ABSTRACT
Fructose 1,6-bisphosphatase (FBPase) (EC 3.1.1.11) catalyzes the hydrolysis of fruc tose 1,6-bisphosphate (FBP) to form fructose 6-phosphate (F6P) and inorganic phos phate in gluconeogenesis [35]. The enzyme requires two divalent cations and a monovalent cation to achieve its maximal activity [36-38]. FBPase is a homotetramer with a subunit molecular mass of 37 kD. The monovalent cation K+ activates the enzyme at low concentrations (.Km = 1 7 mM) and inhibits at high concentrations (K{ = 68 mM) [36]. The structure of FBPase in complex with the substrate analog, 2,5anhydro-D-glucitol 1,6-bisphosphate (AhG-l,6-P2), and K+ or Tl+ [39] revealed three K+ or Tl+ sites. Sites 1 and 2 coincide with positions occupied by Mn2+ in the AhG1,6-P2 -AMP-Mn2+ complex [40]. It is therefore probable that the monovalent cation bound at site 3 occupies the position where activation by monovalent cations occurso naturally [39]. The site 3 potassium coordinates to Glu280 and Arg276, and is 3.3 A from metal site 2. Villeret et al. [39] suggest that K+ at site 1 is responsible for monovalent cation inhibition of the enzyme.
