In enzymology, a protein-glutamine glutaminase () is an enzyme that catalyzes the chemical reaction
Thus, the two substrates of this enzyme are protein L-glutamine (a glutaminyl residue on a protein) and H<sub>2</sub>O, whereas its two products are protein L-glutamate (a glutamyl reside on a protein) and NH<sub>3</sub>.
This enzyme belongs to the family of hydrolases, those acting on carbon-nitrogen bonds other than peptide bonds, specifically in linear amides. The systematic name of this enzyme class is protein-L-glutamine amidohydrolase. Other names in common use include peptidoglutaminase II, glutaminyl-peptide glutaminase, destabilase, and peptidylglutaminase II.
In food production, this enzyme can increase the solubility and potentially reduce the off-flavor of proteins and peptides by forming negatively-charged glutamyl residues. It does not create free glutamin, hence does not add to the umami flavor.