Metalloenzymes and signal transduction: The protein serine/threonine phosphatases, a novel class of binuclear metal-containing enzymes

The serine/threonine protein phosphatases are important regulatory enzymes involved in signal transduction pathways in eukaryotic organisms. These enzymes include protein phosphatases 1, 2A, and 2B (also known as calcineurin). Recent structural data have indicated that the serine/threonine protein phosphatases are novel metalloenzymes containing a dinuclear metal ion cofactor at the active site. The dinuclear metal site is situated in a unique protein fold, a beta-alpha-beta-alpha-beta motif which provides the majority of Ligands to the metal ions. A similar fold is also seen in plant purple acid phosphatases, which also contain a dinuclear iron-zinc cofactor. In these enzymes, the two metal ions are bridged by a solvent molecule and a carboxylate group from an aspartic acid residue, juxtaposing the two metal ions to within 3.0-4.0 Angstrom of each other. A similar motif has been identified in a number of other enzymes which exhibit phosphoesterase activity, implicating several of them as metalloenzymes which contain dinuclear metal ion cofactors.