The Role of Mutations at the Side Door on the Thermal Stability and Structural Flexibility of the pnbCE Enzyme

We have investigated the role of a specific gate residue, LEW362, located at the side-door of the pnbCE enzyme, on the thermal stability and structural flexibility of the enzyme. pnbCE is a bacterial carboxylesterase enzyme from Bacillus subtilis, which has a structural resemblance as well as similar catalytic behavior to the mammalian carboxylesterases. Mutations at the side-door residue 362 of pnbCE are known to alter the catalytic activity of this enzyme. Using molecular dynamics simulations at several temperatures, we have studied the mechanism through which mutations at position 362 of pnbCE affect the structure and dynamics of this enzyme. We have identified two coil residues, SER218 and GLN276, whose interactions with residue 362 in wild-type and mutant pnbCE enzymes control the dynamics of the side-door domain of pnbCE. A hydrogen bond between the GLN276 and ARG362 residues in the arginine substituted (L362R) pnbCE mutant enzyme appears to he responsible tor locking the side-door domain region of the L362R enzyme, thus lowering the catalytic rates of the L362R mutant pnbCE enzyme compared to the wild-type. Similarly, a hydrogen bond formed between SER218 and ARG362 in I362R provides thermal stability to the arginine substituted mutant enzyme. This hydrogen bond is not as prevalent in the wild-type or other mutated pnbCE's, making them more prone to structural fluctuations upon increasing temperature. A trade-off between thermal stability of pnbCE and flexibility of the side-door domain region appears to be a best compromise for effectively controlling the catalytic properties of mutated pnbCE enzymes.