This is a study of a unique event in enzymology, involving a protein we first characterized in 1985 that is the key to a process fundamental to survival in higher forms of life, the synthesis of glycogen. The protein is glycogenin. Its amount and location control the amount of glycogen the cell can synthesize and where the glycogen is deposited, and its presence in a functioning form seems essential for the economy of the cell. Therefore we need to understand glycogenin's key role in the biogenesis of glycogen. The protein is unique in that it is autocatalytic. By using UDPglucose to attach a chain of glucose units to itself-the first one via a novel bond to tyrosine -glycogenin creates the priming chain on which glycogen synthase and branching enzyme build glycogen. Glycogenin remains part of the glycogen as a cryptic enzyme and, if released by glycogen depletion, becomes active again, ready to help restore glycogen. We wish to know how glycogenin synthesis is regulated, whether its activity is controlled by covalent modification (phosphorylation/dephosphorylation) and the location on the peptide chain of the several important functional sites, Mn2+ -binding (Mn2+ is an activator), the UDPglucose-binding site and the site(s) where inhibitory nucleotides bind. Progress has been greatly assisted by the ability to express wild-type and mutant glycogenins. in the coming project period we will use our array of recombinant proteins to study the structure and functioning of Glycogenin. Then we will examine how it interacts with glycogen synthase and try to define the complex that it has been claimed, is essential for glycogen synthesis to take place. This will include a study of a new form of glycogen synthase we discovered, that acts between glycogenin and the already known synthase to synthesize a second, intermediate 400 kDa form of glycogen, proglycogen, which goes on to become depot glycogen. In parallel tot he in vitro experiments, we will employ the immature frog oocyte as a """"""""living test tube"""""""" in which to study glycogenin and its participation in glycogen synthesis. The oocyte's own rudimentary glycogen synthesizing system is ideally suited to this purpose. Overall, our aim is to define precisely the initial events in glycogen synthesis. pursuing health related matters, we are localizing sequencing the human gene for glycogenin. We regard glycogenin as a plausible, so- far-untested candidate enzyme, aberrations in which may cause the abnormalities of glycogen metabolism seen in Type II diabetes. Using blood from Type II patients we will search for possible mutations in the glycogenin gene and attempt to express and study any glycogenin mutant thereby detected. Our work also has direct interest for exercise physiology and sports medicine in terms of the advantages of glycogen supercompensation for endurance. Glycogenin and proglycogen are the keys to supercompensation.
Showing the most recent 10 out of 21 publications
