Catalytic efficiency of signal peptidase I of Escherichia coli is comparable to that of members of the serine protease family.

A method for estimating the activity of bacterial signal peptidase I (SPase I) was used to determine its activation energy (E[act]). Pro-OmpA-nuclease A, a hybrid secretory precursor, was purified to homogeneity under denaturing conditions and used as a substrate. This substrate was used to determine the activity of SPase I at different temperatures. The results show that the conformation of the mature domain of the substrate pro-OmpA-nuclease A has no discernible effect on the activity of SPase I. The activity data at a range of temperatures were then used to determine the activation energy using the Arrhenius equation. We have estimated E(act) to be 10.4 +/- 0.6 kcal/mol. This work indicates that SPase I is as catalytically efficient as the His-Ser-Asp family of proteases.