plastids originated in a single endosymbiosis, the so-called "chromalveolate hypothesis." Here we describe a novel molecular character that supports the chromalveolate hypothesis. Fructose-1,6-bisphosphate aldolase (FBA) is a glycolytic and Calvin cycle enzyme that exists as two nonhomologous types, class I and class II. Red algal plastid-targeted FBA is a class I enzyme related to homologues from plants and green algae, and it would be predicted that the plastid-targeted FBA from algae with red algal secondary endosymbionts should be related to this class I enzyme. However, we show that plastid-targeted FBA of heterokonts, cryptomonads, haptophytes, and dinoflagellates (all photosynthetic chromalveolates) are class II plastid-targeted enzymes, completely unlike those of red algal plastids. The chromalveolate enzymes form a strongly supported group in FBA phylogeny, and their common possession of this unexpected plastid characteristic provides new evidence for their close relationship and a common origin for their plastids.     

Human and animal hepatocytes in vitro with extrapolation in vivo

Human and animal hepatocytes are now being used as an in vitro technique to aid drug discovery by predicting the in vivo metabolic pathways of drugs or new chemical entities (NCEs), identifying drug-metabolizing enzymes and predicting their in vivo induction. Because of the difficulty of establishing whether the cytotoxic susceptibility of human hepatocytes to xenobiotics/drugs in vitro could be used to predict in vivo human hepatotoxicity, a comparison of the susceptibility of the hepatocytes of human and animal models to six chemical classes of drugs/xenobiotics in vitro have been related to their in vivo hepatotoxicity and the corresponding activity of their metabolizing enzymes. This study showed that the cytotoxic effectiveness of 16 halobenzenes towards rat hepatocytes in vitro using higher doses and short incubation times correlated well with rat hepatotoxic effectiveness in vivo with lower doses/longer times. The hepatic/hepatocyte xenobiotic metabolizing enzyme activities of various animal species and human have been reviewed for use by veterinarians and research scientists. Where possible, recommendations have been made regarding which animal hepatocyte model is most applicable for modeling the susceptibility to xenobiotic induced hepatotoxicity of those humans with slow versus rapid metabolizing enzyme polymorphisms. These recommendations are based on the best human fit for animal drug/xenobiotic metabolizing enzymes in terms of activity, kinetics and substrate/inhibitor specificity. The use of human hepatocytes from slow versus rapid metabolizing individuals for drug metabolism/cytotoxicity studies; and the research use of freshly isolated rat hepatocytes and "Accelerated Cytotoxicity Mechanism Screening" (ACMS) techniques for identifying drug/xenobiotic reactive metabolites are also described. Using these techniques the molecular hepatocytotoxic mechanisms found in vitro for seven classes of xenobiotics/drugs were found to be similar to the rat hepatotoxic mechanisms reported in vivo.     

Molecular characterization of four genes involved in sulfur metabolism in <Emphasis Type="Italic">Porphyra purpurea</Emphasis> (Roth) C. Agardh

Sulfated polysaccharides (carrageenans and agars) are among the most important products of red algae that are used as food additives as well as in molecular biology research. The quality and value of the product is greatly dependent on the levels and sites of sulfation of the polysaccharides. Little information is currently available on the molecular details of sulfur metabolism in red algae. Considering the economic importance of sulfated polysaccharide, elucidating the molecular details of sulfur metabolism in these organisms could help in future endeavors to improve algal commercial value, e.g., through genetic engineering. A cDNA library from the red alga Porphyra purpurea (Roth) C. Agardh was used to isolate four cDNAs with homology to genes encoding known sulfur assimilation enzymes: sulfate adenyltransferase (ATP sulfurylase), adenosine 5′-phosphosulfate kinase (APSK), sulfite reductase, and cysteine synthase. These cDNAs were characterized with respect to their molecular properties and a cDNA with homology to APSK was used to functionally complement an Escherichia coli auxotroph APSK⁻ mutant. The other cDNAs are being similarly characterized with respect to their ability to produce functional enzymes. Elucidation of the regulation of expression of these genes will aid in future research to determine the biochemical and genetic details of the sulfate assimilation pathway as well as its genetic manipulation in red algae.     

Thematic review series: lipid posttranslational modifications. Lysosomal metabolism of lipid-modified proteins

Much is now understood concerning the synthesis of prenylated and palmitoylated proteins, but what is known of their metabolic fate? This review details metabolic pathways for the lysosomal degradation of S-fatty acylated and prenylated proteins. Central to these pathways are two lysosomal enzymes, palmitoyl-protein thioesterase (PPT1) and prenylcysteine lyase (PCL). PPT1 is a soluble lipase that cleaves fatty acids from cysteine residues in proteins during lysosomal protein degradation. Notably, deficiency in the enzyme causes a neurodegenerative lysosomal storage disorder, infantile neuronal ceroid lipofuscinosis. PCL is a membrane-associated flavin-containing lysosomal monooxygenase that metabolizes prenylcysteine to prenyl aldehyde through a completely novel mechanism. The eventual metabolic fates of other lipidated proteins (such as glycosylphosphatidylinositol-anchored and N-myristoylated proteins) are poorly understood, suggesting directions for future research.     

Enzymes of triacylglycerol synthesis and their regulation

Since the pathways of glycerolipid biosynthesis were elucidated in the 1950's, considerable knowledge has been gained about the enzymes that catalyze the lipid biosynthetic reactions and the factors that regulate triacylglycerol biosynthesis. In the last few decades, in part due to advances in technology and the wide availability of nucleotide and amino acid sequences, we have made enormous strides in our understanding of these enzymes at the molecular level. In many cases, sequence information obtained from lipid biosynthetic enzymes of prokaryotes and yeast has provided the means to search the genomic and expressed sequence tag databases for mammalian homologs and most of the genes have now been identified. Surprisingly, multiple isoforms appear to catalyze the same chemical reactions, suggesting that each isoform may play a distinct functional role in the pathway of triacylglycerol and phospholipid biosynthesis. This review focuses on the de novo biosynthesis of triacylglycerol in eukaryotic cells, the isoenzymes that are involved, their subcellular locations, how they are regulated, and their putative individual roles in glycerolipid biosynthesis.     

Annotation of genes involved in glycerolipid biosynthesis in Chlamydomonas reinhardtii: discovery of the betaine lipid synthase BTA1Cr

Lipid metabolism in flowering plants has been intensely studied, and knowledge regarding the identities of genes encoding components of the major fatty acid and membrane lipid biosynthetic pathways is very extensive. We now present an in silico analysis of fatty acid and glycerolipid metabolism in an algal model, enabled by the recent availability of expressed sequence tag and genomic sequences of Chlamydomonas reinhardtii. Genes encoding proteins involved in membrane biogenesis were predicted on the basis of similarity to proteins with confirmed functions and were organized so as to reconstruct the major pathways of glycerolipid synthesis in Chlamydomonas. This analysis accounts for the majority of genes predicted to encode enzymes involved in anabolic reactions of membrane lipid biosynthesis and compares and contrasts these pathways in Chlamydomonas and flowering plants. As an important result of the bioinformatics analysis, we identified and isolated the C. reinhardtii BTA1 (BTA1Cr) gene and analyzed the bifunctional protein that it encodes; we predicted this protein to be sufficient for the synthesis of the betaine lipid diacylglyceryl-N,N,N-trimethylhomoserine (DGTS), a major membrane component in Chlamydomonas. Heterologous expression of BTA1Cr led to DGTS accumulation in Escherichia coli, which normally lacks this lipid, and allowed in vitro analysis of the enzymatic properties of BTA1Cr. In contrast, in the bacterium Rhodobacter sphaeroides, two separate proteins, BtaARs and BtaBRs, are required for the biosynthesis of DGTS. Site-directed mutagenesis of the active sites of the two domains of BTA1Cr allowed us to study their activities separately, demonstrating directly their functional homology to the bacterial orthologs BtaARs and BtaBRs.     

Microalgal triacylglycerols as feedstocks for biofuel production: perspectives and advances

Microalgae represent an exceptionally diverse but highly specialized group of micro-organisms adapted to various ecological habitats. Many microalgae have the ability to produce substantial amounts (e.g. 20–50% dry cell weight) of triacylglycerols (TAG) as a storage lipid under photo-oxidative stress or other adverse environmental conditions. Fatty acids, the building blocks for TAGs and all other cellular lipids, are synthesized in the chloroplast using a single set of enzymes, of which acetyl CoA carboxylase (ACCase) is key in regulating fatty acid synthesis rates. However, the expression of genes involved in fatty acid synthesis is poorly understood in microalgae. Synthesis and sequestration of TAG into cytosolic lipid bodies appear to be a protective mechanism by which algal cells cope with stress conditions, but little is known about regulation of TAG formation at the molecular and cellular level. While the concept of using microalgae as an alternative and renewable source of lipid-rich biomass feedstock for biofuels has been explored over the past few decades, a scalable, commercially viable system has yet to emerge. Today, the production of algal oil is primarily confined to high-value specialty oils with nutritional value, rather than commodity oils for biofuel. This review provides a brief summary of the current knowledge on oleaginous algae and their fatty acid and TAG biosynthesis, algal model systems and genomic approaches to a better understanding of TAG production, and a historical perspective and path forward for microalgae-based biofuel research and commercialization.     

dEMBF: A Comprehensive Database of Enzymes of Microalgal Biofuel Feedstock

Microalgae have attracted wide attention as one of the most versatile renewable feedstocks for production of biofuel. To develop genetically engineered high lipid yielding algal strains, a thorough understanding of the lipid biosynthetic pathway and the underpinning enzymes is essential. In this work, we have systematically mined the genomes of fifteen diverse algal species belonging to Chlorophyta, Heterokontophyta, Rhodophyta, and Haptophyta, to identify and annotate the putative enzymes of lipid metabolic pathway. Consequently, we have also developed a database, dEMBF (Database of Enzymes of Microalgal Biofuel Feedstock), which catalogues the complete list of identified enzymes along with their computed annotation details including length, hydrophobicity, amino acid composition, subcellular location, gene ontology, KEGG pathway, orthologous group, Pfam domain, intron-exon organization, transmembrane topology, and secondary/tertiary structural data. Furthermore, to facilitate functional and evolutionary study of these enzymes, a collection of built-in applications for BLAST search, motif identification, sequence and phylogenetic analysis have been seamlessly integrated into the database. dEMBF is the first database that brings together all enzymes responsible for lipid synthesis from available algal genomes, and provides an integrative platform for enzyme inquiry and analysis. This database will be extremely useful for algal biofuel research. It can be accessed at http://bbprof.immt.res.in/embf.     

Hitchhikers guide to the vacuole-mechanisms of cargo sequestration in the Cvt and autophagic pathways

In the yeast Saccharomyces cerevisiae, precursor aminopeptidase I and alpha-mannosidase are selectively targeted via the Cvt and the autophagic pathways to the vacuole. Two studies now uncover the molecular basis of the selective sequestration of these two enzymes.     

Deciphering the uniqueness of Mucoromycotina cell walls by combining biochemical and phylogenomic approaches

Most fungi from the Mucoromycotina lineage occur in ecosystems as saprobes, although some species are phytopathogens or may induce human mycosis. Mucoromycotina represent early diverging models that are most valuable for understanding fungal evolution. Here we reveal the uniqueness of the cell wall structure of the Mucoromycotina Rhizopus oryzae and Phycomyces blakesleeanus compared with the better characterized cell wall of the ascomycete Neurospora crassa. We have analysed the corresponding polysaccharide biosynthetic and modifying pathways, and highlight their evolutionary features and higher complexity in terms of gene copy numbers compared with species from other lineages. This work uncovers the presence in Mucoromycotina of abundant fucose‐based polysaccharides similar to algal fucoidans. These unexpected polymers are associated with unusually low amounts of glucans and a higher proportion of chitin compared with N. crassa. The specific structural features are supported by the identification of genes potentially involved in the corresponding metabolic pathways. Phylogenomic analyses of genes encoding carbohydrate synthases, polysaccharide modifying enzymes and enzymes involved in nucleotide‐sugar formation provide evidence for duplication events during evolution of cell wall metabolism in fungi. Altogether, the data highlight the specificity of Mucoromycotina cell walls and pave the way for a finer understanding of their metabolism.     

藻类挥发性有机化合物研究进展

在水域生态系统中, 藻类释放的挥发性有机化合物(VOCs)种类众多, 主要有萜烯类、醛类、醇类、酯类、酮类、脂肪族烃、芳香族、硫化物和卤化物。这些VOCs通过不同的次生代谢途径形成, 可为多种环境因素所诱导产生。在逆境胁迫下, VOCs具有提高藻细胞抗逆性的作用。当VOCs释放到水体中后, 可为同种藻细胞传递胁迫信息, 使感受细胞做好防御准备。对于异种藻细胞, VOCs通过化感作用抑制其生长, 从而保证VOCs释放者的竞争优势。此外, VOCs还可驱避捕食者, 保护藻细胞免受伤害。未被感受者利用的VOCs经挥发进入大气层后, 会参与水域上空二次有机气溶胶的形成。通过对藻类VOCs的种类、形成途径、诱导释放因素及其生态作用进行综述, 以期对藻类VOCs的研究有所帮助。     

ALGAE AS USEFUL TOOLS IN CELLULOSE RESEARCH

This presentation will review the 1976 discovery of the enzyme complex in cellulose biosynthesis in Oocystis apiculata. A linear terminal complex (TC) was found to be associated with a microfibril, and from other freeze fracture applications, TCs have been found in many different algal genera. In fact, the algae have the most diverse and complex TCs among all organisms. TC diversity in terms of the evolution of cellulose biogenesis will be discussed. Combining the latest information from biochemistry and molecular genetics, the multiplicity of cellulose biogenesis will be reviewed. Cellulose molecular weight, crystalline structure, and mode of glycosylation for polymer formation all indicate that cellulose biogenesis is an extremely complex process. Major questions still remain, and the enzymes for cellulose biosynthesis have yet to be crystallized and their structure elucidated; however, the wealth of new information on cellulose structure and biosynthesis from algae to vascular plants, including bacteria and tunicates, all point to a very exciting and useful area of research. The algae have played key roles in our understanding of nature's most abundant macromolecule.     

Glycobiology of surface layer proteins

Over the last two decades, a significant change of perception has taken place regarding prokaryotic glycoproteins. For many years, protein glycosylation was assumed to be limited to eukaryotes; but now, a wealth of information on structure, function, biosynthesis and molecular biology of prokaryotic glycoproteins has accumulated, with surface layer (S-layer) glycoproteins being one of the best studied examples. With the designation of Archaea as a second prokaryotic domain of life, the occurrence of glycosylated S-layer proteins had been considered a taxonomic criterion for differentiation between Bacteria and Archaea. Extensive structural investigations, however, have demonstrated that S-layer glycoproteins are present in both domains. Among Gram-positive bacteria, S-layer glycoproteins have been identified only in bacilli. In Gram-negative organisms, their presence is still not fully investigated; presently, there is no indication for their existence in this class of bacteria. Extensive biochemical studies of the S-layer glycoprotein from Halobacterium halobium have, at least in part, unravelled the glycosylation pathway in Archaea; molecular biological analyses of these pathways have not been performed, so far. Significant observations concern the occurrence of unusual linkage regions both in archaeal and bacterial S-layer glycoproteins. Regarding S-layer glycoproteins of bacteria, first genetic data have shed some light into the molecular organization of the glycosylation machinery in this domain. In addition to basic S-layer glycoprotein research, the biotechnological application potential of these molecules has been explored. With the development of straightforward molecular biological methods, fascinating possibilities for the expression of prokaryotic glycoproteins will become available. S-layer glycoprotein research has opened up opportunities for the production of recombinant glycosylation enzymes and tailor-made S-layer glycoproteins in large quantities, which are commercially not yet available. These bacterial systems may provide economic technologies for the production of biotechnologically and medically important glycan structures in the future.     

甘油歧化为1,3-丙二醇的代谢及关键酶研究进展

微生物发酵生产1,3-丙二醇因对环境友好而成为研究热点。通过对发酵菌种、代谢途径、调节子和关键酶的分析,阐述了微生物转化甘油为1,3-丙二醇的分子机理。尤其对还原途径的限速酶-甘油脱水酶的分子结构及再激活因子进行了详细分析,为菌种的遗传改造提供了理论依据。     

Elucidating the composition and conservation of the autophagy pathway in photosynthetic eukaryotes

Aquatic photosynthetic eukaryotes represent highly diverse groups (green, red, and chromalveolate algae) derived from multiple endosymbiosis events, covering a wide spectrum of the tree of life. They are responsible for about 50% of the global photosynthesis and serve as the foundation for oceanic and fresh water food webs. Although the ecophysiology and molecular ecology of some algal species are extensively studied, some basic aspects of algal cell biology are still underexplored. The recent wealth of genomic resources from algae has opened new frontiers to decipher the role of cell signaling pathways and their function in an ecological and biotechnological context. Here, we took a bioinformatic approach to explore the distribution and conservation of TOR and autophagy-related (ATG) proteins (Atg in yeast) in diverse algal groups. Our genomic analysis demonstrates conservation of TOR and ATG proteins in green algae. In contrast, in all 5 available red algal genomes, we could not detect the sequences that encode for any of the 17 core ATG proteins examined, albeit TOR and its interacting proteins are conserved. This intriguing data suggests that the autophagy pathway is not conserved in red algae as it is in the entire eukaryote domain. In contrast, chromalveolates, despite being derived from the red-plastid lineage, retain and express ATG genes, which raises a fundamental question regarding the acquisition of ATG genes during algal evolution. Among chromalveolates, Emiliania huxleyi (Haptophyta), a bloom-forming coccolithophore, possesses the most complete set of ATG genes, and may serve as a model organism to study autophagy in marine protists with great ecological significance.