Effect of N-terminal mutation of human lysozyme on enzymatic activity

Using mutant signal peptide introduced Pro at the C-terminal region, 5 kinds of N-terminal mutants of human lysozyme (HLY) were produced. The genes coding chicken lysozyme signal peptide - human lysozyme (HLY) hybrid preproteins were altered as follows: -2Leu to Pro and -lGly to either Ala, Val, Leu, Asn or Lys, and were expressed in yeast cells. The N-terminal sequence data of HLYs secreted from yeast cells, showed that each signal peptide was cleaved only after -2Pro and either Ala, Val, Leu, Asn or Lys was added at their N-terminals of HLYs. This result suggested that the cleavage site of signal peptide can be shifted by the introduction of turn-promoting residue like Pro and N-terminal sequence of protein can be altered. The differences of additional residues were reflected in lytic activities of HLYs. Especially, Lys-HLY exhibited significantly wide optimal pH range and resistance to higher ionic strength conditions than other HLYs including native HLY. This result indicates that the importance of positive charge on the HLY surface to interact with substrate.