Cellular Metabolism, Regulation, and Homeostasis

The simplest mechanism for generating ATP is phosphagen mobilization. In vertebrate tissues such as muscle containing creatine phosphate (PCr) this mobilization is catalyzed by creatine phosphokinase (CPK), a process which requires no 02 and can be written as follows  

Other phosphagens (such as arginine phosphate, lombrocine phosphate, tauromyocine phosphate) are found in many invertebrate muscles. 

Fermentation, or the partial (02 independent) catabolism of substrates to anaerobic end products, is a second means of forming ATP. In animals, the commonest fermentative pathway is that of anaerobic glycolysis (figure 2.1). At high pH ( 8.0), the summed reaction can be written as follows  

In animal fermentations, an organic molecule (e.g., pyruvate) serves as a terminal proton and electron acceptor, forming an organic end product (e.g., lactate). In contrast, 02 is required as a terminal acceptor for the complete oxidation of substrates such as glucose, glycogen, fatty acids, or amino acids. As discussed in chapter 3, 02 was not always available as one of the substrates for oxidative metabolism and organisms in primordial times had to rely on anaerobic metabolic processes. 

The developed H+ concentration gradient plus an electric potential across the membrane supply the driving force for ATP synthesis from ADP and Pi, a thermodynamically unfavorable reaction catalyzed by ATP synthase (Karrasch and Walker, 1999). The latter is a mitochondrial enzyme located on, and spanning, the inner mitochondrial membrane. At least when in submitochondrial particles, ATP synthase saturation kinetics involve ADP positive site-site interactions in catalysis. One group has proposed that ADP saturation in vivo also shows site-site interactions ( , the interaction or Hill coefficient increasing from 1, meaning no interaction, to 2) however, others have not found this, so this issue at this time must be considered to remain unresolved. 

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