Cell Metabolism - Bioprocess Engineering: Kinetics, Biosystems, Sustainability and Reactor Design - page 540

concentrations of the molecules in the cell, the fluxes are nearly independent of the substrate
concentration at least during balanced growth periods. The enzymes or proteins assisting the
metabolic reactions along the pathways are saturated with the molecules of importance at
each step. Therefore, the strengths of the fluxes, or arrows, are indications of the levels of
the enzymes available in the cell for the particular reaction pathways.
For most animal cells, glucose and glutamine are the major carbon and energy sources.
Both nutrients are required; glucose provides pentose sugars via the PP pathway, glucos-
amine-6-phosphate and the widely used precursor glyceraldehyde-3-phosphate. While other
sugars such as fructose or galactose can be used in place of glucose, glucose is more rapidly
utilized. Glutamine is required for the synthesis of purines and the formation of guanine
nucleotides. Glutamine is also the primary source of nitrogen in the cell, via transamidation
and transamination reactions. Both carbon sources are required, although asparagine may
replace glutamine in some cells. Glutamine enters the cell and may be deamidated to gluta-
mate in the cytosol or in the mitochondria.
Figure 10.35 shows the interrelationship of major metabolic pathway in mammalian
cells. As can be seen on Fig. 10.35 , the metabolism of both glucose and glutamine are
interrelated; however, glutamine typically provides most of the energy required by the
cell through respiration. In all mammalian cells, glucose is metabolized to pyruvate. In
normal (i.e. nontumor) cells, pyruvate is converted to acetyl-CoA and oxidized via the
TCA cycle. The ATP produced by mitochondrial respiration regulates glycolysis as a result
of its inhibition of phosphofructokinase (PFK). Glucose-6-phosphate then accumulates
and regulates the phosphorylation of glucose via its action on hexokinase. When oxygen
is less available, the production of ATP in the mitochondria is reduced and PFK is deregu-
lated. More glucose-6-phosphate is consumed, increasing the glucose flux into the cell. As
oxidative phosphorylation is restricted at low oxygen concentrations, pyruvate is con-
verted to lactic acid as a means of regenerating NAD þ from the NADH generated by
glycolysis.
In contrast to normal mammalian cells, cultured cells, tumor cells, and proliferating cells
exhibit high rates of aerobic glycolysis. High rates of lactate production, similar to normal
cells under oxygen limitation, are found. Transformed cells have glycolytic enzymes which
exhibit altered regulation as a result of the action of protein kinases (resulting from proto-
oncogene expression). The number of glucose transporters, responsible for movement of
glucose into the cell, is increased when the cell is transformed. Thus, there is a greater poten-
tial for glucose uptake. A hexokinase isozyme is found bound to the mitochondria and
exhibits reduced inhibition by glucose-6-phosphate and ATP. This decreased inhibition
results in less control of glucose entry into the cell. Pyruvate kinase (PK) also shows a reduced
affinity for PEP. Thus, the increased flux of glucose into the cell results in higher levels of
glucose-6-phosphate (G6P) and fructose-6-phosphate (F6P).
Higher concentrations of fructose-1,6-diphosphate and fructose-2,6-diphosphate conse-
quently occur, overcoming the normal regulation of PFK. As PK is also less tightly regulated,
there is a higher flux of carbon to pyruvate. The respiratory capacity of the cell is limited, and
the excess pyruvate is metabolized to lactate as a means of regenerating NAD þ . As the
glucose concentration is increased from about 5 m mol/l to about 5 mmol/l, the specific
rate of glucose consumption increases significantly. Below about 0.5 mmol/l, over half the
glucose consumed by rat hepatomas, for example, is incorporated into nucleotides, but