Yeast galactokinase in closed conformation can switch between catalytic and signal transducer states.

S.cerevisiae galactokinase (ScGal1p), in closed conformation catalyzes the phosphorylation of galactose to galactose 1-phopshate using ATP as the phosphate donor as well as allosterically activates the GAL genetic switch in response to galactose and ATP as ligands. How both kinase and signaling activities of ScGal1p are associated with closed conformation of the protein is not understood. Conformational sampling of ScGal1p indicated that this protein samples closed kinase and closed non-kinase conformers. Closed non-kinase conformers are catalytically incompetent to phosphorylate galactose and act as a bonafide signal transducer. It was observed that toggling of side chain of highly conserved K266 of ScGal1p between S171and catalytic base D217 is responsible for transitioning of ScGal1p between signal transducer and kinase states. Interestingly in ScGal3p, the paralog of ScGal1p, which has only signal transduction activity and lacks kinase activity, a H bond between a non-conserved Y433, and a highly conserved Y57, gets broken during MD simulation. The corresponding H-bond present in ScGal1p between residues Y441 and Y63 respectively, remains intact throughout the simulations of ScGal1p.Therefore, we predicted that K266 and Y441 have a role in bifunctionality of ScGal1p. To test the above predictions, we monitored the signaling and kinase activity of ScGal1K266Rp and ScGal1Y441Ap variants. Signaling activity increased in both ScGal1Y441Ap and ScGal1K266Rp variants as compared to ScGal1wtp, whereas the kinase activity increased in ScGal1Y441Ap, but decreased in ScGal1K266Rp Based on the above, we propose that K266 and Y441 are crucial for conferring bifunctionality to ScGal1p. Author summaryGalactokinase of S.cerevisiae(ScGal1p), the first enzyme of Leloir pathway of galactose metabolism, phosphorylates galactose using ATP as the phosphate donor. ScGal1p also functions as a signal transducer of GAL regulon wherein galactose and ATP allosterically activate galactokinase. The active form of galactokinase, then sequesters the repressor ScGal80p, to activate the GAL switch. ScGal1p has a single site each for binding to galactose and ATP. How ScGal1p, a monomeric protein, performs the above two mutually exclusive activities using the same set of substrates/ligands, with the same site acting as the active site for enzymatic activity as well as allosteric site for signal transduction activity is unclear. Our findings are that this protein has a distinct conformational state for functioning as a signal transducer and a distinct conformational state for functioning as a kinase. A highly conserved lysine residue (K266) present only in fungal galactokinases, triggers the interconversion between catalysis and signal transduction states. This interconversion is subdued by H bond between Y441 and Y63. These studies suggest that the two activities of ScGal1p are fine tuned by evolution to regulate metabolism through transcriptional control.