Metabolism energy charge

Chapter 23 The integration and control of metabolism Energy charge... 

Thus ATP is the effective controller of metabolism but because AMP + ADP + ATP is constant, it is really the ratio of adenine nucleotides which is important This ratio is termed die adenylate charge or energy charge and is expressed as ... 

The theoretical limits are 1.0 (all ATP) and 0 (all AMP) with a normal working range of 0.75 to 0.9. The involvement of energy charge in the integration and regulation of metabolism is considered further in die BIOTOL text entitled Biosynthesis and the Integration of Cell Metabolism. ... 

Some specific solutes diffuse down electrochemical gradients across membranes more rapidly than might be expected from their size, charge, or partition coefficients. This facilitated diffusion exhibits properties distinct from those of simple diffusion. The rate of facilitated diffusion, a uniport system, can be saturated ie, the number of sites involved in diffusion of the specific solutes appears finite. Many facihtated diffusion systems are stereospecific but, fike simple diffusion, require no metabolic energy. 

P. H. Saglio, P. Raymond, and A. Pradet, Metabolic activity and energy charge of excised maize root tips under anoxia control by soluble sugars. Plant Physiology 7 6 l()53 (1980). 

Hansen, J.I., T. Mustafa, and M. Depledge. 1992b. Mechanisms of copper toxicity in the shore crab, Carcinus maenas. B. Effects on key metabolic enzymes, metabolites and energy charge potential. Mar. Biol. 114 259-264. 

E. E. Selkov, Stabilization of energy charge, generation of oscillation and multiple steady states in energy metabolism as a result of purely stoichiometric regulation. Eur. J. Biochem. 59, 151 157 (1975). 

An index of the phospho anhydride (i.e.,P—O—P) bond content of the adenine nucleotides of a cell, based on a hypothetical modeP that attempts to explain the metabolic basis for control of ATP utilization and regeneration. Later studies demonstrated that the energy charge model is overly simplistic and that its principles are unlikely to constitute a useful model for the control of energy metabolism within biological systems. 

The values for the energy charge could conceivably vary from 0 to 1, as illustrated in figure 11.11. In fact, however, the values for real cells are usually held within very narrow limits. The result is a general stabilizing effect on the cellular metabolism. In much the same way, voltage regulation is necessary for the effective operation of many electronic devices. Now let us see how the limits are maintained. 

This transfer of reducing equivalents is essential for maintaining the favorable NAD+/NADH ratio required for the oxidative metabolism of glucose and synthesis of glutamate in brain (McKenna et al., 2006). The malate-aspartate shuttle is considered the most important shuttle in brain. It is particularly important in neurons. It has low activity in astrocytes. This shuttle system is fully reversible and linked to amino acid metabolism with the energy charge and citric acid cycle of neuronal cells. 

Tolerance of the fruit to anoxia is evidenced by the maintenance of a high energy charge (an index of the cell energy status), very close to that of berries in air for 4-7 days. In addition, mitochondria retain much of their ability to return to respiratory metabolism on exposure to air after... 

Many solute properties are intertwined with those of the ubiquitous solvent, water. For example, the osmotic pressure term in the chemical potential of water is due mainly to the decrease of the water activity caused by solutes (RT In aw = —V ri Eq. 2.7). The movement of water through the soil to a root and then to its xylem can influence the entry of dissolved nutrients, and the subsequent distribution of these nutrients throughout the plant depends on water movement in the xylem (and the phloem in some cases). In contrast to water, however, solute molecules can carry a net positive or negative electrical charge. For such charged particles, the electrical term must be included in their chemical potential. This leads to a consideration of electrical phenomena in general and an interpretation of the electrical potential differences across membranes in particular. Whether an observed ionic flux of some species into or out of a cell can be accounted for by the passive process of diffusion depends on the differences in both the concentration of that species and the electrical potential between the inside and the outside of the cell. Ions can also be actively transported across membranes, in which case metabolic energy is involved. 

Many reactions in metabolism are controlled by the energy status of the cell. One index of the energy status is the energy charge, which is proportional to the mole fraction of ATP plus half the mole fraction of ADP, given that ATP contains two anhydride bonds, whereas ADP contains one. Hence, the energy charge is defined as... 

Figure 14.18. Energy Charge Regulates Metabolism. High concentrations of ATP inhibit the relative rates of a typical ATP-generating (catabolic) pathway and stimulate the typical ATP-utilizing (anabolic) pathway.

---