How do expression level and solubility confound functional comparisons?

Why

Suppose two enzyme variants are expressed in equal culture volumes. Variant B gives twice the absorbance change, but an immunoblot shows it produces three times as much soluble enzyme. Its per-molecule activity could actually be lower.

Quantify total expression, soluble fraction, recovered protein concentration and aggregation, then measure initial reaction rates in a range where substrate is not exhausted. Normalize to molar active enzyme if this can be estimated reliably. Ordinary total protein concentration may still include inactive material.

Run an orthogonal substrate or detection method when assay interference is plausible. Solubility changes can also cause apparent improvements in binding display assays, where a more abundant display protein may bind more fluorescent ligand without better intrinsic affinity. Include parent and reference controls across preparations and replicate on separate days.

Treat titer, folding, activity and stability as separate engineering endpoints and select according to the application.