Chymosin Pseudogene

Chymosin (EC 3.4.23.4) is an aspartyl proteinase with an exceedingly high specificity. The enzyme cleaves only the phenylalanine-methionine peptide bond between amino acid residues 105 and 106 in kappa-casein, leading to the coagulation of milk. In the biotechnological industry, chymosin has a wide use in cheese manufacturing as the coagulant of milk. Chymosin is synthesized in the chief and mucous neck cells of the gastric glands in the fourth stomach of newborn calves. The enzyme is synthesized as an enzymatically inactive precursor, preprochymosin. In the process of secretion, preprochymosin, comprising 381 amino acids, is processed by the signal peptidase into an inactive 365-amino acid prochymosin. At low pH, prochymosin undergoes autocatalytic cleavage of 42 N-terminal amino acids, yielding active chymosin. Chymosin is also found in other mammalian species, e.g., the newborn pig, cat, seal, and lamb. The presence of chymosin in humans is a matter of controversy. Ord et al. (1990) cloned a human genomic sequence homologous to the bovine prochymosin gene. The sequence showed a 1-bp deletion and a 2-bp deletion in the human sequence corresponding to bovine prochymosin exons 4 and 6, respectively. There also was a terminator codon in the open reading frame corresponding to bovine prochymosin exon 5. Thus, this genomic sequence apparently represents a human prochymosin pseudogene. No functional gene was identified. Using DNA obtained from human/hamster somatic cell hybrids as a PCR template, Kolmer et al. (1991) mapped the human prochymosin pseudogene to chromosome 1. Foltmann (1992) gave a review.