PRIMARY STRUCTURE OF ASPERGILLOPEPSIN-I DEDUCED FROM NUCLEOTIDE-SEQUENCE OF THE GENE AND ASPARTIC ACID-76 IS AN ESSENTIAL ACTIVE-SITE OF THE ENZYME FOR TRYPSINOGEN ACTIVATION

The coding region of the aspergillopepsin I (EC 3.4.23.18) gene occupies 1340 base pairs of the genomic DNA and is separated into four exons by three introns. The predicted amino-acid sequence of aspergillopepsin I consists of 325 residues and is 32% and 27% homologous with those of human pepsin and calf chymosin. The cDNA of the gene prepared from mRNA has been cloned and expressed in yeast cells. To identify the residue of the substrate binding pocket in determining the specificity of aspergillopepsin I towards basic substrates, this residue was replaced with a serine residue by site-directed mutagenesis. The mutation is a single amino-acid change, Asp-76 converted to Ser-D76S, in the enzyme. The striking feature of this is that only the trypsinogen activating activity was destroyed. We therefore concluded that Asp-76 is the binding site towards basic substrates.