Conformationally constrained oligosaccharides as probes for carbohydrate-protein interactions
Galan Hurtado, Maria del Carmen
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In this study, a range of conformationally constrained N-acetyl lactosamine derivatives (compounds 4-7) and their open analogues (compouns 2 and 3) were employed to probe how small changes in the conformational properties of glycosyl acceptors can affect the activity of a range of different transferases. The conformational properties of the cyclic disaccharides were studied using a combination of NOE and trans-glycosidic heteronuclear coupling constant measurements and molecular mechanics simulations. The apparent kinetic parameters of sialylation or fucosylation of acceptors 1-7 by a-2,6-, a-2,3-sialyltransferase, a-1,3-fucosyltransferases IV and VI were determined using reported assays and the results correlated with the conformational properties of these derivatives. This dissertation first demonstrates that conformationally restricted oligosaccharides can act as acceptors for glycosyltransferases. The kinetic parameters of transfer to 4, 5, and 6 by ˜-1,3-fucosyltransferases IV and VI and a-2,6- and a-2,3-sialyltransferases showed that for each enzyme compound 6 is markedly the best substrate indicating that all four transferases recognize LacNAc in a low minimum energy conformation. A small change in conformationally properties such as in compound 5 resulted in a significant loss of catalytic activity but different enzymes responded differently and the fucosyltransferases lost less activity than the sialyltransferases. Correlating the apparent kinetic parameters of conformational constrained 6 and 7 and their reference compounds 2 and 3 further support that different enzymes respond differently to conformational changes in an acceptor substrate and that small variations in the conformational properties of the acceptors result in large differences in catalytic efficiency. Furthermore these data also indicate that the fucosyltransferases recognize LacNAc in the A-conformer whereas the B-conformer is required for the sialyltransferases. It is also shown for the first time that the synthesis of conformationally constrained oligosaccharides can be simplified by employing a chemo-enzymatic approach. Conformationally constrained trisaccharides analogs of 6’-sialyllactosamine and Lewis x were prepared, whereby a lactosamine derivative, which has a methylene amide tether between C-6 and C-2’, was enzymatically sialylated and fucosylated using rat liver a-2,6- sialyltransferase or recombinant human a-1,3-fucosyltansferase V, respectively.