Production, purification and characterization of thermomycolase, the extracellular serine protease of the thermophilic fungus Malbranchea pulchella var. sulfurea

Abstract

The thermophilic fungus Malbranchea pulchella produces a single extracellular, alkaline, serine protease when grown at 45 °C, on 2% casein as sole carbon source. The growth-associated production of protease in submerged cultures was inhibited by addition of glucose, amino acids, or yeast extract. A simple four-step purification which yields homogeneous protease in 78% yield is described. The protease has an isoelectric point of 6.0, a pH optimum of 8.5, and is completely inhibited by serine protease inhibitors. A specificity study with small synthetic ester substrates indicated that the protease preferentially hydrolyzed bonds situated on the carboxyl side of aromatic or apolar amino acid residues which are not β-branched, positively charged or of the D configuration. Peptidase substrates and others such as N-acetyl-L-tyrosine-ethyl ester were not hydrolyzed. The protease was stable over a broad range of pH (6.5–9.5 at 30 °C, 20 h), and was particularly thermostable (t_1/2 = 110 min at 73 °C, pH 7.4) in the presence of Ca²⁺ (10 mM). Macromolecules and Ca²⁺ also provide protection against the significant autolysis which occurs at pure protease concentrations greater than 0.01 mg/ml, as well as against surface denaturation which is enhanced by the presence of a silicone antifoam agent. Hence the stability of protease in submerged cultures is rationalized.