On the design of a constitutively active peptide asparaginyl ligase for facile protein conjugation.

Peptide asparaginyl ligases (PALs) are precision tools for peptide cyclization, cell-surface labelling, protein semi-synthesis and protein conjugation. PALs are expressed as inactive proenzymes requiring low pH activation. During activation, a large portion of the cap domain of the proenzyme that covers the substrate binding site is proteolytically removed, exposing the active site to solvent and releasing a population of heterogenous active enzymes. The availability of a readily active ligase not requiring acid-activation and subsequent purification of active forms would facilitate manufacturing and streamline applications. Here, we engineered the OaAEP1b-C247A hyperactive ligase via serial truncations along the linker connecting the cap and core domain of the proenzyme. Recombinant expression of the truncated constructs was carried out in E. coli. Following a solubilization/refolding protocol, one truncated construct termed "OaAEP1b-C247A-𝚫351" could be overexpressed in the insoluble fraction, purified, and displayed a level of ligase activity comparable to the acid-activated OaAEP1b-C247A enzyme. This constitutively active protein can be stored for up to two years at -80°C and readily used for peptide cyclization and protein conjugation. We were able to express and purify a stable constitutively active asparaginyl ligase that can be stored for months without significant activity loss. The removal of the low pH proenzyme activation step eliminates the heterogeneity introduced by this procedure. The yield of purified recombinant active ligase that can be routinely obtained per 100 millilitres of E. coli cell culture is about 0.9 mg. This recombinant active ligase can be used to carry out protein conjugation.