A Two-Step Refolding of Acid-Denatured Microbial Transglutaminase Escaping from the Aggregation-Prone Intermediate

Rational design of a disulfide bridge increases the thermostability of microbial transglutaminase

Microbial transglutaminase (MTG) has numerous industrial applications in the food and pharmaceutical sectors. Unfortunately, the thermostability of MTG is too low to tolerate the desired conditions used in many of these commercial processes. In a previous study, we used protein engineering to improve the thermostability of MTG. Specifically, we generated a T7C/E58C mutant of MTG from Streptomyces mobaraensis that displayed enhanced resistance to thermal inactivation. In this study, a rational structure-based approach was adopted to introduce a disulfide bridge to further increase the thermostability of MTG. In all, four new mutants, each containing a novel disulfide bond, were engineered. Of these four mutants, D3C/G283C showed the most promising thermostability with a significantly higher ∆T₅₀ (defined as the temperature of incubation at which 50% of the initial activity remains) of + 9 °C by comparison to wild-type MTG. Indeed, D3C/G283C combined enhanced thermostability with a 2.1-fold increased half-life at 65 °C compared with the wild-type enzyme. By structure-based rational design, we were able to create an MTG variant which might be useful for expanding the scope of application in food. KEY POINTS: • Microbial transglutaminase (MTG) is an enzyme used in many food applications • The applicability of MTG to various industrial processes other than the food sector is being investigated • Improvement of thermostability was confirmed for the disulfide bridge mutant D3C/G283C.

Effect of introducing a disulfide bridge on the thermostability of microbial transglutaminase from Streptomyces mobaraensis

Microbial transglutaminase (MTG) has been used extensively in academic research and the food industry through cross-linking or posttranslational modification of proteins. In our previous paper, the activity-increased MTG mutants were obtained by means of rational mutagenesis and random mutagenesis coupled with the newly developed screening system. In addition, the improvement of heat resistance of MTG is needed to expand further its industrial applications. Here, a structure-based rational enzyme engineering approach was applied to improve the thermostability of MTG by introducing an artificial disulfide bridge. As a result of narrowing down candidates using a rational approach, we successfully engineered a disulfide bridge into the N-terminal region of MTG by substituting Thr-7 and Glu-58 with cysteine. The T7C/E58C mutant was observed to have a de novo disulfide bridge and showed an increased melting temperature (Tm value) of 4.3 °C with retained enzymatic activity. To address the benefit-gained reason, we focused on the Cβ temperature factor of the amino-acid residues that might form a disulfide bridge in MTG. Introducing the disulfide bridge had no remarkable effect on the mutant aiming to stabilize the high temperature factor. On the other hand, the mutation was effective on the relatively stable region. The introduction of a disulfide bridge may therefore be effective to stabilize further the relatively stable part. This finding is considered to be useful for the rational design of mutants aiming at heat resistance of proteins.Key Points• Microbial transglutaminase (MTG) is used as a binder in the food industry.• MTG has the potential for use in the manufacturing of various commercial materials.• Enhanced thermostability was observed for the disulfide bridge mutant, T7C/G58C.

Salt-induced formations of partially folded intermediates and amyloid fibrils suggests a common underlying mechanism

Amyloid fibrils are misfolded forms of proteins and are involved in various diseases. They have been studied extensively with the aim to obtain a comprehensive understanding of protein folding and misfolding and to use this knowledge to develop therapeutic strategies against the associated diseases. Salt conditions are important factors determining the formation and stability of amyloid fibrils. In the 1990s, salt effects were studied extensively to understand the conformational stability of acid-denatured proteins, and the results of these studies revealed the role of electrostatic repulsion in forming the compact intermediate states. In this review, we compare the effects of salts on the compact intermediate states with those on the formation of amyloid fibrils under acidic conditions. The results argue that both protein folding and misfolding are driven by the same forces, although the resultant conformations are distinct because they are monomeric and multimeric reactions, respectively.

DNA induced aggregation of stem bromelain; a mechanistic insight

Negatively charged species such as nucleic acids have commonly been found to be associated with the proteinaceous deposits in the tissues of patients with amyloid diseases.

Impact of structural stability of cold adapted Candida antarctica lipase B (CaLB): in relation to pH, chemical and thermal denaturation

The effect of pH on the conformational behavior of Candida antartica lipase B (CaLB) has been monitored by spectroscopic and calorimetric studies.

Protonation favors aggregation of lysozyme with SDS

Different proteins have different amino acid sequences as well as conformations, and therefore different propensities to aggregate. Electrostatic interactions have an important role in the aggregation of proteins as revealed by our previous report (J. M. Khan et al., PLoS One, 2012, 7, e29694). In this study, we designed and executed experiments to gain knowledge of the role of charge variations on proteins during the events of protein aggregation with lysozyme as a model protein. To impart positive and negative charges to proteins, we incubated lysozyme at different pH values of below and above the pI (∼11). Negatively charged SDS was used to 'antagonize' positive charges on lysozyme. We examined the effects of pH variations on SDS-induced amyloid fibril formation by lysozyme using methods such as far-UV circular dichroism, Rayleigh scattering, turbidity measurements, dye binding assays and dynamic light scattering. We found that sub-micellar concentrations of SDS (0.1 to 0.6 mM) induced amyloid fibril formation by lysozyme in the pH range of 10.0-1.0 and maximum aggregation was observed at pH 1.0. The morphology of aggregates was fibrillar in structure, as visualized by transmission electron microscopy. Isothermal titration calorimetry studies demonstrated that fibril formation is exothermic. To the best of our current understanding of the mechanism of aggregation, this study demonstrates the crucial role of electrostatic interactions during amyloid fibril formation. The model proposed here will help in designing molecules that can prevent or reverse the amyloid fibril formation or the aggregation.

pH-dependent activation of Streptomyces hygroscopicus transglutaminase mediated by intein

Microbial transglutaminase (MTG) from Streptomyces is naturally secreted as a zymogen (pro-MTG), which is then activated by the removal of its N-terminal proregion by additional proteases. Inteins are protein-intervening sequences that catalyze protein splicing without cofactors. In this study, a pH-dependent Synechocystis sp. strain PCC6803 DnaB mini-intein (SDB) was introduced into pro-MTG to simplify its activation process by controlling pH. The recombinant protein (pro-SDB-MTG) was obtained, and the activation process was determined to take 24 h at pH 7 in vitro. To investigate the effect of the first residue in MTG on the activity and the cleavage time, two variants, pro-SDB-MTG(D1S) and pro-SDB-MTG(ΔD1), were expressed, and the activation time was found to be 6 h and 30 h, respectively. The enzymatic property and secondary structure of the recombinant MTG and two variants were similar to those of the wild type, indicating that the insertion of mini-intein did not affect the function of MTG. This insignificant effect was further illustrated by molecular dynamics simulations. This study revealed a controllable and effective strategy to regulate the activation process of pro-MTG mediated by a mini-intein, and it may have great potential for industrial MTG production.

Biotechnological applications of transglutaminases

In nature, transglutaminases catalyze the formation of amide bonds between proteins to form insoluble protein aggregates. This specific function has long been exploited in the food and textile industries as a protein cross-linking agent to alter the texture of meat, wool, and leather. In recent years, biotechnological applications of transglutaminases have come to light in areas ranging from material sciences to medicine. There has also been a substantial effort to further investigate the fundamentals of transglutaminases, as many of their characteristics that remain poorly understood. Those studies also work towards the goal of developing transglutaminases as more efficient catalysts. Progress in this area includes structural information and novel chemical and biological assays. Here, we review recent achievements in this area in order to illustrate the versatility of transglutaminases.