Purification and properties of glycerol-3-phosphate dehydrogenase from the liver of the hibernating ground squirrel, Urocitellus richardsonii.

Abstract

Cytosolic glycerol-3-phosphate dehydrogenase (G3PDH, EC 1.1.1.8) is an important branch point enzyme connecting lipid metabolism and carbohydrate metabolism. We investigated the dynamic nature of G3PDH by purifying the enzyme from the liver of Richardson's ground squirrel (Urocitellus richardsonii), a hibernating species, and analyzing its structural and functional changes during hibernation. Kinetic parameters of purified G3PDH from ground squirrel liver were characterized at 37, 22 and 5°C and compared between euthermic and hibernating states. Relative to euthermic liver G3PDH, hibernator liver G3PDH had a decreased affinity for its substrate, glycerol-3-phosphate (G3P), at 37°C and 22°C. However, at 5°C, there was a significant increase in the affinity for G3P in the hibernating form of the enzyme, relative to the euthermic form. Furthermore, the structure of G3PDH in the species' hibernating state showed greater thermal stability compared to its structure in the euthermic state. Western blot analysis revealed greater tyrosine phosphorylation in hibernator G3PDH as compared to euthermic G3PDH. In addition, using the protein sequence of the hibernating thirteen-lined ground squirrel (Ictidomys tridecemlineatus) and bioinformatics tools, a three-dimensional model of G3PDH was built to identify the potential phosphorylation site (83Tyr) responsible for the differential phosphorylation between euthermic and hibernator G3PDH. The structural and functional changes in G3PDH support the enzyme's function at a low core body temperature experienced during the species hibernating season.