DNA polymerase beta: contributions of template-positioning and dNTP triphosphate-binding residues to catalysis and fidelity.

The specific catalytic roles of two groups of DNA polymerase beta active site residues identified from crystal structures were investigated: residues possibly involved in DNA template positioning (Lys280, Asn294, and Glu295) and residues possibly involved in binding the triphosphate moiety of the incoming dNTP (Arg149, Ser180, Arg183, and Ser188). Eight site-specific mutants were constructed: K280A, N294A, N294Q, E295A, R149A, S180A, R183A, and S188A. Two-dimensional NMR analysis was employed to show that the global conformation of the mutants has not been perturbed significantly. Pre-steady-state kinetic analyses with single-nucleotide gapped DNA substrates were then performed to obtain the rate of catalysis at saturating dNTP (k(pol)), the apparent dissociation constant for dNTP (K(d)), catalytic efficiency k(pol)/K(d), and fidelity. Of the three template-positioning residues, Asn294 and Glu295 (but not Lys280) contribute significantly to k(pol). Taken together with other data, the results suggest that these two residues help to stabilize the transition state during catalysis even though they interact with the DNA template backbone rather than directly with the incoming dNTP or the opposite base on the template. Furthermore, the fidelity increases by up to 19-fold for N294Q due to differential k(pol) effects between correct and incorrect nucleotides. Of the four potential triphosphate-binding residues, Ser180 and Arg183 contribute significantly to k(pol) while the effects of R149A are relatively small and are primarily on K(d), and Ser188 appears to play a minimal role in the catalysis by Pol beta. These results identify several residues important for catalysis and quantitate the contributions of each of those residues. The functional data are discussed in relation to the prediction on the basis of available crystal structures.