James Richard Fromm
Two pentose sugars, ribose and 2-deoxyribose, react with certain nitrogenous bases to form the biologically significant nucleosides. A nucleoside molecule contains one nitrogenous base and one pentose sugar. The only two different kinds of nitrogenous bases commonly found in nucleosides are the pyrimidines and the purines. There are only five of them, three pyrimidines and two purines, which are commonly found in living organisms.
These purine and pyrimidine bases can attach to sugars through the amine hydrogens at the bottom of the structures shown. Pyrimidine bases have a single six-membered ring while purine bases have the structures with two fused rings shown. Their names and abbreviations are adenine (A), guanine (G), thymine (T), cytosine (C), and uracil (U).
The nucleosides take their names from the names of the bases and sugars which comprise them. The four major ribose nucleosides are adenosine, guanosine, cytidine, and uridine; the four major 2-deoxyribose nucleosides are 2-deoxyadenosine, 2-deoxyguanosine, 2-deoxycytidine, and 2-deoxythymine.
A nucleoside can react with phosphoric acid at one of the sugar ring hydroxyl groups to lose water. When it does so it is said to form a mononucleotide such as adenosine monophosphate (AMP). Adenosine monophosphate can react with additional inorganic phosphate to form high-energy phosphate bonds in adenosine diphosphate (ADP) and adenosine triphosphate (ATP). Hydrolysis of these phosphate bonds returns the energy used to form them. Living organisms use ATP as an energy carrier, since movement of the ATP and ADP molecules is equivalent to the movement of energy.
The most ubiquitous and perhaps the most important energy carrier in living organisms is the nucleotide adenosine triphosphate, normally abbreviated ATP. The name adenosine triphosphate denotes adenine coupled to one of the ribose sugars, with three phosphate groups attached in a chain structure.
The actual high-energy bonds in ATP are the bonds linking the phosphate groups to each other. Thus the hydrolysis of such a bond releases energy while the formation of it requires energy, and it is in the form of ATP that energy is moved around the organism to the point at which it is required.
While ATP and, to a much lesser extent, other similar molecules serve as energy transport molecules, the amount of ATP present in an organism at any one time is comparatively small. Thus sustained muscular work, for example, could not be provided by a handy supply of ATP stored in the muscle. Muscular work is a complex process in which ATP gives up its energy and is thus converted into adenosine diphosphate (ADP) and inorganic phosphate (H3PO4 or, more correctly, simply Pi since it is not in acid form):
ATP ADP + Pi + energy.
In higher animals, such as humans, muscles all contain some stored energy in the form of creatine phosphate. Lower animals, such as invertebrates, use other compounds. The storage is based upon a reaction whose equilibrium constant is about 0.6 catalyzed by creatine kinase:
creatine phosphate + ADP creatine + ATP.
This reaction is catalyzed by the enzyme creatine kinase, one of the many enzymes present in the human body; its concentration in human blood serum is about 10-7 mol/liter.
Living organisms use, through the intermediate of ATP, energy for a wide variety of things. Some of it must be expended to produce more energy, as in the oxidation of glucose. Some of it is used in muscular work. Some is used to produce light, as in the firefly; much of it appears as waste heat. Some of it is used in the interesting area of osmotic work.
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