Evaluation of the milk clotting properties of an aspartic peptidase secreted by Rhizopus microsporus

Abstract

Traditionally, chymosin has been used for milk-clotting, but this naturally occurring enzyme is in short supply and its use has raised religious and ethical concerns. Because milk-clotting peptidases are a promising substitute for chymosin in cheese preparation, there is a need to find and test the specificity of these enzymes. Here, we evaluated the milk-clotting properties of an aspartic peptidase secreted by Rhizopus microsporus. The molecular mass of this enzyme was estimated at 36 kDa and Pepstatin A was determined to be an inhibitor. Optimal activity occurred at a pH of 5.5 and a temperature range of 50–60 °C, but the peptidase was stable in the pH range of 4–7 and a temperature as low as 45 °C. Proteolytic activity was significantly reduced in the presence of Cu2+ and Al3+. When enzyme substrates based on FRET were used, this peptidase exhibited the highest catalytic efficiency for Abz-KNRSSKQ-EDDnp (4,644 ± 155 mM−1.s−1), Abz-KLRSSNQ-EDDnp (3,514 ± 130 mM−1.s−1), and Abz-KLRQSKQ-EDDnp (3,068 ± 386 mM−1.s−1). This study presents a promising peptidase for use in cheese making, due to its high stability in the presence of Ca2+ and broad pH range of 4–7, in addition to its ability to efficiently clot milk.