Abstract of the PDB Structure's related Publication:
The essential tRNA-specific adenosine deaminase catalyzes the deamination of adenosine to inosine at the wobble position of tRNAs. This modification allows for a single tRNA species to recognize multiple synonymous codons containing A, C, or U in the last (3'-most) position and ensures that all sense codons are appropriately decoded. We report the first combined structural and kinetic characterization of a wobble-specific deaminase. The structure of the Escherichia coli enzyme clearly defines the dimer interface and the coordination of the catalytically essential zinc ion. The structure also identifies the nucleophilic water and highlights residues near the catalytic zinc likely to be involved in recognition and catalysis of polymeric RNA substrates. A minimal 19 nucleotide RNA stem substrate has permitted the first steady-state kinetic characterization of this enzyme (k(cat) = 13 +/- 1 min(-)(1) and K(M) = 0.83 +/- 0.22 microM). A continuous coupled assay was developed to follow the reaction at high concentrations of polynucleotide substrates (>10 microM). This work begins to define the chemical and structural determinants responsible for catalysis and substrate recognition and lays the foundation for detailed mechanistic analysis of this essential enzyme.
E.coli tadA displays sequence similarity to the yeast tRNA deaminase subunit Tad2p. It is an omodimeric enzyme-containing zinc. Recombinant tadA protein forms homodimers and is sufficient for site-specific inosine formation at the wobble position of tRNA-Arg2, the only tRNA having this modification in prokaryotes. Inosine modification allows for a single tRNA species to recognize multiple synonymous codons containing A, C, or U in the last (3'-most) position, and ensures that all sense codons are appropriately decoded. Essential for cell viability (
Wolf et al. 2002).