Regulation of glycogen synthetase in a freeze-avoiding insect: Role in cryoprotectant glycerol metabolism

Glycogen synthetase D was purified 131-fold from autumn-collected larvae of the freeze-avoiding gall moth Epiblema scudderiana; final specific activit y was 16.1 U/mg protein. Arrhenius plots suggested a conformational change in the enzyme at temperatures below 9 degrees C with activation energy bein g 2.8-fold higher over the lower (3-9 degrees C) versus higher (9-30 degree s C) assay range. The enzyme also showed reduced affinity for substrates an d effecters at low assay temperatures; Km UDPG and glycogen were 1.8- and 4 .4-fold higher at 4 degrees C than at 22 degrees C and Ka for glucose-6-pho sphate was 2.9-fold higher. These properties would suppress enzyme function at low temperature and thereby facilitate the cold-stimulated conversion o f glycogen reserves into glycerol by helping to block the recycling of carb on into glycogen.