Systems Level Regulation of Cardiac Energy Fluxes Via Metabolic Cycles: Role of Creatine, Phosphotransfer Pathways, and AMPK Signaling - Systems Biology of Metabolic and Signaling Networks - page 270

unidirectionally toward PCr synthesis utilizing mitochondrial ATP supplied by
ANT (direct channeling). This process moves ADP back into mitochondria,
because of the differential permeability of VDAC in interaction with tubulin that
impedes ADP release from mitochondria. These coupled reactions of oxidative
phosphorylation and PCr synthesis in MI are effectively regulated by Cr (Fig. 11.8 ).
In the presence of an extra-mitochondrial ADP trapping system (pyruvate kinase,
PK; phosphoenolpyruvate, PEP), Cr addition rapidly increases the respiration rate
to its maximal value, revealing a preferential accessibility of the ADP produced by
MtCK to matrix ATPase, not to the cytosolic trapping system. Metabolic control
analysis of mitochondrial respiration in permeabilized cardiac cells showed high
flux control coefficients (FCC) for reactions involving ADP recycling coupled to
MtCK and PCr production (Fig. 11.9a ). Actually, the sum of control coefficients
exceeds the theoretical value for linear systems by a factor of 4 (Tepp et al. 2011 ).
This can be interpreted in terms of MtCK-controlled reactions in MI acting as very
effective amplifiers of metabolic signals from cytoplasm (Tepp et al. 2011 ; Aon and
Cortassa 2012 ). According to Kholodenko, Westerhoff, and their coworkers, the
sum of the FCC of the metabolic pathway components exceeding one indicates a
direct channeling in the pathway (Moreno-Sanchez et al. 2008 ). On the contrary, in
isolated heart mitochondria and permeabilized cardiac fibers the sum of FCC of
respiratory chain complexes, ATP synthase, and metabolite carriers, estimated
under conditions of respiration activated by ADP, is close to 1, corresponding to
a linear metabolic pathway (Moreno-Sanchez et al. 2008 ; Kuznetsov et al. 1996 ;
Doussiere et al. 1984 ; Fell and Thomas 1995 ; Groen et al. 1982 ). The high
efficiency of energy flux control in MI makes this supercomplex a key site for the
feedback of metabolic regulation of mitochondrial respiration in cardiac cells (Saks
et al. 2012 ; Tepp et al. 2011 ).
Figure 11.9b depicts the possible role of both Cr and ADP in the control of
respiration in situ. Extra- and intra-mitochondrial ADP in the regulation of respira-
tion was studied by MgATP titration in the absence or presence of Cr, i.e., activated
MtCK (Saks et al. 2012 ; Guzun et al. 2009 ; Guzun and Saks 2010 ; Timohhina
et al. 2009 ). The influence of mitochondrial ADP alone on respiration was
estimated by removing extra-mitochondrial ADP through the PEP-PK trapping
system mimicking glycolytic ADP consumption. From Fig. 11.9b we can see that
stimulation of the extra-mitochondrial ADP producing system by MgATP alone
cannot effectively activate respiration. The high apparent K m for exogenous
MgATP (157.8

M) corresponds to the apparent K m of myofibrillar ATPase
reaction for MgATP. However, when oxidative phosphorylation is stimulated by
both extra- and intra-mitochondrial ADP (in the presence of Cr to activate MtCK
and MM-CK in myofibrils), the respiration rate increases rapidly up to maximal
values and the apparent K m for ATP decreases from 157.8

40.1 μ

40.1 μ Mto

24.9 M. Removal of extra-mitochondrial ADP by PEP-PK provokes an

increase of K m for MgATP up to 2.04

0.8 μ

0.10 mM. These results show that local
endogenous ADP in ICEUs is an important regulatory factor of respiration but only
in the presence of Cr and activated MtCK. The stimulatory effect of respiration by
endogenous ADP is strongly amplified by functional coupling of MtCK with ANT