Crystal Structures of Creatininase Reveal the Substrate Binding Site and Provide an Insight into the Catalytic Mechanism.

ABSTRACT
Creatininase from Pseudomonas putida is a member of the urease-related amidohydrolase superfamily. The crystal structure of the Mn-activated enzyme has been solved by the single isomorphous replacement method at 1.8 Å resolution. The structures of the native creatininase and the Mnactivated creatininase-creatine complex have been determined by a dfference Fourier method at 1.85 Å and 1.6 Å resolution, respectively.
We found the disc-shaped hexamer to be roughly 100 Å in diameter and 50 Å in thickness and arranged as a trimer of dimers with 32 (D₃) point group symmetry. The enzyme is a typical Zn²⁺ enzyme with a binuclear metal center (metal1 and metal2). Atomic absorption spectrometry and X-ray crystallography revealed that Zn²⁺ at metal1 (Zn1) was easily replaced with Mn²⁺ (Mn1). In the case of the Mn-activated enzyme,metal1 (Mn1) has a square-pyramidal geometry bound to three protein ligands of Glu34, Asp45, and His120 and two water molecules. Metal2(Zn2) has a well-ordered tetrahedral geometry bound to the three protein ligands of His36, Asp45, and Glu183 and a water molecule. The crystal structure of the Mn-activated creatininase?creatine complex, which is the first structure as the enzyme?substrate/inhibitor complex of creatininase,reveals that significant conformation changes occur at the flap (between the α5 helix and the α6 helix) of the active site and the creatine is accommodated in a hydrophobic pocket consisting of Trp174, Trp154, Tyr121,Phe182, Tyr153, and Gly119. The high-resolution crystal structure of the creatininase-creatine complex enables us to identify two water molecules (Wat1 and Wat2) that are possibly essential for the catalytic mechanism of the enzyme. The structure and proposed catalytic mechanism of the creatininase are different from those of urease-related amidohydrolase superfamily enzymes. We propose a new two-step catalytic mechanism possibly common to creatininases in which the Wat1 acts as the attacking nucleophile in the water-adding step and the Wat2 acts as the catalytic acid in the ring-opening step.