3.1.3.89 | free state and two complexes with natural substrates, to 2.1 A resolution. The free-state structure contains a large cavity accommodating the metal-coordinating HD motif of residues H33, H68, D69, and D137 and other conserved residues, i.e. R18, E72, and D77. These residues are important for activity. Residue R18 stabilizes the phosphate on the Co2+, and residue D77 forms a strong hydrogen bond critical for binding the ribose. The indole side chain of W19 is located close to the 2'-carbon atom of the deoxyribose moiety and is proposed to act as the selectivity switch for deoxyribonucleotide. The nucleotide bases of both dAMP and TMP make no specific hydrogen bonds with the protein, explaining the lack of nucleotide base selectivity. The E72A substrate complex structures also suggest a plausible single-step nucleophilic substitution mechanism |
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