Study of the function of pyridoxal 5'-phosphate in glycogen phosphorylase

Chang, Yen-Chung
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Biochemistry, Biophysics and Molecular Biology

Phosphorylase reconstituted with pyridoxal, 6-fluoro-pyridoxal (6-FPAL), and 6-fluoropyridoxal phosphate (6-FLPL) were studied in this dissertation work to gain more insight of the involvement of the 5'-phosphoryl, 3-OH, and 1-nitrogen groups of pyrodoxal 5'-phosphorylase (PLP) in the catalytic process of glycogen phosphorylase. And the conformational changes around the coenzyme binding site during the "T state" (DBLARR) "R state" transition of glycogen phosphorylase was also examined;An analysis of apparent kinetic parameters of varied anions for the activation of pyridoxal phosphorylase and ('19)F NMR spectral study of the interaction between fluorophosphate and pyridoxal enzyme suggested that the 5'-phosphoryl group of enzyme-bound PLP does not affect the binding of glucose-1-P to the enzyme, that this phosphoryl group is unlikely to participate in any acid-base reaction essential for catalysis, and that the phosphoryl group could be distorted into a trigonal-bipyramidal geometry during catalysis, and that the phosphoryl group could be distorted into a trigonal-bipyramidal geometry during catalysis. Kinetic and NMR, UV-VIS, and fluorescence spectral studies of phosphorylase reconstituted with 6-FPAL and 6-FPLP reconfirmed that the enzyme-bound PLP is a neutral enolimine tautermer and suggested that the proton of the 3-OH in PLP is unlikely to be involved in any proton shuttle during catalysis. However, the ring nitrogen of PLP may interact with the protein, and this interaction may be important for efficient catalysis. Studies of nuclear relaxation mechanisms of the fluorine nucleus in 6-FPAL and 6-FPLP enzymes indicated that the protein structure around the coenzyme bind site undergoes substantial changes during the "T" to "R" transition. In the "R" state phosphorylase, the coenzyme is bound more tightly to the protein than in the "T" state enzyme, and the configuration of PLP, the relative orientation of the 5'-phosphate and 6-proton of the coenzyme, could be different in these two conformers.