Biology Reference
In-Depth Information
C J
Ei
C Ei ¼
HPI
ð
0
:
4
Þ
and GLUT
ð
¼
0
:
2
Þ
in AS-30D hepatoma cells; and glycogen
C J
E i
C Ei ¼
C Ei ¼
degradation
in HeLa cells.
Another relevant finding was that in tumor cells exposed to hypoxia or hypoglycemia
the main controlling steps remained the same with only slight quantitative changes in
the values of the control coefficients (Mar´n-Hern´ndez et al. 2011 ).
The model enabled identification of mechanisms by which these enzymes and
transporters control the glycolytic flux in each cell type. In AS-30D cells, HK over-
expression, as compared to healthy cells (up to 300-fold), and binding to the
mitochondrial outer membrane could explain the diminished flux control displayed
by this enzyme. Moreover, the HK over-expression and its ambiguous/ubiquitous
character in cancer cells promote increased G6P levels that potently inhibit the
enzyme, thus avoiding excessive ATP consumption that might compromise cell
viability (Mar´n-Hern´ndez et al. 2006 ). GLUT exerts high flux control as a
consequence of its low in vivo catalytic efficiency (both low V max and affinity for
glucose). Despite being one of the fastest enzymes, HPI is also potently inhibited by
physiological concentrations of several glycolytic and PPP intermediates, which is
likely related with its strategic location at the G6P crossroad involving the glyco-
lytic, glycogen, and PP pathways.
The standard concentration of 25 mM glucose present in the culture medium of
HeLa cells promotes glycogen accumulation and over-expression of GLUT1, the
isoform with low affinity for glucose ( K m ¼
ð
0
:
57
Þ;
GLUT
ð
0
:
39
Þ
and HK
ð
¼
0
:
08
Þ
9 mM). Consequently, the glycogen
degradation branch exerts high control on the glycolytic flux. In contrast, AS-30D
ascites cells that grow in the intraperitoneal cavity of rats where the glucose
concentration is about 0.026 mM (Rodr ´ guez-Enr ´ quez et al. 2000 ) express the
GLUT isoform with high affinity for glucose (GLUT3, K m ¼
0.5 mM; Rodr ´ guez-
Enr´quez et al. 2009 ) and maintain a low glycogen content. Under these conditions,
the AS-30D cells rely more on extracellular glucose for glycolysis to proceed,
leading to high HK control and negligible control by glycogen degradation (Mar´n-
Hern´ndez et al. 2011 ).
9.6 The Pattern of Enzyme Isoforms Expression and the
Control Distribution of Glycolysis
Most of the glycolytic enzymes and transporters in mammalian cells, except for
HPI and triose phosphate isomerase (TPI), have two to four different isoforms, each
one with specific kinetic characteristics (Mar´n-Hern´ndez et al . 2009 ). It is well
documented that the HIF-1
transcription factor, which is stabilized by low oxygen
conditions and found in high levels in tumor cells, upregulates tumor glycolysis.
This stimulation happens through increasing transcription of a particular set of
glycolytic protein isoforms that have higher affinity for substrates and catalytic
capacity ( V max ) in the forward reaction along with decreased sensitivity to their
reaction products and physiological inhibitors (Mar ´ n-Hern ´ ndez et al. 2009 ).
α
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