Structure and dynamics of sphingomyelin bilayer: insight gained through systematic comparison to phosphatidylcholine

Sphingomyelin, one of the main lipid components of biological membranes, is actively involved in various cellular processes such as protein trafficking and signal transduction. In particular, specific lateral domains enriched in sphingomyelin and cholesterol have been proposed to play an important functional role in biomembranes, although their precise characteristics have remained unclear. A thorough understanding of the functional role of membranes requires detailed knowledge of their individual lipid components. Here, we employ molecular dynamics simulations to conduct a systematic comparison of a palmitoylsphingomyelin (PSM, 16:0-SM) bilayer with a membrane that comprises dipalmitoylphosphatidylcholine (DPPC) above the main phase transition temperature. We clarify atomic-scale properties that are specific to sphingomyelin due to its sphingosine moiety, and further discuss their implications for SM-rich membranes. We find that PSM bilayers, and in particular the dynamics of PSM systems, are distinctly different from those of a DPPC bilayer. When compared with DPPC, the strong hydrogen bonding properties characteristic to PSM are observed to lead to considerable structural changes in the polar headgroup and interface regions. The strong ordering of PSM acyl chains and specific ordering effects in the vicinity of a PSM-water interface reflect this issue clearly. The sphingosine moiety and related hydrogen bonding further play a crucial role in the dynamics of PSM bilayers, as most dynamic properties, such as lateral and rotational diffusion, are strongly suppressed. This is most evident in the rotational motion characterized by spin-lattice relaxation times and the decay of hydrogen bond autocorrelation functions that are expected to be important in complexation of SM with other lipids in many-component bilayers. A thorough understanding of SM bilayers would greatly benefit from nuclear magnetic resonance experiments for acyl chain ordering and dynamics, allowing full comparison of these simulations to experiments.     

Tissue distribution of S-adenosylmethionine and S-adenosylhomocysteine in the rat. Effect of age, sex and methionine administration on the metabolism of S-adenosylmethionine, S-adenosylhomocysteine and polyamines

In the presence of ATP and Mg2+, ATP sulphurylase from Saccharomyces cerevisiae catalysed the conversion of selenate into a compound with the electrophoretic and acid-lability properties of adenosine 5'-sulphatophosphate. Structural characterization, involving extensive purification of adenosine 5'-selenophosphate, proved impossible. However, we showed ATP-, Mg2+- and ATP sulphurylase-dependent, and inorganic pyrophosphatase-stimulated, production of elemental selenium from selenate in the presence of GSH (reduced glutathione). Since selenate was not reduced by GSH, this reaction proved that ATP sulphurylase had formed an active selenate. The enzyme catalysed formation of elemental selenium had the same kinetics and GSH-dependency as the non-enzymic reduction of selenite to elemental selenium by GSH. In the presence of inorganic pyrophosphatase, 2 mol of Pi was released for each mol of 'active selenate' formed. This was shown by a spectrophotometric assay for elemental selenium. The observed reactivity with thiols and the instability of the enzymic product were those predicted for selenium anhydrides. By analogy with the chemistry of sulphur, the product of the thiolytic cleavage of a selenium anhydride would be converted into selenite. The selenite would then be reduced by the thiol to elemental selenium. We conclude that ATP sulphurylase can catalyse the formation of adenosine 5'-selenophosphate. The anhydride can be reduced by thiols in a manner similar to the reduction of selenite. These results probably explain the ability of mammals, lacking a sulphate reductase system, to incorporate selenium from selenate into seleno-amino acids.     

Genetic variation in the hTAS2R38 taste receptor and food consumption among Finnish adults

Genetic variation in bitter taste receptors, such as hTAS2R38, may affect food preferences and intake. The aim of the present study was to investigate the association between bitter taste receptor haplotypes and the consumption of vegetables, fruits, berries and sweet foods among an adult Finnish population. A cross-sectional design utilizing data from the Cardiovascular Risk in Young Finns cohort from 2007, which consisted of 1,903 men and women who were 30–45 years of age from five different regions in Finland, was employed. DNA was extracted from blood samples, and hTAS2R38 polymorphisms were determined based on three SNPs (rs713598, rs1726866 and rs10246939). Food consumption was assessed with a validated food frequency questionnaire. The prevalence of the bitter taste-sensitive (PAV/PAV) haplotype was 11.3 % and that of the insensitive (AVI/AVI) haplotype was 39.5 % among this Finnish population. PAV homozygotic women consumed fewer vegetables than did the AVI homozygotic women, 269 g/day (SD 131) versus 301 g/day (SD 187), respectively, p = 0.03 (multivariate ANOVA). Furthermore, the intake of sweet foods was higher among the PAV homozygotes of both genders. Fruit and berry consumption did not differ significantly between the haplotypes in either gender. Individuals perceive foods differently, and this may influence their patterns of food consumption. This study showed that the hTAS2R38 taste receptor gene variation was associated with vegetable and sweet food consumption among adults in a Finnish population.     

Induced hypertension for the treatment of acute MCA occlusion beyond the thrombolysis window: case report

Background  

A minority of stroke patients is eligible for thrombolytic therapy. Small pilot case series have hinted that elevation of incident arterial blood pressure might be associated with a favorable prognosis either in acute or subacute stroke. However, these patients were not considered for thrombolytic therapy and were not followed – up systematically. We used pharmacologically induced hypertension in a stroke patient with middle cerebral artery (MCA) occlusion ineligible for thrombolysis that was followed-up by radiological, clinical and functional outcome assessment.     

The layered fold of the TSR domain of P. falciparum TRAP contains a heparin binding site

Thrombospondin-related anonymous protein, TRAP, has a critical role in the hepatocyte invasion step of Plasmodium sporozoites, the transmissible form of the parasite causing malaria. The extracellular domains of this sporozoite surface protein interact with hepatocyte surface receptors whereas its intracellular domain acts as a link to the sporozoite actomyosin motor system. Liver heparan sulfate proteoglycans have been identified as potential ligands for TRAP. Proteoglycan binding has been associated with the A- and TSR domains of TRAP. We present the solution NMR structure of the TSR domain of TRAP and a chemical shift mapping study of its heparin binding epitope. The domain has an elongated structure stabilized by an array of tryptophan and arginine residues as well as disulfide bonds. The fold is very similar to those of thrombospondin type-1 (TSP-1) and F-spondin TSRs. The heparin binding site of TRAP-TSR is located in the N-terminal half of the structure, the layered side chains forming an integral part of the site. The smallest heparin fragment capable of binding to TRAP-TSR is a tetrasaccharide.     

SH3 domain ligand binding: What’s the consensus and where’s the specificity?

An increasing number of SH3 domain-ligand interactions continue to be described that involve the conserved peptide-binding surface of SH3, but structurally deviate substantially from canonical docking of consensus motif-containing SH3 ligands. Indeed, it appears that that the relative frequency and importance of these types of interactions may have been underestimated. Instead of atypical, we propose referring to such peptides as type I or II (depending on the binding orientation) non-consensus ligands. Here we discuss the structural basis of non-consensus SH3 ligand binding and the dominant role of the SH3 domain specificity zone in selective target recognition, and review some of the best-characterized examples of such interactions.     

Loosening of globular structure under alkaline pH affects accessibility of β-lactoglobulin to tyrosinase-induced oxidation and subsequent cross-linking

Globular proteins such as β-lactoglobulin (BLG) are poorly accessible to enzymes. We have studied susceptibility of BLG to oxidation by Trichoderma reesei (TrTyr) and Agaricus bisporus (AbTyr) tyrosinases and subsequent intermolecular cross-linking with respect to pH-induced structural changes. We evaluated pH-induced structural changes in BLG using circular dichroism, tryptophan fluorescence and small angle X-ray scattering (SAXS) measurements, where after these results were correlated with the analysis of cross-linking by sodium dodecyl sulphate polyacrylamide gel electrophoresis. Oxygen consumption measurement and changes in radii of gyration determined by SAXS during the enzyme-induced oxidation at the respective reaction conditions were also followed. Intermolecular cross-linking of BLG by TrTyr was found at pH 9 but not at pH 7.5. AbTyr was unable to catalyze cross-linking at pH 7.5 or pH 9. Increased accessibility and cross-linking by TrTyr was addressed to loosening of the three dimensional structure of the protein, increased flexibility of the backbone as well as partial hydrolysis. In addition to basic research of the effect of protein folding on enzymatic cross-linking the research results have significance on the exploitation of TrTyr at alkaline conditions.