Structure, cooperativity and inhibition of the inosine 5′‐monophosphate‐specific phosphatase from Saccharomyces cerevisiae

The nucleoside inosine is a main intermediate of purine nucleotide catabolism in Saccharomyces cerevisiae and is produced via the dephosphorylation of inosine monophosphate (IMP) by IMP‐specific 5′‐nucleotidase 1 (ISN1), which is present in many eukaryotic organisms. Upon transition of yeast from oxidative to fermentative growth, ISN1 is important for intermediate inosine accumulation as purine storage, but details of ISN1 regulation are unknown. We characterized structural and kinetic behavior of ISN1 from S. cerevisiae (ScISN1) and showed that tetrameric ScISN1 is negatively regulated by inosine and adenosine triphosphate (ATP). Regulation involves an inosine‐binding allosteric site along with IMP‐induced local and global conformational changes in the monomer and a tetrameric re‐arrangement, respectively. A proposed interaction network propagates local conformational changes in the active site to the intersubunit interface, modulating the allosteric features of ScISN1. Via ATP and inosine, ScISN1 activity is likely fine‐tuned to regulate IMP and inosine homeostasis. These regulatory and catalytic features of ScISN1 contrast with those of the structurally homologous ISN1 from Plasmodium falciparum, indicating that ISN1 enzymes may serve different biological purposes in different organisms. Tetrameric ScISN1 is an allosteric enzyme that undergoes inosine monophosphate (IMP)‐dependent conformational changes. The changes in the vicinity of the active site propagated to adjacent subunits via a long‐range interaction network, resulting in allosteric features. Adenosine triphosphate (ATP) and inosine act as negative effectors to ScISN1, consistent with a physiological event for a fermentative state of Saccharomyces cerevisiae, in which inosine/IMP homeostasis is maintained under conditions of IMP production from adenosine monophosphate (AMP).