Impact of Mutating the Key Residues of a Bifunctional 5,10-Methylenetetrahydrofolate Dehydrogenase-Cyclohydrolase from <i>Escherichia coli</i> on Its Activities

Methylenetetrahydrofolate dehydrogenase-cyclohydrolase (FolD) catalyzes interconversion of 5,10-methylene-tetrahydrofolate and 10-formyl-tetrahydrofolate in the one-carbon metabolic pathway. In some organisms, the essential requirement of 10-formyl-tetrahydrofolate may also be fulfilled by formyltetrahydrofolate synthetase (Fhs). Recently, we developed an <i>Escherichia coli</i> strain in which the <i>folD</i> gene was deleted in the presence of <i>Clostridium perfringens fhs</i> (<i>E. coli</i> Δ<i>folD</i>/p-<i>fhs</i>) and used it to purify FolD mutants (free from the host-encoded FolD) and determine their biological activities. Mutations in the key residues of <i>E. coli</i> FolD, as identified from three-dimensional structures (D121A, Q98K, K54S, Y50S, and R191E), and a genetic screen (G122D and C58Y) were generated, and the mutant proteins were purified to determine their kinetic constants. Except for the R191E and K54S mutants, others were highly compromised in terms of both dehydrogenase and cyclohydrolase activities. While the R191E mutant showed high cyclohydrolase activity, it retained only a residual dehydrogenase activity. On the other hand, the K54S mutant lacked the cyclohydrolase activity but possessed high dehydrogenase activity. The D121A and G122D (in a loop between two helices) mutants were highly compromised in terms of both dehydrogenase and cyclohydrolase activities. <i>In vivo</i> and <i>in vitro</i> characterization of wild-type and mutant (R191E, G122D, D121A, Q98K, C58Y, K54S, and Y50S) FolD together with three-dimensional modeling has allowed us to develop a better understanding of the mechanism for substrate binding and catalysis by <i>E. coli</i> FolD