Phosphoric anhydride bonds

Hydrolases catalyze the hydrolytic cleavage of C-0, C-N, C-C, and some other bonds including phosphoric anhydride bonds. They possess several attractive features, such as broad substrate selectivity and high stereospecificity. This has made them a popular choice for the conduction of many biotransformafions as well as a powerful addition to the organic chemistry toolbox. Hydrolases also often catalyze several related reactions, such as condensations and alcoholysis. 

Compounds of one acid with another are referred to as acid anhydrides. A particularly large amount of energy is required for the formation of an acid—anhydride bond. Phosphoric anhydride bonds therefore play a central role in the storage and release of chemical energy in the cell (see p. 122). Mixed anhydrides between carboxylic acids and phosphoric acid are also very important energy-rich metabolites in cellular metabolism. 

Because the primary metabolic function of ATP is to drive reactions, biochemists often refer to it as a "high-energy molecule" or an "energy storehouse." These terms don t mean that ATP is somehow different from other compounds they mean only that ATP releases a large amount of energy when its P-O-P (phosphoric anhydride) bonds are broken and a phosphate group is transferred. 

Condensed phosphates, other than branched phosphates, are stable in neutral aqueous solution at room temperature. The hydrolysis of the P-O-P bond in linear polyphosphates such as Graham s salt liberates energy equivalent to approximately 10 kcal/mol (Yoshida, 1955a,b Van Wazer, 1958), i.e. the same amount of energy as is liberated in the hydrolysis of the terminal phosphoric anhydride bonds in the adenosine 5 -triphosphate (ATP) molecule. Hydrolysis of the cyclotriphosphate also liberates this same amount of energy (Meyerhof etal, 1953). 

It is possible, however, that in some cases utilization of PolyP does not involve hydrolysis to P , but rather phosphate transfer without loss of the energies of the phosphoric anhydride bonds to other compounds. It seems unlikely that the energy stored in PolyPs would be dissipated without being utilized for energy-requiring processes. 

ATP consists of a nitrogenous base (adenine) and a phosphate ester of the five-carbon sugar ribose (Figure 15.5). The triphosphate group attached to libose is made up of three phosphate groups bonded to one another by phosphoric anhydride bonds. When two phosphate groups react with one another, a water molecule is lost. Because water is lost, the resulting bond is called a phosphoric anhydride, or phosphoanhydride, bond. 

The hydrolysis of the phosphoric anhydride bond of ATP is accompanied by the release of energy that Is used for biochemical reactions In the cell. 

An alcohol can react with phosphoric acid to produce a phosphate ester iphosphoester). When two phosphate groups are joined, the resulting bond is a phosphoric anhydride bond. These two functional groups are important to the structure and fimction o( adenosine triphosphate (ATP), the universal energy currency of all cells. Cellular enzymes can carry out a reaction between a thiol and a carboxylic acid to produce a thioester. This reaction is essential for the activation of acyl groups in carbohydrate and fatty acid metabolism. Coenz)une A is the most important thiol involved in these pathways. 

How does an organism ensure that glycogen synthesis and glycogen breakdown do not operate simultaneously If this were to occur, the main result would be the hydrolysis of UTP, which would waste chemical energy stored in the phosphoric anhydride bonds. A major controlling factor lies in the behavior of glycogen phosphorylase. This enzyme is subject not only to allosteric control but also to another control feature covalent modification. We saw an earlier example of this kind of control in the sodium-potassium pump in Section 8.6. In that example, phosphorylation and dephosphorylation of an enzyme determined whether it was active, and a similar effect takes place here. 

EC 3 Hydrolases catalyze hydrolytic reactions These enzymes catalyze the hydrolytic cleavage of C—O, C—N, C—C and other bonds (e.g. phosphoric anhydride bonds), although the systematic name always includes hydrolase. The name of the substrate suffixed with -ase is used in many cases (especially common names). A number of hydrolases are known to catalyze not only hydrolytic removal of a particular group from their substrate but likewise transfer of this group to suitable acceptor molecules. However, in most cases the reaction with water as the acceptor was discovered earlier and are now considered as hydrolases. 

Adenosine triphosphate (ATP) contains two phosphoric anhydride bonds, and considerable energy is released when ATP is hydrolyzed to adenosine diphosphate (ADP) and further to adenosine monophosphate (AMP). These reactions are used to provide energy for other biological reactions. 

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