Purification and Characterization of Aspartic Protease Produced fromAspergillus oryzaeDRDFS13 under Solid-State Fermentation

Abstract

Aspartic proteases (E.C.3.4.23.) are endopeptidases with molecular masses ranging between 30-45 kDa. They depend on aspartic acid residues for their catalytic activity and show maximal activity at low pH. Thus the main objective of the present study was to purify and characterize aspartic protease from locally identified fungi by solid-state fermentation. The aspartic protease in the current study was obtained from A. oryzae DRDFS13 under SSF. The crude enzyme extract was purified by size-exclusion (SEC) and ion-exchange (IEC) chromatography. The protein contents of crude enzyme and IEC fractions were determined by BCA methods while the presence of N-glycosylation was checked using Endo-H. Inhibition studies were conducted using protease inhibitors. The milk-clotting activity (MCA), protease activity (PA); molecular weight and enzyme kinetics were determined using standard methods. Optimum temperature and stability, optimum pH and stability, and the effect of cations on MCA were assessed using standard methods. The maximum MCA (477.11 U/mL) was recorded from IEC fraction A8. The highest specific activity (183.50 U/mg), purification fold (6.20) and yield (9.2%) were also obtained from the same fraction (IEC A8). The molecular weight of 40 kDa was assigned for the purified enzyme (IEC A8). However, its molecular weight was decreased to 30 KDa upon deglycosylation assay which infers that the protein is glycosylated. Incubation of the pure enzyme (IEC A8) with pepstatin A caused a 94 % inhibition on MCA. The dialyzed enzyme showed a Km and Vmax values of 17.50 mM and 1369 U, respectively. The enzyme showed maximum MCA at 60 oC and pH 5.0 with stability at pH 4.5-6.5 and temperature 35-45 oC. Most cat-ions stimulate the activity of the enzyme; moreover, the highest MCA was detected at 50 mM of MnSO4. Furthermore, the results obtained in the present study confirmed that the aspartic protease enzyme produced from A. oryzae DRDFS13 and purified in ion-exchange chromatography could be used as a substitute source of rennet enzyme for cheese production.