R67, the other dihydrofolate reductase: Rational design of an alternate active site configuration

R67 dihydrofolate reductase (DHFR) bears no sequence or structural homologies with chromosomal DHFRs. The gene for this enzyme produces subunits that are 78 amino acids long, which assemble into a homotetramer possessing 222 symmetry. More recently, a tandem array of four gene copies linked in-frame was constructed, which produces a monomer containing 312 amino acids named Quad3. Asymmetric mutations in Quad3 have also been constructed to probe the role of Q67 and K32 residues in catalysis. This present study mixes and matches mutations to determine if the Q67H mutation, which tightens binding approximately 100-fold to both dihydrofolate (DHF) and NADPH, can help rescue the K32M mutation. While the latter mutation weakens DHF binding over 60-fold, it concurrently increases k_cat by a factor of 5. Two Q67H mutations were added to gene copies 1 and 4 in conjunction with the K32M mutation in gene copies 1 and 3. Addition of these Q67H mutations tightens binding 40-fold, and the catalytic efficiency (k_cat/K_m(DHF)) of the resulting protein is similar to that of Quad3. Since these Q67H mutations can mostly compensate for the K32M lesion, K32 must not be necessary for DHF binding. Another multimutant combines the K32M mutation in gene copies 1 and 3 with the Q67H mutation in all gene copies. This mutant is inhibited by DHF but not NADPH, indicating that NADPH binds only to the wild type half of the pore, while DHF can bind to either the wild type or mutant half of the pore. This inhibition pattern contrasts with the mutant containing only the Q67H substitution in all four gene copies, which is severely inhibited by both NADPH and substrate. Since gene duplication and divergence are evolutionary tools for gaining function, these constructs are a first step toward building preferences for NADPH and DHF in each half of the active site pore of this primitive enzyme.