PrA (EC 3.4.23.25), encoded by PEP4 in S. cerevisiae, is the most
important and massive proteolytic enzyme in yeast (Rothman,
Hunter, Valls, & Stevens, 1986; Rupp & Wolf, 1995). During beer
production, yeast cells release PrA, which breaks down proteins
resulting in abundant amino acids in beer (Teichert, Mechler,
Mülle, & Wolf, 1989). Many studies have shown that the deletion
of PEP4 gene in industrial brewer’s yeast helps beer maintain foam
stability, as this decreases the concentration of PrA (Hao et al.,
2008; Lu et al., 2012).