Structural insights into the catalytic mechanism and thermostability of microbial esterases from marine sediment

Microbial esterases have been widely used in industry, pharmaceuticals and food production. Metagenomics is a powerful tool to screen novel enzymes with industrial potential from various environments. By this method, two novel HSL esterases, E25 and E40, were identified from a marine sediment sample. E25 is mesophilic (optimum temperature at 50°C), salt-tolerant, slightly alkaline (optimum pH at 8.5) for its activity, and capable of hydrolyzing short chain monoesters (C2-C10). Structural and mutational analysis indicated that E25 has some catalytic profiles different from other HSLs. Dimerization is essential for E25 to exert its catalytic activity by keeping the accurate orientation of the catalytic Asp282 within the catalytic triad. E40, the other HSL esterase, exhibited the maximal activity at 45°C and was quite thermolabile, with a half-life of only 2 min at 40°C, which may be an adaptation of E40 to the permanently cold sediment environment. Structural and mutational analysis revealed that the absence of interdomain hydrophobic interactions between loop1 and α7 leads to the thermolability of E40. A comparative analysis of the structures of E40 and other thermolabile and thermostable HSLs suggests that the interdomain hydrophobic interactions between loop1 and α7 are a key element for the thermostability of microbial HSLs. Our study sheds light on protein folding and evolution of HSLs and also on protein engineering of thermolabile HSLs with industrial potential.