Catalysis of Thermostable Alcohol Dehydrogenase Improved by Engineering the Microenvironment through Fusion with Supercharged Proteins

The enzymatic microenvironment can impact biocatalytic activity; however, these effects can be difficult to investigate as mutations and fusions can introduce multiple variables and overlapping effects. The fusion of a supercharged protein is a potentially facile means to alter the enzymatic microenvironment. We have investigated complexes made between a thermostable alcohol dehydrogenase (AdhD) and superfolding green fluorescent protein (sfGFP) mutants with extreme surface charges. Three charged sfGFP variants, -30, 0, and +36 were covalently attached to AdhD through the SpyCatcher/SpyTag system. Specific rates for the NAD(+)-dependent oxidation of butane-2,3-diol were significantly increased in the -30 sfGFP complex, a mixed effect was seen for the 0 sfGFP complexes, and the rates were unaffected by +36 sfGFP complexation. Reactions performed at various pH values (7.8-9.8) and salt concentrations (7.75-500 mm) showed that there was a complex interplay between these effects that was consistent with fusion proteins affecting the local ionic strength, as opposed to the local pH. Steady-state kinetic analyses were performed with the -30 and 0 AdhD-sfGFP complexes. The overall catalytic efficiency was dependent on the charge of the fused sfGFP variant; the -30 sfGFP fusions exhibited the largest beneficial effects at pH 8.8. The impact of the fusions on the apparent ionic strength provides further insight into the effects of charged patches observed on metabolon-forming enzyme complexes.