The high group transfer potential of ATP enables it to act as a donor of high-energy phosphate to form those compounds below it in Table 1. Likewise, with the necessary enzymes, ADP can accept phosphate groups 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 1), 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.

Table1. Standard Free Energy of Hydrolysis of Some Organophosphates of Biochemical Importance

Fig1. Role of ATP/ADP cycle in transfer of high energy phosphate.

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

1. Oxidative phosphorylation is the greatest quantitative source of ~ⓟ in aerobic organisms. ATP is generated in the mitochondrial matrix as O2 is reduced to H₂O by electrons passing down the respiratory chain.

2. Glycolysis. A net formation of two ~ⓟ  results from the formation of lactate from one molecule of glucose, generated in two reactions catalyzed by phosphoglycerate kinase and pyruvate kinase, respectively.

 3. The citric acid cycle. One ~ⓟ  is generated directly in the cycle at the succinate thiokinase step.

Phosphagens act as storage forms of group transfer potential and include creatine phosphate, which occurs in vertebrate skeletal muscle, heart, spermatozoa, and brain, and arginine phosphate, which occurs 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 an energy store (Figure 2).

Fig2. Transfer of high-energy phosphate between ATP and creatine.

When ATP acts as a phosphate donor to form com pounds of lower free energy of hydrolysis (see Table 1), the phosphate group is invariably converted to one of low energy. For example, the phosphorylation of glycerol to form glycerol-3-phosphate:

ATP Allows the Coupling of Thermodynamically Unfavorable Reactions to Favorable Ones

Endergonic reactions cannot proceed without an input of free energy. For example, the phosphorylation of glucose to glucose 6-phosphate, the first reaction of glycolysis:

is highly endergonic and cannot proceed under physiologic conditions. Thus, in order to take place, the reaction must be coupled with another—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:

Adenylyl kinase is important for the maintenance of energy homeostasis in cells because it allows:

 1. The group transfer potential in ADP to be used in the syn thesis of ATP.

2. The AMP formed as a consequence of activating reactions involving ATP to be rephosphorylated to ADP.

3. AMP to increase in concentration when ATP becomes depleted so that it is able to act as a metabolic (allosteric) signal to increase the rate of catabolic reactions, which in turn lead to the generation of more ATP.

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

ATP can also be hydrolyzed directly to AMP, with the release of PPi (see Table 1). This occurs, for example, in the activation of long-chain fatty acids.

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 PPi , catalyzed by inorganic pyrophosphatase, a reaction that itself has a large ΔG0′ of −19.2 kJ/mol. Note that activations via the pyrophosphate pathway result in the loss of two ~ⓟ rather than one, as occurs when ADP and Pi are formed.

A combination of the above reactions makes it possible for phosphate to be recycled and the adenine nucleotides to inter change (Figure 3).

Fig3. Phosphate cycles and interchange of adenine nucleotides.

Other Nucleoside Triphosphates Participate in Group Transfer Potential

 By means of the nucleoside diphosphate (NDP) kinases, UTP, GTP, and CTP can be synthesized from their diphosphates, for example, UDP reacts with ATP to form UTP.

All of these triphosphates take part in phosphorylations in the cell. Similarly, specific nucleoside monophosphate (NMP) kinases catalyze the formation of NDP from the corresponding monophosphates. Thus, adenylyl kinase is a specialized NMP kinase.

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Living Organisms Package Transition Metals Within Organometallic Complexes

Androgen Biosynthesis

Androgens and Related Molecules

Two-Dimensional Polyacrylamide Gel Electrophoresis

Isoelectric Focussing Gels

Native (Buffer) Gels

Sodium Dodecyl Sulfate (SDS)-Polyacrylamide Gel Electrophoresis

Transition Metals are Essential for Health

Individual Regulatory Events Combine to Form Sophisticated Control Networks

Regulation of Enzyme Quantity

Biochemical Evaluation of Cobalamin and Folate Deficiencies