Organic compounds of which organisms are made have more free energy and more reduced than are the raw materials that the organism takes in. Reduced organic compounds are also energy rich materials that heterotrophs use as energy sources. The general problem an organism faces is the need to make one of these more energetic compounds by driving a reaction in the uphill or endergonic direction. This is done by coupling such reactions to exergonic reactions that release somewhat more energy, so the two reactions taken together still have the decrease in free energy. Coupling generally occurs through a group transfer process in which some atom of group of atoms is passed from one compound to another. ATP is the major source of free energy because it can transfer the phosphoryl group so readily.
However, apart from ATP, other phosphorylated nucleotides found in cells are guanosine triphosphate and uridine triphosphate. They are used to ‘drive’ certain endergonic reactions: their hydrolysis is coupled with an endergonic reaction so the overall reaction is exergonic. Therefore the distinguishing feature of compounds like ATP is that they have relatively high transfer potentials, so they can easily add their distinctive groups to other compounds. Some of these compounds are phosphorylated thus activating them, while others transfer groups that must be shuffled around in metabolism. . ATP can thus be considered as a coenzyme as it transfers a phosphate group to drive reactions.
Compounds like ATP that are capable of driving many reactions uphill energetically have been called “high energy compounds” or they are said to contain “high energy bonds”. However, neither description is accurate. There is nothing unusual about ATP as a chemical substance. Energy is not stored in chemical bonds.. Another explanation would be the presence of a group transfer potential such as the phosphate group. For example, a compound is phosphorylated, (with the removal of a water molecule) and the reverse of this process, hydrolysis, releases energy. This energy is known as the phosphoryl transfer potential. Thus the major source of free energy in all cells is ATP, a compound that can drive many endergonic reactions because of the large amount of free energy released when it transfers its phosphoryl group, hence activating substrates into their conversion into other compounds .
ATP hydrolysis is favoured i.e. it has a strong tendency to transfer its terminal phosphoryl group in the presence of an appropriate catalyst(ATPase), a reaction associated with the release of 30.5 kj mol-1 of ATP. There are many reasons for this tendency: firstly, the repulsion between the four negative charges in ATP4- is reduced because two negative charges are removed with phosphate; secondly, the H+ ion which is released when ATP is hydrolysed reacts with the OH- ions to form water- this is a highly favoured reaction. Also the charge distribution on ADP +Pi is more stable than that on ATP.
ATP is the central molecule in metabolism. ATP is important not just for driving biosynthetic reactions such as nitrogen fixation and bioluminescence; it is also used as a source of energy for virtually all kinds of movements, including small motions of cellular structures, locomotion of single cells and muscle contraction, where ATP hydrolysis changes the position of the myosin ‘head” relative to actin, It also supplies energy for the transport of ions and molecules across cell membranes and so it underlies the whole activity of the nervous system. Active transport systems are driven by the phosphorylation of membrane-bound proteins. In urea synthesis, ATP hydrolysis drives the ornithine cycle which removes toxic ammonia. In protein synthesis, ATP is used to load amino acids onto transfer RNA. ATP is also involved in photosynthesis, such as in the Calvin cycle where its hydrolysis drives the cyclic reduction of carbon dioxide to triose phosphate.
It is possible to use glucose as an energy source during cell metabolism since energy for ATP synthesis is released from the breakdown of glucose. However the breakdown of glucose cannot be used directly to power the cell’s work i.e. the breakdown is not coupled to the cell’s endergonic reactions, therefore all cells use ATP as their energy source for metabolism. This is because: firstly, energy release from ATP hydrolysis is instantaneous: catabolism of glucose takes some time. Secondly, catabolism of one molecule of ATP releases only a small amount of energy: catabolism of one molecule of glucose releases much more. In addition, linking all endergonic reactions to ATP hydrolysis means that the cell can economise on enzymes. Only one enzyme is needed to hydrolyse ATP whereas many are needed to release the energy contained in a glucose molecule. These three reasons-instantaneous access to energy that is released in small, controllable amounts using only one enzyme to release it-probably explain why ATP has become the universal energy currency in cells. Thus, ATP is a better immediate energy source than glucose during cell metabolism.
Besides being an energy currency, a co-enzyme and a reservoir of potential chemical energy, ATP and its derivatives are also involved in the feedback mechanism control for cellular respiration by acting as allosteric activators or inhibitors. Allosteric enzymes at certain points in the respiratory pathway respond to inhibitors and activators to set the pace of glycolysis and the Krebs cycle. Phosphofructokinase, the enzyme that catalyses step 3 of glycolysis, is one such enzyme. It is stimulated by ADP and AMP but inhibited by ATP and citrate. This feedback regulation adjusts the rate of respiration as the cell’s catabolic and anabolic demands change.
Furthermore, ATP is also involved is the second messenger mechanism of hormone action. When the hormone binds to the receptor site in the cell membrane it activates the receptor protein to become the enzyme adenyl cyclase. This converts ATP to cyclic AMP. Cyclic AMP in turn triggers a wide variety of responses.
It can be seen that without ATP, conversion of one substance to the next is non-spontaneous and thus unlikely to occur. Playing such a necessary role in a cell, it must be continually produced.
ATP is synthesized by substrate level phosphorylation where a phosphate group is transferred from a phosphorylated compound to ADP, or by chemiosmosis where ATP synthesis is catalysed by the enzyme ATPase. A gradient of hydrogen ions is generated across the membrane of a mitochondrion (in respiration) or a chloroplast (in photosynthesis) by a proton pump driven by energy from respiration of foods (oxidative phosphorylation) or from absorption of light photophosphorylation) respectively. The membrane is impermeable to hydrogen ions. The accumulation of hydrogen ions on one side of the membrane is a source of potential energy for ATP synthesis. The enzyme catalyzing the hydrolysis of ATP to ADP and Pi occurs in the membrane. The hydrogen ions are able to pass back across the membrane, through a channel in the ATPase. Flow of hydrogen ions through this channel causes reversal of ATP hydrolysis.
Thus although an organism at work uses ATP continuously, ATP is a renewable resource that can be regenerated by the addition of phosphate to ADP. This ATP cycle moves at an astonishing pace. For example, a working muscle cell recycles its entire pool of ATP about once each minute. That turnover represents 10 million molecules of ATP consumed and regenerated per second per cell.
In conclusion, the production of ATP can never cease, for energy is needed to drive cellular work, be it chemical, mechanical or transport since the immediate source of energy that powers cellular work is ATP. Cellular reactions, without the free energy to kick start, will take place too slowly to sustain life. It is such an important molecule that no other molecule can replace its functions in an organism.
Done by : Stephanie Ow
Class: 2S03A
Group 1
Comments: this is a well written essay which covers the important details about ATP from its structure, function and its other non energy coupling roles. In the actual essay it would be good to include a diagram of ATP and the coupling to illustrate your point but you did give a good written description. Well done.