A mutated bovine prochymosin zymogen can be activated without proteolytic processing at low pH

Area of Science:

• Biochemistry
• Molecular Biology
• Enzymology

Background:

• Prochymosin is a zymogen, an inactive precursor to the enzyme chymosin.
• Zymogen activation involves structural changes and often proteolytic cleavage.
• Prochymosin has two distinct autolysis sites, leading to pseudochymosin at pH 2 and chymosin at pH 4-5.

Purpose of the Study:

• To investigate the role of the amino-terminal propeptide in prochymosin zymogen activation.
• To determine if proteolytic processing is essential for zymogen activation.
• To examine the independence of the two autolysis sites in prochymosin.

Main Methods:

• Site-directed mutagenesis was used to alter the nucleotide sequence encoding the prochymosin propeptide.
• Specific codons at one autolysis site were modified (one deletion, two substitutions).
• The genetically modified prochymosin was subjected to activation conditions at different pH values (2.0 and 4.5).

Main Results:

• The engineered prochymosin exhibited normal activation and proteolytic processing at pH 4.5.
• At pH 2.0, only partial activation was observed, with no evidence of proteolytic processing.
• This indicates that the modified site is not required for activation at pH 4.5.

Conclusions:

• Zymogen activation of prochymosin does not strictly require proteolysis.
• The two distinct autolysis sites of prochymosin can function independently.
• The propeptide region plays a crucial role in mediating pH-dependent activation pathways.