Highenergy Phosphates Act As The Energy Currency Of The Cell

ATP is able to act as a donor of high-energy phosphate to form those compounds below it in Table 10-1. Likewise, with the necessary enzymes, ADP can accept high-energy phosphate to form ATP from those compounds above ATP in the table. In effect, an ATP/ ADP cycle connects those processes that generate ~© to those processes that utilize ~© (Figure 10-6), continuously consuming and regenerating ATP. This occurs at a very rapid rate, since the total ATP/ADP pool is extremely small and sufficient to maintain an active tissue for only a few seconds.

There are three major sources of ~© taking part in energy conservation or energy capture:

(1) Oxidative phosphorylation: The greatest quantitative source of ~© in aerobic organisms. Free energy

Figure 10-5. Structure of ATP, ADP, and AMP showing the position and the number of high-energy phosphates (~©).
Figure 10-6. Role of ATP/ADP cycle in transfer of high-energy phosphate.
Figure 10-7. Transfer of high-energy phosphate between ATP and creatine.

comes from respiratory chain oxidation using molecular O2 within mitochondria (Chapter 11).

(2) Glycolysis: A net formation of two ~© results from the formation of lactate from one molecule of glucose, generated in two reactions catalyzed by phospho-glycerate kinase and pyruvate kinase, respectively (Figure 17-2).

(3) The citric acid cycle: One ~© is generated directly in the cycle at the succinyl thiokinase step (Figure

Phosphagens act as storage forms of high-energy phosphate and include creatine phosphate, occurring in vertebrate skeletal muscle, heart, spermatozoa, and brain; and arginine phosphate, occurring in invertebrate muscle. When ATP is rapidly being utilized as a source of energy for muscular contraction, phosphagens permit its concentrations to be maintained, but when the ATP/ADP ratio is high, their concentration can increase to act as a store of high-energy phosphate (Figure

When ATP acts as a phosphate donor to form those compounds of lower free energy of hydrolysis (Table 10-1), the phosphate group is invariably converted to one of low energy, eg,

ATP Allows the Coupling of Thermodynamically Unfavorable Reactions to Favorable Ones

The phosphorylation of glucose to glucose 6-phos-phate, the first reaction of glycolysis (Figure 17-2), is highly endergonic and cannot proceed under physiologic conditions.

(1) Glucose+P ^ Glucose 6-phosphate+ H2O kJ mol)

To take place, the reaction must be coupled with an-other—more exergonic—reaction such as the hydrolysis of the terminal phosphate of ATP.

When (1) and (2) are coupled in a reaction catalyzed by hexokinase, phosphorylation of glucose readily proceeds in a highly exergonic reaction that under physiologic conditions is irreversible. Many "activation" reactions follow this pattern.

Adenylyl Kinase (Myokinase) Interconverts Adenine Nucleotides

This enzyme is present in most cells. It catalyzes the following reaction:

AL!LPivLvI

This allows:

(1) High-energy phosphate in ADP to be used in the synthesis of ATP.

(2) AMP, formed as a consequence of several activating reactions involving ATP, to be recovered by rephosphorylation to ADP.

(3) AMP to increase in concentration when ATP becomes depleted and act as a metabolic (allosteric) signal to increase the rate of catabolic reactions, which in turn lead to the generation of more ATP (Chapter 19).

When ATP Forms AMP, Inorganic Pyrophosphate (PPi) Is Produced

This occurs, for example, in the activation of long-chain fatty acids (Chapter 22):

This reaction is accompanied by loss of free energy as heat, which ensures that the activation reaction will go to the right; and is further aided by the hydrolytic splitting of PPj, catalyzed by inorganic pyrophosphatase, a reaction that itself has a large AG0' of -27.6 kJ/

F IhCftflrfMD ; ^IlSHEBft-lAnöt r-----------------------,

Figure 10-8. Phosphate cycles and interchange of adenine nucleotides.

Figure 10-8. Phosphate cycles and interchange of adenine nucleotides.

mol. Note that activations via the pyrophosphate pathway result in the loss of two ~© rather than one ~© as occurs when ADP and P; are formed.

FvTBPHQSPI-lffrflSE

A combination of the above reactions makes it possible for phosphate to be recycled and the adenine nucleotides to interchange (Figure 10-8).

Other Nucleoside Triphosphates Participate in the Transfer of High-Energy Phosphate

By means of the enzyme nucleoside diphosphate kinase, UTP, GTP, and CTP can be synthesized from their diphosphates, eg,

All of these triphosphates take part in phosphoryla-tions in the cell. Similarly, specific nucleoside monophosphate kinases catalyze the formation of nucleoside diphosphates from the corresponding monophosphates.

Thus, adenylyl kinase is a specialized monophosphate kinase.

Diabetes 2

Diabetes 2

Diabetes is a disease that affects the way your body uses food. Normally, your body converts sugars, starches and other foods into a form of sugar called glucose. Your body uses glucose for fuel. The cells receive the glucose through the bloodstream. They then use insulin a hormone made by the pancreas to absorb the glucose, convert it into energy, and either use it or store it for later use. Learn more...

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