Structures of enterococcal glycerol kinase in the absence and presence of glycerol: Correlation of conformation to substrate binding and a mechanism of activation by phosphorylation

The first structure of a glycerol kinase from a Gram-positive organism, Enterococcus casseliflavus, has been determined to 2.8 Angstrom resolution in the presence of glycerol and to 2.5 Angstrom resolution in the absence of substrate. The substrate-induced closure of 7degrees is significantly smaller than that reported for hexokinase, a model for substrate-mediated domain closure that has been proposed for glycerol kinase. Despite the 78% level of sequence identity and conformational similarity in the catalytic cleft regions of the En. casseliflavus and Escherichia coli glycerol kinases, remarkable structural differences have now been identified. These differences correlate well with their divergent regulatory schemes of activation by phosphorylation in En. casseliflavus and allosteric inhibition in E. coli. On the basis of our structural results, we propose a mechanism by which the phosphorylation of a histidyl residue located 25 Angstrom from the active site results in a 10-15-fold increase in the activity of the enterococcal glycerol kinase.