Distal Structural Elements Coordinate a Conserved Base Flipping Network

One of the most dramatic illustrations of enzymatic promotion of a high-energy intermediate is observed in DNA modification and repair enzymes where an individual base is rotated (flipped) 180 degrees around the deoxyribose phosphate backbone and into the active site. While the end states have been extensively characterized, experimental techniques have yet to yield a full description of the base flipping process and the role played by the enzyme. The CS cytosine methyltransferase M.HhaI coordinates an ensemble of reciprocal DNA and enzyme rearrangements to efficiently flip the target cytosine from the DNA helix. We sought to understand the role of individual amino acids during base flipping. Our results demonstrate that M.HhaI initiates base flipping before closure of the catalytic loop and utilizes the conserved serine 85 in the catalytic loop to accelerate flipping and maintain distortion of the DNA backbone. Serine 87, which forms specific contacts within the DNA helix after base flipping, is not involved in the flipping process or in maintaining the catalytically competent complex. At the base of the catalytic loop, glycine 98 acts as a hinge to allow conformational dynamism of the loop and mutation to alanine inhibits stabilization of the closed loop. Our results illustrate how an enzyme utilizes numerous, distal residues in concert to transform substrate recognition into catalysis.