Metabolic flux analysis using stoichiometric models for Aspergillus niger: comparison under glucoamylase-producing and non-producing conditions.

Abstract Aspergillus niger AB1.13 cultures with glucoamylase production (with d -glucose as substrate) and without glucoamylase production (with d -xylose as substrate) were characterized by metabolic flux analysis. Two comprehensive metabolic models for d -glucose- as well as for d -xylose-consumption were used to quantify and compare the metabolic fluxes through the central pathways of carbon metabolism at different pH-values. The models consist of the most relevant metabolic pathways for A. niger including glycolysis, pentose-phosphate pathway, citrate cycle, energy metabolism and anaplerotic reactions comprising two intracellular compartments, the cytoplasm and mitochondrion. When d -xylose was used as the sole carbon source, the relative flux of the substrate through the oxidative pentose-phosphate pathway (PPP) via G6P-dehydrogenase was unaffected by the pH-value of the culture medium. About 30% of d -xylose consumed was routed through the oxidative PPP. In contrast, the flux of d -glucose (i.e., under glucoamylase-producing conditions) through the oxidative PPP was remarkably higher and, in addition was significantly affected by the pH-value of the culture medium (40% at pH 5.5, 56% at pH 3.7, respectively). Summarizing, the flux through the PPP under glucoamylase producing conditions was 30–90% higher than for non-producing conditions.