Direct measurement of the pKa of aspartic acid 26 in Lactobacillus casei dihydrofolate reductase:: Implications for the catalytic mechanism

The ionization stale of aspartate 26 in Lactobacillus casei dihydrofolate reductase has been investigated by selectively labeling the enzyme with [C-13 gamma] aspartic acid and measuring the C-13 chemical shifts in the ape, folate-enzyme, and dihydrofolate-enzyme complexes. Our results indicate that no aspartate residue has a pK(a) greater than similar to 4.8 in any of the three complexes studied. The resonance of aspartate 26 in the dihydrofolate-enzyme complex has been assigned by site-directed mutagenesis; aspartate 26 is found to have a pK(a) value of less than 4 in this complex. Such a low pK(a) value makes it most unlikely that the ionization of this residue is responsible for the observed pH profile of hydride ion transfer [apparent pK(a) = 6.0; Andrews, J., Fierke, C. A., Birdsall, B., Ostler, G., Feeney, J., Roberts, G. C. K., and Benkovic, S. J. (1989) Biochemistry 28, 5743-5750]. Furthermore, the downfield chemical shift of the Asp 26 C-13 gamma resonance in the dihydrofolate-enzyme complex provides experimental evidence that the pteridine ring of dihydrofolate is polarized when bound to the enzyme. We propose that this polarization of dihydrofolate acts as the driving force for protonation of the electron-rich O4 atom which occurs in the presence of NADPH. After this protonation of the substrate, a network of hydrogen bonds between O4, N5 and a bound water molecule facilitates transfer of the proton to N5 and transfer of a hydride ion from NADPH to the C6 atom to complete the reduction process.