Structural basis for regulated inhibition and substrate selection in yeast glycogen synthase

Abstract

Glycogen synthase (GS) is the rate limiting enzyme in the synthesis of glycogen. Eukaryotic GS catalyzes the transfer of glucose from UDP-glucose to the non-reducing ends of glycogen and its activity is negatively regulated by phosphorylation and allosterically activated by glucose-6-phosphate (G6P). A highly conserved cluster of six arginine residues on the C-terminal domain controls the responses toward these opposing signals. Previous studies had shown that tetrameric enzyme exists in three conformational states which are linked to specific structural changes in the regulatory helices that carry the cluster of arginines. These helices are found opposite and anti-parallel to one another at one of the subunit interfaces. The binding of G6P beneath the regulatory helices induces large scale conformational changes which open up the catalytic cleft for better substrate access. We solved the crystal structure of the enzyme in its inhibited state and found that the tetrameric and regulatory interfaces are more compacted compared to other states. The structural consequence of the tighter interfaces within the inhibited state of the tetramer is to lower the ability of glycogen chains to access to the catalytic cleft. Based on these observations, we developed a novel regulatory feature in yeast GS by substituting two of its conserved arginine residues on the regulatory helix with cysteines that permits its activity to be controlled by reversible oxidation/reduction of the cysteine residues which mimics the effects of reversible phosphorylation. In addition to defining the structural changes that give rise to the inhibited states, we also used X-ray crystallography to define the mechanism by which the enzyme discriminates between different UDP-sugar donors to be used as substrates in the catalytic mechanism of yeast GS. We found that only donor substrates can adopt the catalytically favorable bent conformation for donor transfer to a growing glycogen chain.