Anticoagulant heparin has been shown to possess important biological functions that vary according to its fine structure. Variability within heparin's structure occurs owing to its biosynthesis and animal tissue-based recovery, and adds another dimension to its complex polymeric structure. The structural variations in chain length and sulfation patterns mediate its interaction with many heparin-binding proteins, thereby, eliciting complex biological responses. The advent of novel chemical and enzymatic approaches for polysaccharide synthesis coupled with high throughput combinatorial approaches for drug discovery have facilitated an increased effort to understand heparin's structure-activity relationships. An improved understanding would offer potential for new therapeutic development through the engineering of polysaccharides. Such a bioengineering approach requires the amalgamation of several different disciplines including carbohydrate synthesis, applied enzymology, metabolic engineering, and process biochemistry.
The chemoenzymatic synthesis of heparan sulfate tetrasaccharide (1) and hexasaccharide (2) with a fluorous tag attached at the reducing end is reported. The fluorous tert-butyl dicarbonate (FBoc) tag did not interfere with enzymatic recognition for both elongation and specific sulfation, and flash purification was performed by standard fluorous solid-phase extraction (FSPE). Based on an FBoc attached disaccharide as acceptor, a series of partial N-sulfated, 6-O-sulfated heparan sulfate oligosaccharides were successfully synthesized employing fluorous techniques.
Contamination in heparin batches during early 2008 has resulted in a significant effort to develop a safer bioengineered heparin using bacterial capsular polysaccharide heparosan and recombinant enzymes derived from the heparin/heparan sulfate biosynthetic pathway. This requires controlled chemical N-deacetylation/ N-sulfonation of heparosan followed by epimerization of most of its glucuronic acid residues to iduronic acid and O-sulfation of the C2 position of iduronic acid and the C3 and C6 positions of the glucosamine residues. A combinatorial study of multi-enzyme, one-pot, in vitro biocatalytic synthesis, carried out in tandem with sensitive analytical techniques, reveals controlled structural changes leading to heparin products similar to animal- derived heparin active pharmaceutical ingredients. Liquid chromatography-mass spectrometry and nuclear magnetic resonance spectroscopy analysis confirms an abundance of heparin’s characteristic trisulfated disaccharide, as well as 3-O-sulfo containing residues critical for heparin binding to antithrombin III and its anticoagulant activity. The bioengineered heparins prepared using this simplified one-pot chemoenzymatic synthesis also show in vitro anticoagulant activity.
Sorption of three divalent toxic metal ions, viz. copper, lead, and zinc, onto a low-cost aluminosilicate mineral, pyrophyllite, was studied in a single component system at four different temperatures. Four isothermsFreundlich, Langmuir, Redlich−Peterson, and Temkinwere examined in order to test their suitability for the given sets of experimental data. A comparison of the linear least-squares method and a trial-and-error nonlinear method of the four isotherms was done. The Langmuir isotherm had four different linear forms, and all of them were individually studied and compared with the others. The parameters of all the four forms of the Langmuir isotherm obtained by the linear method were different. Langmuir-I is the most popular form of the Langmuir isotherm having the highest coefficient of determination values (0.996886) compared with other linear forms of the Langmuir isotherm. The Redlich−Peterson isotherm produced a higher coefficient of determination values (0.999234) in comparison to all the isotherms studied.
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