Double mutations far from the active site affect cold activity in an Antarctic halophilic β-galactosidase

The Antarctic haloarchaeon, Halorubrum lacusprofundi, contains a polyextremophilic family 42 β-galactosidase, which we are using as a model for cold-active enzymes. Divergent amino acid residues in this 78 kDa protein were identified through comparative genomics and hypothesized to be important for cold activity. Six amino acid residues were previously mutated and five were shown by steady-state kinetic analysis to have altered temperature-dependent catalytic activity profiles via effects on K_m and/or k_cat compared to the wild-type enzyme. In this follow-up study, double-mutated enzymes were constructed and tested for temperature effects, including two new tandem residue pairs (N180T/A181T and T383A/S384A), and pairwise combination of the single residue mutations (N251D, F387L, I476V, and V482L). All double-mutated enzymes were found to be more catalytically active at moderate and/or less active at colder temperatures than wild-type, with both K_m and k_cat effects observed for the two tandem mutations. For pairwise combinations, a K_m effect was seen when the surface exposed F387L mutation located in a domain A TIM barrel α helix 19 Å from the active site was combined with two internal residues, N251D or V482L. When another surface exposed mutation I476V located in a coiled region of domain B 25 Å from the active site was paired with N251D or V482L, a k_cat effect was observed. These results indicate that temperature-dependent kinetic effects may be complex and subtle and are mediated by a combination of a small number of residues distant from the active site via changes to the hydration shell and/or perturbation of internal packing.