Physicochemical properties of proteinases from selected psychrotrophic bacteria

Abstract

SUMMARYThe physicochemical properties of eight extracellular proteinases secreted by psychrotrophic bacteria of dairy origin have been studied. Seven of these proteinases were able to withstand ultra heat treatment (UHT) with D values at 140 °C ranging from 2 to 300 s. The sixPseudomonas fluorescensproteinases were glycoproteins of mol. wt 47000–49500. The twoSerratia marcescensproteinases, of mol. wt of 51000, did not contain carbohydrate but in other respects were similar to the Pseudomonas proteinases. The proteinases were inhibited by various metal chelators and all contained Ca and Zn in similar proportions. Their amino acid compositions were similar, with alanine as the N-terminal group, cysteine completely absent and very low levels of methionine. Isoelectric points ranged from 5·10 to 8·25. Their physical and chemical properties enabled them to be classified as alkaline metalloendopeptidases. A similarity index (S▵n) was used to predict sequence homology between ten proteinases of known amino acid composition. Comparisons of S▵n of these proteinases showed only minor sequence differences except for those ofPs. fluorescensMC60. Heat resistance could not be related wholly to similarities in protein sequence, but could be related both to the strength of stabilizing Ca2+–protein interactions and to the randomness inherent within the folding of the peptide chain.