New mechanism for broad substrate-specificity of enzymes

Graduate School of Pharmaceutical Sciences, Kumamoto University* Graduate School of Pharmaceutical Sciences, Hokkaido University** Department of Chemistry, Tokyo Metropolitan University*** Graduate School of Engineering, Kyoto University**** Medical Institute of Bioregulation, Kyushu University*****
â—‹Teruya Nakamura* Hiroyuki Kamiya** Masaki Mishima*** Sachiko Toma* Shinji Ikemizu* Masahiro Shirakawa**** Yusaku Nakabeppu***** Yuriko Yamagata*

Oxidized nucleotides cause replicational and transcriptional errors because of their misincorporations into DNA and RNA. In oxidized nucleotides, 8-oxo-dGTP and 2-oxo-dATP are known to induce frequent transversion mutations. E.coli MutT specifically hydrolyzes 8-oxo-dGTP to 8-oxo-dGMP and prevents the A:T to C:G transversion caused by 8-oxoguanine. On the other hand, human MutT homolog-1 (hMTH1) protein has broad substrate-specificity for several oxidized purine nucleotides such as 8-oxo-dGTP, 2-oxo-dATP and 8-oxo-dATP. Mice lacking the mth1 gene exhibit an increased occurrence of spontaneous carcinogenesis especially in the liver, and to a lesser extent, in the lung and stomach, thus suggesting that the accumulation of oxidized purine nucleotides triggers such malignant transformation in vivo.
In addition to the determination of the solution structure of hMTH1 (apo form)*, we have solved the crystal structures of hMTH1 in complexes with 8-oxo-dGMP, 8-oxo-dGTP and 2-oxo-dATP. The hMTH1-8-oxo-dGMP and hMTH1-8-oxo-dGTP structures compared with the previously determined MutT-8-oxo-dGMP structure reveal the structural basis of the difference in substrate-specificity for 8-oxo-dGTP between hMTH1 and MutT. The hMTH1-2-oxo-dATP and hMTH1-8-oxo-dGTP structures suggest that hMTH1 recognizes the different oxidized purine nucleotides by the exchange of the protonation site in the neighboring aspartate residues. This is the first mechanism found for broad substrate-recognition of enzymes. This mechanism is supported by our mutational studies.
* Mishima et al., J. Biol. Chem. 279, 33806-33815 (2004)