Purification and characterization of a novel milk-clotting metalloproteinase from Paenibacillus spp. BD3526

In this study, a milk-clotting enzyme (MCE) isolated from Paenibacillus spp. BD3526 was purified and characterized. The MCE was purified 8.9-fold with a 10.11% recovery using ammonium sulfate precipitation and anion-exchange chromatography and the specific milk-clotting activity (MCA) reached 6791.73 SU/mg. The enzyme was characterized as a 35 kDa metalloproteinase, and the zymogen of which was encoded by a 1671 bp gene named zinc metalloproteinase precursor (zmp) with a predicted molecular weight of 59.6 kDa. The optimal temperature for MCA and proteolytic activity (PA) was 65 °C and 60 °C, respectively. The enzyme was stable over a pH range of 5.0–9.0 and at temperatures below 50 °C. The MCA was completely inactivated when the enzyme was heated at 60 °C for 30 min, and the PA was totally inactivated for 20 and 10 min when the enzyme was heated at 55 °C and 60 °C, respectively. The BD3526 enzyme was preferentially active towards κ-casein (κ-CN) and β-casein (β-CN), as determined by sodium dodecyl sulfate-polyacrylamide gels (SDS-PAGE), whereas the hydrolysis of αs-casein (αs-CN) was slow and comparable to that caused by chymosin and asparatic acid proteinase from Rhizomucor miehei. The cleavage site of the metalloproteinase in κ-CN was located at the Met106–Ala107 bond, as determined by mass spectrometry analysis.