Lipid intermediates

Lipids have critical roles in nervous system structure and function. Synaptic complexes and myelin are characterized by unique lipid compositions that contribute to the specialized properties of these nervous system structures. Multiple signaling pathways involving lipid intermediates regulate cell differentiation and synaptic transmission. 

Glycosyl transfer to hydrophobic acceptors by polyenzyme systems is a possibility. Transfer of sugar to endogenous lipid intermediates present in microsomal material from liver has been implied in the incorporation of glucose, mannose, and iV-acetylglucosamine into endogenous glycoprotein (A4, B6, B7, B8, D2, Mil, R7, T5). Dolichol monophosphate... 

One of the most impressive findings has been the discovery of lipid intermediates in the biosynthesis of polysaccharides (see Refs. 2 and 465.) At least two structurally different types of these compounds exist the intermediate may be an isoprenoid alcohol ester of the glycosyl pyrophosphate or the analogous derivative of the glycosyl phosphate. Derivatives of the first type are formed by reaction between the sugar nucleotide and the alcohol phosphate, for example, undecaprenyl phosphate (120), which participates in the biosynthesis of Salmonella lipopolysaccharide.466... 

A unique, membrane-associated, sialyltransferase system in E. coli has been described that catalyzes the synthesis of sialic acid polymers with the aid of sialyl-monophosphonoundecaprenol.293 In mammalian systems, no evidence was obtained lor the involvement of lipid intermediates in sialic acid transfer, or in the fonnation of sialyl-(2— -8) link-... 

The enzymes catalyzing these reactions are membrane-bound, and are stimulated by exogenous dolichol phosphate. It has been suggested, and now shown43 (at least in the formation of GlcNAc-PP-Dol by enzymes from yeast), that Dol-P stimulates the incorporation of sugar residues into the lipid intermediates because it serves as a substrate. 

The polyprenols obtained from tobacco and Arum maculatum (solanesol and spadicol), which are all E, are the exceptions. Another interesting fact is that, in most seed-plants, the polyprenols are allylic, as is bacterial undecaprenol. Saturated or slightly modified polyprenols, such as animal dolichol, seem to be present only in fungi. Any of those plant polyprenols, in a phosphorylated form, can be potential sugar acceptors for transfer reactions. Experiments were performed with phosphorylated allylic polyprenols and membrane preparations from Phaseolus aureus. D-Mannose from GDP-D-mannose was incorporated into the exogenous, polyprenyl phosphates,29,49 52 but it was not possible to establish unequivocally the role of these polyprenols as lipid intermediates. 

No experimental evidence is as yet available that would throw light on the turnover of lipid intermediates. Nevertheless, the cycle of dolichol in... 

In fungi, and in higher plants, the cell wall is a dynamic structure, changing during growth and differentiation, or in response to environmental factors. The main polysaccharides found in plant cell-walls are listed in this Section. The biosynthesis of those which are synthesized by way of lipid intermediates (see Table V) will be analyzed later. 

Polymer Precursor Source Product linkage Lipid intermediates References... 

The mechanism of synthesis of polysaccharides is a controversial issue. After discovery, by Cardini s group, that starch may be polymerized on a protein,152,153 it was suggested153 that all nascent, polysaccharide chains might be covalently associated with a protein. Connected with the formation of glycoprotein is the involvement of lipid intermediates. We shall analyze the biosynthetic pathways of polysaccharides where partial or complete evidence of this kind of mechanism has been educed. 

Glycoproteins Fonnd in Plants, Type of Linkage between the Peptide and Saccharide Moieties, and Involvement of Lipid Intermediates in Their Biosynthesis... 

Little is known about the regulation mechanisms of the synthesis of complex carbohydrate in plants, through lipid intermediates. However, partial evidence indicates that lipid-mediated glycosylation in proteins could be a regulatory step. When glycosylation of carboxypeptidase Y is inhibited... 

Finally, details of the synthesis of heteropolysaccharides in plants are as yet completely unknown. The structural similarities among some plant gums and such bacterial exopolysaccharides as xanthan gum suggest that similar mechanisms may be operative in bacteria and in plants. Lipid intermediates could be suggested as potential glycosyl donors in the formation of plant gums and mucilages. 

In recognition of the difficulty in obtaining the lipid intermediates in quantities to facilitate detailed mechanistic study of the peptidoglycan biosynthetic enzymes, we initiated an effort directed toward a chemical synthesis of both lipid I and lipid II. As was the case with the fermentation/isolation protocols, we anticipated several technical challenges that would need to be addressed in order to reach the target compounds. These challenges, and their solutions, will be discussed in the sections that follow. 

Similar concerns regarding the stability of our activated lipid intermediate led us to reject a method that involves the in situ preparation of lipid-derived phosphoric anhydrides.22 The common theme for each of the methods illustrated above is that each involves activation of the lipid intermediate for the diphosphate coupling reaction. Given the expense of the undecaprenyl monophosphate precursor, we sought a method that would entail activation of the carbohydrate fragment for capture by an undecaprenyl monophosphate nucleophile. This, in principle, would also allow the opportunity for the coupling reaction to take place under basic reaction conditions and enable us to preserve the chemical integrity of our expensive lipid precursor. 

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