UBC Theses and Dissertations
Protein - carbohydrate interactions in glycogen phosphorylase Street, Ian Philip
It has long been observed that some organo-fluorine compounds exhibit enhanced biological activity over their non-fluorinated precursors, however reasons for these unusual properties still remain poorly understood. An explanation which has been widely used relates to the ability of the C-F fragment of the analog to participate in hydrogen-bonding interactions with its protein receptor. For this reason, fluorinated carbohydrates have been used as hydrogen-bonding probes with a number of proteins. Thus there exists a need for a systematic investigation into the hydrogen-bonding ability of the C-F fragment, and the enzyme glycogen phosphorylase provides an excellent subject for such a study. The glucopyranose binding site in the inactive (T-state) conformation of the enzyme has been well characterised and high resolution crystallographic data is available. Thus by comparison of kinetic and crystallographic data for the natural effectors and the fluorinated substrate analogs considerable insight into the hydrogen bonding ability of the C-F fragment and the nature of carbohydrate-protein interactions should be gained. Little is known about the active (R-state) conformation of the enzyme and about the T-state to R-state transition. Use of fluorinated analogs of the enzymes natural substrate, glucose-l-phosphate, could also shed light on these questions. With these aims in mind, all of the isomeric mono-fluorinated derivatives of glucose and glucose-l-phosphate have been synthesised. Some deoxy and difluorinated analogs of glucose and mannose have also been prepared. Kinetic results obtained using the analogs of glucose indicate that the 3 and 6 positions of the sugar participate in strong hydrogen-bonding interactions with the protein while the other positions are only involved in relatively weak interactions. These results agree well with recent X-ray crystallographic data. None of the analogs of glucose-l-phosphate exhibited any substrate activity. The 2-deoxyfluoro analog had a similar affinity to glucose-1-phosphate and therefore probably binds in the same mode. The lack of substrate activity in this case can be explained by the destabi1isation of the putative oxo-carbonium ion intermediate at C(l), by the adjacent fluorine substituent. The other analogs of glucose-l-phosphate showed lower affinity for the enzyme. The similar inhibition constants obtained for these compounds suggested a binding mode in which the glucopyranose ring contributes little to the overall binding energy. This has led to the proposal of a molecular mechanism for the T-state to R-state transition.
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