The nature of the putative general acid His187 in the reaction catalyzed by Escherichia coli uracil DNA glycosylase (UDG) was investigated using X-ray crystallography and NMR spectroscopy. The crystal structures of H187Q UDG, and its complex with uracil, have been solved at 1.40 and 1.60 A resolution, respectively. The structures are essentially identical to those of the wild-type enzyme, except that the side chain of Gln187 is turned away from the uracil base and cannot interact with uracil O2. This result provides a structural basis for the similar kinetic properties of the H187Q and H187A enzymes. The ionization state of His187 was directly addressed with (1)H-(15)N NMR experiments optimized for histidine ring spin systems, which established that His187 is neutral in the catalytically active state of the enzyme (pK(a) <5.5). These NMR experiments also show that His187 is held in the N(epsilon)()2-H tautomeric form, consistent with the crystallographic observation of a 2.9 A hydrogen bond from the backbone nitrogen of Ser189 to the ring N(delta)()1 of His187. The energetic cost of breaking this hydrogen bond may contribute significantly to the low pK(a) of His187. Thus, the traditional view that a cationic His187 donates a proton to uracil O2 is incorrect. Rather, we propose a concerted mechanism involving general base catalysis by Asp64 and electrophilic stabilization of the developing enolate on uracil O2 by a neutral His187.

The DNA repair enzyme uracil DNA glycosylase (UDG) catalyzes the hydrolysis of premutagenic uracil residues from single-stranded or duplex DNA, producing free uracil and abasic DNA. Here we report the high-resolution crystal structures of free UDG from Escherichia coli strain B (1.60 A), its complex with uracil (1.50 A), and a second active-site complex with glycerol (1.43 A). These represent the first high-resolution structures of a prokaryotic UDG to be reported. The overall structure of the E. coli enzyme is more similar to the human UDG than the herpes virus enzyme. Significant differences between the bacterial and viral structures are seen in the side-chain positions of the putative general-acid (His187) and base (Asp64), similar to differences previously observed between the viral and human enzymes. In general, the active-site loop that contains His187 appears preorganized in comparison with the viral and human enzymes, requiring smaller substrate-induced conformational changes to bring active-site groups into catalytic position. These structural differences may be related to the large differences in the mechanism of uracil recognition used by the E. coli and viral enzymes. The pH dependence of k(cat) for wild-type UDG and the D64N and H187Q mutant enzymes is consistent with general-base catalysis by Asp64, but provides no evidence for a general-acid catalyst. The catalytic mechanism of UDG is critically discussed with respect to these results.

The DNA repair enzyme uracil DNA glycosylase (UDG) pinches the phosphodiester backbone of damaged DNA using the hydroxyl side chains of a conserved trio of serine residues, resulting in flipping of the deoxyuridine from the DNA helix into the enzyme active site. We have investigated the energetic role of these serine-phosphodiester interactions using the complementary approaches of crystallography, directed mutagenesis, and stereospecific phosphorothioate substitutions. A new crystal structure of UDG bound to 5'-HO-dUAAp-3' (which lacks the 5' phosphodiester group that interacts with the Ser88 pinching finger) shows that the glycosidic bond of dU has been cleaved, and that the enzyme has undergone the same specific clamping motion that brings key active site groups into position as previously observed in the structures of human UDG bound to large duplex DNA substrates. From this structure, it may be concluded that glycosidic bond cleavage and the induced fit conformational change in UDG can occur without the 5' pinching interaction. The S88A, S189A, and S192G "pinching" mutations exhibit 360-, 80-, and 21-fold damaging effects on k(cat)/K(m), respectively, while the S88A/S189A double mutant exhibits an 8200-fold damaging effect. A free energy analysis of the combined effects of nonbridging phosphorothioate substitution and mutation at these positions reveals the presence of a modest amount of strain energy between the compressed 5' and 3' phosphodiester groups flanking the bound uridine. Overall, these results indicate a role for these serine-phosphodiester interactions in uracil flipping and preorganization of the sugar ring into a reactive conformation. However, in contrast to a recent proposal [Parikh, S. S., et al. (2000) Proc Natl. Acad. Sci. 94, 5083], there is no evidence that conformational strain of the glycosidic bond induced by serine pinching plays a major role in the 10(12)-fold rate enhancement brought about by UDG.