Site-specific derivatization of avidin using microbial transglutaminase

Transglutaminase in Foods and Biotechnology

Stabilization and reusability of enzyme transglutaminase (TGM) are important goals for the enzymatic process since immobilizing TGM plays an important role in different technologies and industries. TGM can be used in many applications. In the food industry, it plays a role as a protein-modifying enzyme, while, in biotechnology and pharmaceutical applications, it is used in mediated bioconjugation due to its extraordinary crosslinking ability. TGMs (EC 2.3.2.13) are enzymes that catalyze the formation of a covalent bond between a free amino group of protein-bound or peptide-bound lysine, which acts as an acyl acceptor, and the γ-carboxamide group of protein-bound or peptide-bound glutamine, which acts as an acyl donor. This results in the modification of proteins through either intramolecular or intermolecular crosslinking, which improves the use of the respective proteins significantly.

Approaching transglutaminase from Streptomyces bacteria over three decades

Transglutaminases are protein cross-linking and protein-modifying enzymes that have attracted considerable interest due to their causal involvement in various diseases and versatility in industrial applications. In particular, microbial transglutaminases (MTG) from Streptomyces bacteria have managed in recent years to evolve from simple food additives to specialized enzymes for the site-directed modification of therapeutic proteins. The review summarizes relevant studies from the beginning dealing with the occurrence, production, structure, catalysis, and substrate molecules of MTG enzymes. It also addresses biotechnological procedures with MTG from S. mobaraensis (SmMTG) as the most prominent representative in focus. Reassessment of the available data revealed unexpected insights into catalysis of SmMTG and other transglutaminases, suggesting selection of glutamine donor proteins by subsites at the front vestibule and the existence of distinct lysine pockets. Flexibility of the SmMTG-accessible glutamine donor substrate regions seems to be more important than the glutamine environment. Nevertheless, residues in close vicinity to glutamines also determine interaction with the SmMTG subsites. The apparent lack of subsites for lysine donor proteins suggests self-assembly of the substrate proteins prior to enzymatic cross-linking. The study of natural substrate proteins, especially their mutual interaction, is proposed to further illuminate catalysis of SmMTG. To this end, structure and function of the characterized substrate proteins from S. mobaraensis are discussed in conclusion.

Enzymatic Methods for the Site-Specific Radiolabeling of Targeting Proteins

Site-specific conjugation of proteins is currently required to produce homogenous derivatives for medicine applications. Proteins derivatized at specific positions of the polypeptide chain can actually show higher stability, superior pharmacokinetics, and activity in vivo, as compared with conjugates modified at heterogeneous sites. Moreover, they can be better characterized regarding the composition of the derivatization sites as well as the conformational and activity properties. To this aim, several site-specific derivatization approaches have been developed. Among these, enzymes are powerful tools that efficiently allow the generation of homogenous protein-drug conjugates under physiological conditions, thus preserving their native structure and activity. This review will summarize the progress made over the last decade on the use of enzymatic-based methodologies for the production of site-specific labeled immunoconjugates of interest for nuclear medicine. Enzymes used in this field, including microbial transglutaminase, sortase, galactosyltransferase, and lipoic acid ligase, will be overviewed and their recent applications in the radiopharmaceutical field will be described. Since nuclear medicine can benefit greatly from the production of homogenous derivatives, we hope that this review will aid the use of enzymes for the development of better radio-conjugates for diagnostic and therapeutic purposes.

Microbial transglutaminase for biotechnological and biomedical engineering

Research on bacterial transglutaminase dates back to 1989, when the enzyme has been isolated from Streptomyces mobaraensis. Initially discovered during an extensive screening campaign to reduce costs in food manufacturing, it quickly appeared as a robust and versatile tool for biotechnological and pharmaceutical applications due to its excellent activity and simple handling. While pioneering attempts to make use of its extraordinary cross-linking ability resulted in heterogeneous polymers, currently it is applied to site-specifically ligate diverse biomolecules yielding precisely modified hybrid constructs comprising two or more components. This review covers the extensive and rapidly growing field of microbial transglutaminase-mediated bioconjugation with the focus on pharmaceutical research. In addition, engineering of the enzyme by directed evolution and rational design is highlighted. Moreover, cumbersome drawbacks of this technique mainly caused by the enzyme's substrate indiscrimination are discussed as well as the ways to bypass these limitations.

Enzymatic labelling of snake venom phospholipase A2 toxins

Almost all animal venoms contain secretory phospholipases A₂ (PLA₂s), 14 kDa disulfide-rich enzymes that hydrolyze membrane phospholipids at the sn-2 position, releasing lysophospholipids and fatty acids. These proteins, depending on their sequence, show a wide variety of biochemical, toxic and pharmacological effects and deserve to be studied for their numerous possible applications, and to improve antivenom drugs. The cellular localization and activity of a protein can be studied by conjugating it with a tag. In this work, we applied an enzymatic labelling method, using Streptomyces mobaraense transglutaminase, on three snake venom PLA₂s: a recombinant neuro- and myotoxic group I PLA₂ from Notechis scutatus scutatus, and two myotoxic group II PLA₂s from Bothrops asper - one of them a natural catalytically inactive variant. We demonstrate that TGase can be used to produce active mono- or bi-derivatives of these three PLA₂s modified at specific Lys residues, and that all three of these proteins, conjugated with fluorescent peptides, are internalized in primary myotubes.

Recent, non-classical, approaches to antibody lysine modification

This review will discuss recent development in the bioconjugation of lysine residues on antibodies. As several chemoselective reagents have already been developed for modifying amine groups, recent strategies now tend to aim at being site-specific. Four general methods have been listed: kinetically controlled, template-directed or enzymatic strategies as well as the use of chemically programmed antibodies.

Transglutaminase-mediated conjugation and nitride-technetium-99m labelling of a bis(thiosemicarbazone) bifunctional chelator

An assessment study involving the use of the transglutaminase (TGase) conjugation method and the nitride-technetium-99m labelling on a bis(thiosemicarbazone) (BTS) bifunctional chelating agent is presented. The previously described chelator diacetyl-2-(N⁴-methyl-3-thiosemicarbazone)-3-(N⁴-amino-3-thiosemicarbazone), H₂ATSM/A, has been functionalized with 6-aminohexanoic acid (ε-Ahx) to generate the bifunctional chelating agent diacetyl-2-(N⁴-methyl-3-thiosemicarbazone)-3-[N⁴-(amino)-(6-aminohexanoic acid)-3-thiosemicarbazone], H₂ATSM/A-ε-Ahx (1), suitable for conjugation to glutamine (Gln) residues of bioactive molecules via TGase. The feasibility of the TGase reaction in the synthesis of a bioconjugate derivative was investigated using Substance P (SP) as model peptide. Compounds 1 and H₂ATSM/A-ε-Ahx-SP (2) were labelled with nitride-technetium-99m, obtaining the complexes [^99mTc][Tc(N)(ATSM/A-ε-Ahx)] (^99mTc1) and [^99mTc][Tc(N)(ATSM/A-ε-Ahx-SP)] (^99mTc2). The chemical identity of ^99mTc1 and ^99mTc2 was confirmed by radio/UV-RP-HPLC combined with ESI-MS analysis on the respective carrier-added products ^99g/99mTc1 and ^99g/99mTc2. The stability of the radiolabelled complexes after incubation in various environments was investigated. All the results were compared with those obtained for the corresponding ⁶⁴Cu-analogues, ⁶⁴Cu1 and ⁶⁴Cu2. The TGase reaction allows the conjugation of 1 with the peptide, but it is not highly efficient due to instability of the chelator in the required conditions. The SP-conjugated complexes are unstable in mouse and human sera. However, indeed the BTS system can be exploited as nitride-technetium-99m chelator for highly efficient technetium labelling, thus making compound 1 worthy of further investigations for new targeted technetium and copper radiopharmaceuticals encompassing Single Photon Emission Computed Tomography and Positron Emission Tomography imaging.

Dual, Site-Specific Modification of Antibodies by Using Solid-Phase Immobilized Microbial Transglutaminase

Microbial transglutaminase (MTG) was stably solid-phase immobilized on glass microbeads by using a second-generation dendronized polymer. Immobilized MTG enabled the efficient generation of site-specifically conjugated proteins, including antibody fragments, as well as whole antibodies through distinct glutamines and, unprecedentedly, also through lysines with various bifunctional substrates with defined stoichiometries. With this method, we generated dual, site-specifically modified antibodies comprising a fluorescent probe and a metal chelator for radiolabeling-a strategy anticipated to design antibodies for imaging and simultaneous therapy. Furthermore, we provide evidence that immobilized MTG features higher siteselectivity than soluble MTG.

Site-selective enzymatic chemistry for polymer conjugation to protein lysine residues: PEGylation of G-CSF at lysine-41

Microbial transglutaminase (mTGase) is an enzyme that catalyzes site-specific protein derivatization at specific glutamines and lysines.

A fluorescence-based array screen for transglutaminase substrates

Transglutaminases (EC 2.3.2.13) form an enzyme family that catalyzes the formation of isopeptide bonds between the γ-carboxamide group of glutamine and the ε-amine group of lysine residues of peptides and proteins. Other primary amines can be accepted in place of lysine. Because of their important physiological and pathophysiological functions, transglutaminases have been studied for 60 years. However, the substrate preferences of this enzyme class remain largely elusive. In this study, we used focused combinatorial libraries of 400 peptides to investigate the influence of the amino acids adjacent to the glutamine and lysine residues on the catalysis of isopeptide bond formation by microbial transglutaminase. Using the peptide microarray technology we found a strong positive influence of hydrophobic and basic amino acids, especially arginine, tyrosine, and leucine. Several tripeptide substrates were synthesized, and enzymatic kinetic parameters were determined both by microarray analysis and in solution.