Modification of β-lactoglobulin by microbial transglutaminase under high hydrostatic pressure: Localization of reactive glutamine residues

Unaided efficient transglutaminase cross-linking of whey proteins strongly impacts the formation and structure of protein alginate particles

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Microbial transglutaminase (MTG) cross-linked >70% β-lactoglobulin (β-Lg) at pH 8.5.

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Initial MTG catalyzed isopeptide bond formation caused partial unfolding of β-Lg.

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>75% of whey protein cross-linked, forming hetero-polymers containing β-Lg.

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50% less alginate is needed to form particles with cross-linked than with native β-Lg.

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Cross-linked β-Lg and alginate formed suspendable hydrophobically driven particles.

There is a dogma within whey protein modification, which dictates the necessity of pretreatment to enzymatic cross-linking of β-lactoglobulin (β-Lg). Here microbial transglutaminase (MTG) cross-linked whey proteins and β-Lg effectively in 50 mM NaHCO₃, pH 8.5, without pretreatment. Cross-linked β-Lg spanned 18 to >240 kDa, where 6 of 9 glutamines reacted with 8 of 15 lysines. The initial isopeptide bond formation caused loss of β-Lg native structure with t_1/2 = 3 h, while the polymerization occurred with t_1/2 = 10 h. Further, cross-linking effects on protein carbohydrate interaction have been overlooked, leaving a gap in understanding of these complex food matrices. Complexation with alginate showed that β-Lg cross-linking decreased onset of particle formation, hydrodynamic diameter, stoichiometry (β-Lg/alginate) and dissociation constant. The complexation was favored at higher temperatures (40 °C), suggesting that hydrophobic interactions were important. Thus, β-Lg was cross-linked without pretreatment and the resulting polymers gave rise to altered complexation with alginate.

Aspects of high hydrostatic pressure food processing: Perspectives on technology and food safety

The last two decades saw a steady increase of high hydrostatic pressure (HHP) used for treatment of foods. Although the science of biomaterials exposed to high pressure started more than a century ago, there still seem to be a number of unanswered questions regarding safety of foods processed using HHP. This review gives an overview on historical development and fundamental aspects of HHP, as well as on potential risks associated with HHP food applications based on available literature. Beside the combination of pressure and temperature, as major factors impacting inactivation of vegetative bacterial cells, bacterial endospores, viruses, and parasites, factors, such as food matrix, water content, presence of dissolved substances, and pH value, also have significant influence on their inactivation by pressure. As a result, pressure treatment of foods should be considered for specific food groups and in accordance with their specific chemical and physical properties. The pressure necessary for inactivation of viruses is in many instances slightly lower than that for vegetative bacterial cells; however, data for food relevant human virus types are missing due to the lack of methods for determining their infectivity. Parasites can be inactivated by comparatively lower pressure than vegetative bacterial cells. The degrees to which chemical reactions progress under pressure treatments are different to those of conventional thermal processes, for example, HHP leads to lower amounts of acrylamide and furan. Additionally, the formation of new unknown or unexpected substances has not yet been observed. To date, no safety-relevant chemical changes have been described for foods treated by HHP. Based on existing sensitization to non-HHP-treated food, the allergenic potential of HHP-treated food is more likely to be equivalent to untreated food. Initial findings on changes in packaging materials under HHP have not yet been adequately supported by scientific data.

Cross-linking of hen egg white lysozyme by microbial transglutaminase under high hydrostatic pressure: localization of reactive amino acid side chains

After incubation of hen egg white lysozyme (HEWL) with microbial transglutaminase (mTG) under high pressure (400-600 MPa for 30 min at 40 °C), the formation of HEWL oligomers was observed via SDS electrophoresis. At atmospheric pressure, HEWL represents no substrate for mTG. Likewise, enzymatic treatment following a pretreatment with high pressure did not lead to oligomerization. Reactive amino acid side chains were identified by peptide mapping after tryptic digestion using RP-HPLC with ESI-TOF-MS. Isopeptide-containing peptide fragments were found only in HEWL samples simultaneously treated with enzyme and pressure. It was found that mTG exclusively cross-links HEWL under high pressure by formation of an isopeptide between lysine at position 1 and glutamine at position 121 in the peptide chain. Therefore, a pressure-induced partial and reversible unfolding of the protein with exposure of lysine and glutamine side chains has to occur, resulting in a site-directed oligomerization of HEWL by mTG. The enzymatic modification of HEWL by mTG under high pressure offers interesting perspectives for further functionalization reactions.

Cross-linking of type I collagen with microbial transglutaminase: identification of cross-linking sites

Collagen is a popular biomaterial. To deal with its lack of thermal stability and its weak resistance to proteolytic degradation, collagen-based materials are stabilized via different cross-linking procedures. Regarding the potential toxicity of residual cross-linking agents, enzyme-mediated cross-linking would provide an alternative and nontoxic method for collagen stabilization. The results of this study show that type I collagen is a substrate for mTG. However, epsilon-(gamma-glutamyl)lysine cross-links are only incorporated at elevated temperatures when the protein is partially or completely denatured. A maximum number of 5.4 cross-links per collagen monomer were found for heat-denatured collagen. Labeling with the primary amine monodansylcadaverine revealed that at least half of the cross-links are located within the triple helical region of the collagen molecule. Because the triple helix is highly ordered in its native state, this finding might explain why the glutamine residues are inaccessible for mTG under nondenaturing conditions.

Affinity of microbial transglutaminase to αs1-, β-, and acid casein under atmospheric and high pressure conditions

Kinetics for the reaction of microbial transglutaminase (MTG) with individual caseins in a TRIS-acetate buffer at pH 6.0 was evaluated under atmospheric pressure (0.1 MPa) and high pressure (400 MPa) at 40 °C. The reaction was monitored under the following limitations: The kinetics from the initial velocities was obtained from nonprogressive enzymatic reactions assuming that the individual catalytic constants of reactive glutamine residues are represented by the reaction between MTG and casein monomers. Enzyme reaction kinetics carried out at 0.1 MPa at 40 °C showed Henri-Michaelis-Menten behavior with maximal velocities of 2.7 ± 0.02 × 10(-3), 0.8 ± 0.01 × 10(-3), and 1.3 ± 0.30 × 10(-3) mmol/L · min and K(m) values of 59 ± 2 × 10(-3), 64 ± 3 × 10(-3), and 50 ± 2 × 10(-3) mmol/L for β-, α(s1)-, and acid casein, respectively. Enzyme reaction kinetics of β-casein carried out at 400 MPa and 40 °C also showed a Henri-Michaelis-Menten behavior with a similar maximal velocity of 2.5 ± 0.33 × 10(-3) mmol/L · min, but, comparable to a competitive inhibition, the K(m) value increased to 144 ± 34 × 10(-3) mmol/L. The reaction of MTG with α(s1)-casein under high pressure did not fit in to Henri-Michaelis-Menten kinetics, indicating the complex influence of pressure on protein-enzyme interactions.