Engineering nitrile hydratase activity into a cysteine protease by a single mutation

Area of Science:

• Enzyme engineering
• Biocatalysis
• Protein chemistry

Background:

• Cysteine proteases, like papain, are enzymes with a catalytic active site.
• Peptide nitriles are substrates that can react with cysteine proteases.
• Understanding cysteine protease catalytic mechanisms is key to enzyme modification.

Purpose of the Study:

• To engineer peptide nitrile hydratase activity into the cysteine protease papain.
• To investigate the role of specific active site mutations in altering enzyme function.
• To enhance the catalytic efficiency of papain for nitrile hydrolysis.

Main Methods:

• Site-directed mutagenesis was used to introduce a single amino acid substitution (Gln19Glu) in the papain active site.
• Enzyme kinetics were measured to determine the catalytic efficiency (kcat/KM) of the wild-type and mutant enzymes.
• pH-dependency studies were conducted to elucidate the catalytic mechanism.

Main Results:

• The papain variant Gln19Glu exhibited significant peptide nitrile hydratase activity (kcat/KM = 1.15 x 10(3) M-1 s-1).
• The Gln19Glu mutation increased the catalytic rate (kcat) by at least 4 x 10(5)-fold at pH 5 compared to wild-type papain.
• The pH-dependency data supported the proposed mechanism involving the Glu19 residue in proton transfer.

Conclusions:

• A single mutation can successfully engineer novel catalytic activity (nitrile hydratase) into a cysteine protease.
• The engineered Glu19 residue acts as a general acid catalyst, facilitating thioimidate hydrolysis.
• This study highlights the power of rational enzyme design based on a deep understanding of catalytic mechanisms.