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 266

compartments involving kinase cycles (Dzeja and Terzic 2003 , 2009 ; Dzeja
et al. 1999 ; Nemutlu et al. 2012 ). Most effective and informative in bioenergetic
studies of phosphoryl transfer has been the use of 18 O transfer (see the Chap. 6 ) .
This method is based on the following two reactions: ATP hydrolysis by water
molecules containing

18 O and ATP resynthesis with formation of [ 18 O]

γ

ATP

(Dzeja and Terzic 2009 ; Nemutlu et al. 2012 ):

18 O H 2 O 18 O Pi

ATP þ ! þ ADP (11.1)

18 O Pi 18 O γ

þ ADP ! ATP (11.2)

Paul Boyer used this method for studying the ATP synthase reaction (Boyer
1997 ). Inclusion of [ 18 O]Pi into [ 18 O] ATP in the presence of uncouplers led him to

the conclusion of the rotational binding change mechanism of mitochondrial ATP
synthesis. Nelson Goldberg, Petras Dzeja, Andr ´ Terzic, and coworkers have
successfully applied this method for studying the kinetics of phosphoryl-transfer
reactions and energy fluxes in vivo by measuring the rates of the following
reactions (Dzeja and Terzic 2003 , 2009 ; Nemutlu et al. 2012 ):
Creatine kinase phosphotransfer:
18 O γ

γ

18 O PCr

ATP þ Cr ! þ ADP (11.3)

Adenylate kinase phosphotransfer:
18 O γ

18 O β 18 O β

ATP þ AMP ! ADP þ ADP ! ATP þ AMP (11.4)

Glycolytic phosphotransfer:

18 O γ 18 O G6P

ATP þ Glucose ! þ ADP (11.5)

If a direct transfer of ATP from mitochondria to MgATPases happens together
with its immediate hydrolysis for contraction as sometimes proposed in the litera-
ture, only isotope transfer reactions 1 and 2 could be observed. In an excellent series
of studies Dzeja's group showed that in normal cardiac cells about 80-85 % of
phosphoryl groups are transferred out from mitochondria by the PCr flux, and about
10-15 % by adenylate kinase, with a minor contribution by glycolysis (Dzeja
et al. 1999 ). In the heart, these fluxes increase linearly with workload energy
demand under conditions of the Frank-Starling law (Saks et al. 2007c ). Figure 11.6
shows that PCr fluxes measured experimentally can be quantitatively simulated
with a mathematical model of compartmentalized energy transfer (Dos Santos
et al. 2000 ; Aliev et al. 2012 ; Aliev and Saks 1997 ; Vendelin et al. 2000 ). This
model was based on the experimental data obtained in studies of mitochondrial PCr
synthesis in permeabilized cardiomyocytes. The role of the adenylate kinase system
becomes important in hypoxia and pathological situations (Dzeja et al. 1999 ).