Purification and characterisation of a serine peptidase from the marine psychrophile strain PA-43.

An extracellular serine peptidase, purified from the culture supernatant of the sub-Arctic psychrophilic bacterium strain PA-43, is monomeric, with a relative molecular mass of 76000, and an unusually low pI of 3.8. The peptidase is active towards N-succinyl AAPF p-nitroanilide and N-succinyl AAPL p-nitroanilide, indicating a chymotrypsin-like substrate specificity. It is inhibited by the serine peptidase inactivator phenylmethylsulfonyl fluoride, but not by EDTA or EGTA, suggesting that added metal ions are not necessary for activity. The enzyme is most active at pH 8.3 and at 55-60 degrees C, although it is unstable at 60 degrees C. It is nevertheless remarkably stable for an enzyme from a psychrophilic microorganism, remaining active after 1 week at 20 degrees C and after five freeze-thaw cycles. Comparison of the N-terminal 40 amino acid residues with other archived sequences revealed highest similarity to the alkaline serine protease (aprx) from Bacillus subtilis.