Gene/Protein Disease Symptom Drug Enzyme Compound
Pivot Concepts:   Target Concepts:
Query: EC:2.4.99.6 (sialyltransferase)
 1,546 document(s) hit in 31,850,051 MEDLINE articles (0.00 seconds)

A series of biotinylated glycan-Asn derivatives has been synthesized containing either no extension arm between biotin and Asn (glycan-biotinyl Asn) or containing HN(CH2)nCO extension arms of differing lengths, where n denotes the number of methylene groups in the arm (glycan-biotinyl[HN(CH2)nCO]Asn, n = 1-5). The glycan structures used were Man6GlcNAc2-, Man5GlcNAc2-, GlcNAcMan5GlcNAc2- and Gal2GlcNAc2Man3GlcNAc2-, the substrates for mannosidase I, GlcNAc transferase I, mannosidase II and sialyltransferase, respectively. Each family of substrates was subjected to the action of its respective enzyme in the absence and in the presence of streptavidin, and the relative rate of processing (in the presence of UDP-GlcNAc and the mannosidase II inhibitor, swainsonine for GlcNAc transferase I and CMP-sialic acid for sialyl transferase) was measured to evaluate the effect of the proximity of the protein matrix on the glycan substrate quality. Mannosidase I was found to be strongly inhibited by the protein matrix in the proximal as well as in the distal positions relative to the glycan substrate. In contrast, GlcNAc transferase I and mannosidase II, which were both strongly inhibited by the proximal substrate complexes (no extension arm) showed complete release of the inhibition even with the shortest (n = 1) extension arm. Sialyl transferase showed inhibition of both reaction steps in the proximal complex, and complete release of the inhibition of the first step, but not the second step, in the distal complexes. The results show that the availability of different glycan substrates in a given protein environment reflects, to a great extent, the nature of each individual enzyme. The mechanisms by which the protein matrix affects glycan processing are proposed to involve simple steric effects, as well as more subtle effects of the protein in permitting or preventing certain active glycan conformations to form.
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PMID:The effect of the protein matrix proximity on glycan reactivity in a glycoprotein model. 788 15

During short incubations of a Golgi apparatus-enriched subcellular fraction from rat liver with UDP-[3H]GlcNAc, label is efficiently transferred to endogenous acceptors. Most of the macromolecular radioactivity is specifically released by peptide-N4-(N-acetyl-beta-glucosaminyl)asparagine amidase, indicating that it is mainly associated with N-linked oligosaccharides. The glycoprotein acceptors are resistant to proteases unless detergent is added in amounts greater than the critical micellar concentration. This shows that the acceptors are within the lumen of intact compartments, which have the correct topological orientation expected for the Golgi apparatus in intact cells. Structural characterization of the radiolabeled N-linked oligosaccharides shows a variety of distinct neutral and anionic species. The neutral chains include bi-, tri-, and tetra-antennary molecules with terminal beta-[3H] GlcNAc residues. In vitro sialylation shows that some of the tetra-antennary chains have beta 1,3-linked Gal residues on their unlabeled antennae. An unknown modification appears to block the action of beta-galactosidase on these galactosylated oligosaccharides. Chasing the labeling reaction with a mixtures of UDP-Gal, CMP-Neu5Ac, and adenosine 3'-phosphate,5'-phosphosulfate causes an increase in the percent of radiolabeled anionic oligosaccharides. Most of the negative charge is due to sialic acid (Sia), and some appears to be in phosphodiester-linked [3H]GlcNAc. The sialylated oligosaccharides are a mixture of bi-, tri-, and tetra-antennary species with 1-3-Sia residues, and some of the [3H]GlcNAc residues are directly covered with unlabeled Gal and Sia residues. This in vitro approach should recapitulate reactions that occur in the biosynthesis of N-linked oligosaccharides in the Golgi apparatus of the intact cell. Since the conditions during labeling do not permit inter-compartmental transport, the oligosaccharides produced should represent the biosynthetic capabilities of individual Golgi compartments. Evidence is presented for a functional association of GlcNAc transferases I, II, and alpha-mannosidase II, with separation from GlcNAc transferase IV and/or V. The structures also indicate co-compartmentalization of several GlcNAc transferase(s) with beta-galactosyltransferase(s) and sialyltransferase(s). The compartmental organization of the Golgi apparatus is discussed in light of these findings.
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PMID:Biosynthesis of oligosaccharides in intact Golgi preparations from rat liver. Analysis of N-linked glycans labeled by UDP-[6-3H]N-acetylglucosamine. 834 99