Human uracil-DNA glycosylase SMUG1 removes uracil residues and some other noncanonical or damaged bases from DNA. Despite the functional importance of this enzyme, its X-ray structure is still unavailable. Previously, we performed homology modeling of human SMUG1 structure and suggested the roles of some amino acid residues in the recognition of damaged nucleotides and their removal from DNA. In this study, we investigated the kinetics of conformational transitions in the protein and in various DNA substrates during enzymatic catalysis using the stopped-flow method based on changes in the fluorescence intensity of enzyme's tryptophan residues and 2-aminopurine in DNA or fluorescence resonance energy transfer (FRET) between fluorophores in DNA. The kinetic mechanism of interactions between reaction intermediates was identified, and kinetic parameters of the intermediate formation and dissociation were calculated. The obtained data help in elucidating the functions of His239 and Arg243 residues in the recognition and removal of damaged nucleotides by SMUG1.
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