Synthesis of 4-nitrophenyl O-(2-acetamido-2-deoxy-beta-D-glucopyranosyl)- (1-->2)-O-(6-O-methyl-alpha-D-mannopyranosyl)-(1-->6)-beta-D- glucopyranoside and its 4',6'-di-O-methyl analog. Potential inhibitors of N-acetylglucosaminyl-transferase V (GnT-V)

From methyl mannosides to methyl octosides by a stepwise homologation with Grignard C(1) reagents

A four-step procedure for homologation of methyl alpha-D-mannofuranoside and alpha-D-mannopyranoside was examined. The reactions consisted in (i) oxidation of the terminal hydroxymethyl group in a protected sugar derivative to an aldehyde; (ii) reaction with allyloxymethylmagnesium chloride (or (phenyldimethyl)silylmethyl-magnesium chloride); (iii) protection of the newly formed secondary alcohol group; (iv) deprotection of the terminal CH(2)OR (or oxidation of the CH(2)SiMe(2)Ph) group. From methyl alpha-D-mannosides, stereoisomeric DalphaD and LalphaD methyl heptosides and from them, methyl octosides of D-threo- and L-erythro-alpha-D-manno configuration were obtained.

Synthetic substrate analogues for UDP-GlcNAc: Man alpha 1-3R beta 1-2-N-acetylglucosaminyltransferase I. Substrate specificity and inhibitors for the enzyme

UDP-GlcNAc:Man alpha 1-3R beta 1-2-N-acetylglucosaminyltransferase I (GlcNAc-T I; EC 2.4.1.101) catalyses the conversion of [Man alpha 1-6(Man alpha 1-3)Man alpha 1-6][Man alpha 1-3]Man beta-O-R to [Man alpha 1-6(Man alpha 1-3)Man alpha 1-6] [GlcNAc beta 1-2Man alpha 1-3]Man beta-O-R (R = 1-4GlcNAc beta 1-4GlcNAc- Asn-X) and thereby controls the conversion of oligomannose to complex and hybrid asparagine-linked glycans (N-glycans). GlcNAc-T I also catalyses the conversion of Man alpha 1-6(Man alpha 1-3)Man beta-O-octyl to Man alpha 1-6(GlcNAc beta 1-2Man alpha 1-3)Man beta-O-octyl. We have therefore tested a series of synthetic analogues of Man"alpha 1-6(Man'alpha 1-3)Man beta-O-octyl as substrates and inhibitors for rat liver GlcNAc-T I. The 2"-deoxy and the 3"-, 4"- and 6"-O-methyl derivatives are all good substrates confirming previous observations that the hydroxyl groups of the Man"alpha 1-6 residue do not play major roles in the binding of substrate to enzyme. In contrasts, all four hydroxyl groups on the Man'alpha 1-3 residue are essential since the corresponding deoxy derivatives either do not bind (2'- and 3'-deoxy) or bind very poorly (4'- and 6'-deoxy) to the enzyme. The 2'- and 3'-O-methyl derivatives also do not bind to the enzyme. However, the 4'-O-methyl derivative is a substrate (KM = 2.6 mM) and the 6'-O-methyl compound is a competitive inhibitor (Ki = 0.76 mM). We have therefore synthesized various 4'- and 6'-O-alkyl derivatives, some with reactive groups attached to an O-pentyl spacer, and tested these compounds as reversible and irreversible inhibitors of GlcNAc-T I. The 6'-O-(5-iodoacetamido-pentyl) compound is a specific time dependent inhibitor of the enzyme. Four other 6'-O-alkyl compounds showed competitive inhibition while the remaining compounds showed little or no binding indicating that the electronic properties of the attached O-pentyl groups influence binding.

Synthesis of beta-D-GlcpNAc-(1-->2)-5a-carba-alpha-D-Man p-(1-->6)-beta-D- Glcp-O(CH2)7CH3: a reactive acceptor analog for N-acetylglucosaminyltransferase-V

The branching enzyme N-acetylglucosaminyltransferase-V (GlcNAcT-V) recognizes the trisaccharide beta-D-GlcpNAc-(1-->2)-alpha-D-Man p-(1-->6)-beta-D-Glcp-O(CH2)7CH3 (1) as its minimum substrate. We report here the chemical synthesis of beta-D-GlcpNAc-(1-->2)-5a- carba-alpha-D-Manp-(1-->6)-beta-D-Glcp-O(CH2)7CH3 (2), a carbocyclic analog of 1 where the ring oxygen of the alpha-D-Manp residue is replaced by a methylene group. Trisaccharide 2 was found to be fully active as an acceptor for GlcNAcT-V, both with the enzyme isolated from hamster kidney and the one cloned from rat kidney. The kinetic parameters Km and Vmax for 1 and 2 were functionally equivalent. A preparative glycosylation reaction was performed using 2 as the acceptor with the cloned rat kidney enzyme. A tetrasaccharide formed by the addition of a Glc pNAc residue was the sole product as detected by 1H NMR spectroscopy and FAB mass spectrometry and was assigned the structure beta-D-GlcpNAc-(1-->2)-[beta-D-GlcpNAc-(1-->6)]-5a- carba-alpha-D-Manp-(1-->6)-beta-D-Glc p-O(CH2)7CH3 (13).

Substrate specificity and inhibition of UDP-GlcNAc:GlcNAc beta 1-2Man alpha 1-6R beta 1,6-N-acetylglucosaminyltransferase V using synthetic substrate analogues

UDP-GlcNAc:GlcNAc beta 1-2Man alpha 1-6R (GlcNAc to Man) beta 1,6- N-acetylglucosaminyltransferase V (GlcNAc-T V) adds a GlcNAc beta 1-6 branch to bi- and triantennary N-glycans. An increase in this activity has been associated with cellular transformation, metastasis and differentiation. We have used synthetic substrate analogues to study the substrate specificity and inhibition of the partially purified enzyme from hamster kidney and of extracts from hen oviduct membranes and acute myeloid leukaemia leukocytes. All compounds with the minimum structure GlcNAc beta 1-2Man alpha 1-6Glc/Man beta-R were good substrates for GlcNAc-T V. The presence of structural elements other than the minimum trisaccharide structure affected GlcNAc-T V activity without being an absolute requirement for activity. Substrates with a biantennary structure were preferred over linear fragments of biantennary structures. Kinetic analysis showed that the 3-hydroxyl of the Man alpha 1-3 residue and the 4-hydroxyl of the Man beta- residue of the Man alpha 1-6(Man alpha 1-3)Man beta-R N-glycan core are not essential for catalysis but influence substrate binding. GlcNAc beta 1-2(4,6-di-O-methyl-)Man alpha 1-6Glc beta-pnp was found to be an inhibitor of GlcNAc-T V from hamster kidney, hen oviduct microsomes and acute and chronic myeloid leukaemia leukocytes.

Synthesis of a di-O-methylated pentasaccharide for use in the assay of N-acetylglucosaminyltransferase III activity

The biantennary oligosaccharide analogue beta-D-Glc pNAc-(1-->2)-alpha-D-Man p-(1-->3)-[beta-D-Glc pNAc-(1-->2)-alpha- D-Man p-(1-->6)]-beta-D-man p-O(CH2)8COOMe (3) is a potential substrate for N-acetylglucosaminyltransferases (GlcNAcTs) III-V which are present in mammalian cells. The di-O-methylated analogue of 3, beta-D-Glc pNAc-(1-->2)-[4-O-methyl-alpha-D-Man p]-(1-->3)-[beta-D-Glc pNAc-(1-->2)-[6-O-methyl-alpha-D-Man p]-(1-->6)]-beta-D-Man p-O-(CH2)8COOMe (5), was prepared by a block synthesis approach involving sequential addition of two O-methylated disaccharide donors to a protected central beta-D-Man residue. The OH groups acted on by GlcNAcT-IV and -V are protected from glycosylation in 5 since they are present as their methyl ethers. Pentasaccharide 5 was found to be an excellent substrate for GlcNAcT-III (EC 2.4.1.144) from rat kidney with Km = 0.15 mM. The product formed by incubation of 5 with a rat kidney extract, in the presence of UDP-GlcNAc, was isolated, structurally characterized by NMR spectroscopy and confirmed to be the expected di-O-methyl hexasaccharide where a beta-D-Glc pNAc residue had been added to OH-4 of the central beta-D-Man p unit.

Synthetic substrate analogues for UDP-GlcNAc: Man alpha 1-6R beta(1-2)-N-acetylglucosaminyltransferase II. Substrate specificity and inhibitors for the enzyme

UDP-GlcNAc:Man alpha 1-6R beta(1-2)-N-acetylglucosaminyltransferase II (GlcNAc-T II; EC 2.4.1.143) is a key enzyme in the synthesis of complex N-glycans. We have tested a series of synthetic analogues of the substrate Man"'alpha 1-6(GlcNAc"beta 1-2Man'alpha 1-3)Man beta-O-octyl as substrates and inhibitors for rat liver GlcNAc-T II. The enzyme attaches N-acetylglucosamine in beta 1-2 linkage to the 2"'-OH of the Man"'alpha 1-6 residue. The 2"'-deoxy analogue is a competitive inhibitor (Ki = 0.13 mM). The 2"'-O-methyl compound does not bind to the enzyme presumably due to steric hindrance. The 3"'-, 4"'- and 6"'-OH groups are not essential for binding or catalysis since the 3"'-, 4"'- and 6"'-deoxy and -O-methyl derivatives are all good substrates. Increasing the size of the substituent at the 3"'-position to pentyl and substituted pentyl groups causes competitive inhibition (Ki = 1.0-2.5 mM). We have taken advantage of this effect to synthesize two potentially irreversible GlcNAc-T II inhibitors containing a photolabile 3"'-O-(4,4-azo)pentyl group and a 3"'-O-(5-iodoacetamido)pentyl group respectively. The data indicate that none of the hydroxyls of the Man"'alpha 1-6 residue are essential for binding although the 2"'- and 3"'-OH face the catalytic site of the enzyme. The 4-OH group of the Man beta-O-octyl residue is not essential for binding or catalysis since the 4-deoxy derivative is a good substrate; the 4-O-methyl derivative does not bind.(ABSTRACT TRUNCATED AT 250 WORDS)