EJB Electronic Journal of Biotechnology
ISSN: 0717-3458 |
Vol.4 No. 1, Issue of April
15, 2001 |
© 2001 by Universidad Católica de Valparaíso
-- Chile |
Received October 27, 2000 / Accepted January 6, 2001 |
Optimization of the
citric acid production by Aspergillus niger through a metabolic
flux balance model
Daniel V.
Guebel
Counseling Biotechnological
Services
Av. San Martin 4927 Dpto. A.
(C1417DSJ)
Buenos Aires. Argentina
Tel: 54-11-45039355
E-mail: dguebel@radar.com.ar
Néstor
V. Torres Darias*
Grupo de Tecnología
Bioquímica y Control Metabólico
Dpto. de Bioquímica y Biología Molecular
Facultad de Biología, Universidad de La Laguna
38206 La Laguna, Tenerife, Islas Canarias, España
Tel and fax: +34-922-318334
E-mail: ntorres@ull.es
*Corresponding author
Financial support:
The work of one of us (N.V.T.D) was supported by a research
grant from the Comisión Interministerial de Ciencia y Tecnología,
contract nº BIO99-0492-C02-02 and also by a research from the Gobierno
de Canarias, contract nº PI2000-071.
Keywords: bioenergetics,
metabolic engineering, metabolic pathway, stoichiometry.
Idiophase, the citric acid producing stage of
Aspergillus niger was mathematically modeled to identify required
genetic manipulations to optimize citric acid production rate. For this
reason, a consistent picture of cell functioning had to be achieved. The
transient idiophase nature was established by stoichiometric analysis. The
main intracellular fluxes were computed by application of material and physiological
constraints (ATP, reduction equivalents, proton motive force) at culture
time 120 hours. The HMP pathway accounts for 16% of the glucose input (carbon
basis), the Krebs cycle for 13% and the citric acid synthesis for the remaining
71%. This profile implies an operative glycerol-P shuttle. It recycles 93%
of the cytosolic glycerol-P to cytosolic DHAP thus coupling the transformation
of cytosolic NADH to mitochondrial FADH. A cellular maintenance energy of
3.7 mmol ATP/g·h was determined. It would be spent in fueling cytoplasmatic
(1.4 mmol H+/g·h) and mitochondrial (1.8 mmol H+/g·h)
H+-ATPase pumps with efficiencies of 0.65 and 1.2 mmol H+/mmol
ATP respectively. The role and extent of the alternative respiration system
activity and polyol excretion is accounted by the model as well. In addition,
the significance of GABA shunt and futile NH4+/NH3
cycle were rejected. According to the developed model, the specific citric
productivity would be increased in 45% by an unique change if glucose influx
were duplicated. Differences with predictions from other model that required
many manipulations are also discussed.
|