Substrate engineering of microbial transglutaminase for site-specific protein modification and bioconjugation

Microbial transglutaminase (MTG), a robust enzyme developed initially for the manipulation of edible proteins in the food industry, has now been widely recognized as a practical protein-modifying reagent in the range of biotechnological applications. In this chapter, we introduce the potential use of MTG through our basic studies on the design of novel glutamine (Gln) donor substrates for lysine (Lys)-specific protein modification. Based on the core structure of a conventional transglutaminase substrate, benzyloxycarbonyl-L-glutaminylglycine (Z-QG), new Gln-donor substrates have been developed for the conjugation of recombinant proteins with different functionalities. The first target site for the substrate engineering was the C-terminal carboxylic group of Z-QG, which is feasibly labeled with functional moieties. For the preparation of protein-nucleic acid conjugates with novel molecular architecture, a new nucleotidyl substrate, Z-QG-(d)UTP, was created. We have also explored substitution of the N-terminal protecting group (Z) with fluorophores and biotin, and found that MTG accepts diverse functional groups at the N-terminus by inserting a short linker, leading to an increase in the utility of MTG in site-specific modification of functional proteins. Our results demonstrated how the design of (small) Gln-donor substrates of MTG can expand the scope of enzymatic manipulation in biomolecular engineering.