HRP was purchased from Sigma-Aldrich (P8375, RZ > 2.5) and stored in 100 mM potassium phosphate buffer (KPi), pH 7. The enzymes LdhA, LacZ and GapA were purified as N-terminal His6-tag fusion proteins from E. coli BL21 strains harbouring the plasmids pCA24N::ldhA, pCA24N::lacZ, and pCA24N::gapA, respectively. Cells were grown in LB medium containing 50 µg ml−1 chloramphenicol. For overexpression, overnight cultures were diluted to an OD600 of 0.05 and incubated at 37°C. At OD600 of 0.5, overexpression was induced with 100 µM isopropyl β-D-thiogalactoside (IPTG). Cultures were shifted to 30°C and harvested after 4 h of growth following the shift. Cell pellets were resuspended in lysis buffer (20 mM sodium phosphate, 500 mM NaCl, 0.2 mg ml−1 DNase, 0.2 mg ml−1 RNase, 0.35 mg ml−1 lysozyme, cOmplete protease inhibitor (Roche); all other components acquired from Sigma-Aldrich). After sonication and centrifugation for 30 min (21 000g), the supernatant was loaded onto a HisTrap FF crude 5 ml column (GE Healthcare). Purification of the His6-tagged proteins was carried out with an ÄKTA pure25 system (GE Healthcare). Proteins were eluted with a linear gradient from 20 to 500 mM imidazole in 20 mM sodium phosphate buffer and 500 mM NaCl. Elution fractions of interest were pooled, concentrated using centrifugal filter units (cut-off 10 kDa), aliquoted, and stored in potassium phosphate buffer (100 mM, pH 7) at −80°C.