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.