ATP cycle

FIGURE 18.8 The ATP cycle in cells. ATP is formed via photosynthesis in phototrophic cells or catabolism in heterotrophic cells. Energy-requiring cellular activities are powered by ATP hydrolysis, liberating ADP and Pj. 

The three control steps in glycolysis are reactions catalysed by HK, PFK-1 and PK. All three reactions involve ADP/ATP cycling and are strongly exergonic suggesting they operate far from the true equilibrium position. Such reactions are physiologically difficult to reverse and so act as metabolic one-way streets . 

Figure 3. Energy transformation half-cycles in fully coupled conservative metabolism. The catabolic half-cycle (k) is coupled to the ATP cycle (p, -p) which is itself coupled to anabolism (a) and work. These processes demand ATP, the energy for the transformation of which is supplied by the oxidation of glucose in the catabolic half-cycle. The system is irreversible so the net work is zero.

The most dramatic illustration of a mass-specific illusion is the comparative heat dissipation of the human erythrocyte and platelet. In mammals, both of these cell types are anucleate and discoid in shape, but the longest dimension of the former is four times that of the latter. Yet heat production of a human erythrocyte was shown to be 10 fW, a sixth that of a human platelet (61 fW see Table 1). The relatively high metabolic activity of platelets is probably due to the need to maintain a considerable phosphagen (phosphocreatine) pool for actomyosin contraction at stimulation and clot retraction. Phosphocreatine is synthesized from creatine using ATP and acts as a demand on the ATP cycle to drive the coupled catabolic half-cycle. On the other hand, ATP requirements of the erythrocyte are relatively small, being mostly confined to active transport of ions at the plasma membrane. 

This reaction is catalyzed by fructose-1,6-bispho-sphatase (FBPase-1). This enzyme is a tetramer (M.W. 164,000) of identical subunits. It is inhibited by AMP, inorganic phosphate, and fructose-2,6-bisphosphate, all of which are allosteric activators of 6-phosphofructo-1-kinase (PFK-1), the competing glycolytic enzyme (Figure 15-5). This inhibition prevents the simultaneous activation of the two enzymes and helps prevent a futile ATP cycle. However, this futile cycle apparently does operate at a low rate in mammalian muscle, and in the bumblebee it is essential for maintaining the flight muscle at 30°C. 

While this book is not the appropriate place for a detailed discussion of the ADP-ATP cycle, perhaps a bit more mechanistic detail is useful. The aerobic metabolism of glucose to produce ATP from ADP can be-considered to consist of three parts the fermentation of glucose to form pyruvate (and a small amount of ATP from ADP), the conversion of the pyruvate to carbon dioxide in the citric acid cycle in which NAD" " and FAD (the oxidized form of flavin adenine dinucleotide) are converted to NADH and FADH2, and their oxidation in the respiratory chain, resulting in the formation of a larger quantity of ATP from ADP. If oxygen is not available, so the reaction is anaerobic, only the first step, formation of pyruvate, occurs with a small amount of ATP production. 

Figure 8.11 The current view of the ATP cycle in muscle contraction. 

The ATP Cycle ATP is formed by photosynthesis or catabolism. Cell activities that require energy are powered by ATP hydrolysis. Ttie Pj represents a phosphate. 

---