Abstract
The active-site metal ion and the associated ligand amino acids in the NADP-linked, tetrameric enzyme Thermoanaerobacter brockii alcohol dehydrogenase (TBADH) were characterized by atomic absorption spectroscopy analysis and site-directed mutagenesis. Our preliminary results indicating the presence of a catalytic zinc and the absence of a structural metal ion in TBADH (Peretz & Burstein. 1989. Biochemistry 28:6549-6555) were verified. To determine the role of the putative active-site zinc, we investigated whether exchanging the zinc for other metal ions would affect the structural and/or the enzymatic properties of the enzyme. Substituting various metal ions for zinc either enhanced or diminished enzymatic activity, as follows: Mn2+ (240%); Co2+ (130%); Cd2+ (20%); Cu2+ or V3+ (< 5%). Site-directed mutagenesis to replace any one of the three putative zinc ligands of TBADH, Cys 37, His 59, or Asp 150, with the non-chelating residue, alanine, abolished not only the metal-binding capacity of the enzyme but also its catalytic activity, without affecting the overall secondary structure of the enzyme. Replacing the three putative catalytic zinc ligands of TBADH with the respective chelating residues serine, glutamine, or cysteine damaged the zinc-binding capacity of the mutated enzyme and resulted in a loss of catalytic activity that was partially restored by adding excess zinc to the reaction. The results imply that the zinc atom in TBADH is catalytic rather than structural and verify the involvement of Cys 37, His 59, and Asp 150 of TBADH in zinc coordination.