Gluconolactonase with low activity towards other sugar lactones, including gulonolactone and galactonolactone. Catalyzes a key step in ascorbic acid (vitamin C) biosynthesis. Can also hydrolyze diisopropyl phosphorofluoridate and phenylacetate (in vitro). Calcium-binding protein. Modulates Ca(2+) signaling, and Ca(2+)-dependent cellular processes and enzyme activities (By similarity). D-glucono-1,5-lactone + H2O = D-gluconate + H(+) Binds 1 divalent metal cation per subunit. Most active with Zn(2+) and Mn(2+) ions. The physiological cofactor is most likely Ca(2+) or Mg(2+). Cofactor biosynthesis; L-ascorbate biosynthesis via UDP-alpha-D-glucuronate pathway; L-ascorbate from UDP-alpha-D-glucuronate: step 3/4. Monomer. Belongs to the SMP-30/CGR1 family.