Physiology and Biochemistry of Sourdough Yeasts - Sourdough Biotechnology - page 154

Fig. 6.1 Combined effect of NaCl and a-amylase supplementation on the rate of volume increase
of doughs inoculated with S. cerevisiae and incubated at 30°C for 3 h (Gianotti et al.
unpublished)
(GDH), which forms glutamate from a-ketoglutarate and ammonium. Whenever
the concentration of ammonium ions is low, glutamine synthase is activated. This
forms glutamine from a-ketoglutarate and ammonium in an ATP-dependent reaction.
Glutamine is absolutely required as the prominent precursor of several biosynthetic
pathways for asparagine, tryptophane, histidine, arginine, carbamoyl-phosphate,
CTP, AMP, GMP, glucosamine and NAD. Saccharomyces cerevisiae does not use
nitrate as a nitrogen source but very few species of yeasts have that capacity. Nitrate
assimilation occurs via the activity of the NADPH-dependent nitrate reductase,
which forms nitrite. Further, nitrite is reduced to ammonium by the NADPH-dependent
nitrite reductase. The type of nutrients available defines the internal metabolic flow,
while their abundance often limits the rate of biomass production and the energy
available for growth [ 17 ] .
The use of the various nitrogen sources is highly regulated. It requires the
synthesis of specific catabolic enzymes and permeases, which are subject to nitrogen
catabolite repression. The de novo synthesis of permeases and catabolic enzymes is
controlled at the level of transcription, and often requires two distinct positive signals.