Phosphate anhydride bond

The pentose phosphates formed in the transketolase reactions—ribose 5-phosphate and xylulose 5-phos-phate—are converted to ribulose 5-phosphate (steps (7) and (3)), which in the final step ( ) of the cycle is phosphorylated to ribulose 1,5-bisphosphate by ribulose 5-phosphate kinase (Fig. 20-13). This is the third very exergonic reaction of the pathway, as the phosphate anhydride bond in ATP is swapped for a phosphate ester in ribulose 1,5-bisphosphate. 

Of particular importance is the conversion of much of the energy that results from photosynthesis, or from the oxidation of fats, carbohydrates, and proteins in cells into formation of phosphate ester bonds (C—O—P) or phosphate anhydride bonds (P—O—P). The energy so stored is used in other reactions, the net result of which is hydrolysis ... 

How do mitochondria couple such different types of reactions as electron transfer and the formation of a phosphate anhydride bond We explore this question in the following sections. 

A perusal of current biochemistry texts suggests that the use of the squiggle has largely died out, but the concept of a group potential and the importance of these phosphate anhydride bonds in energy storage and transfer is universally accepted. Lipmann shared the Nobel Prize in Physiology/Medicine in 1953 with Hans Krebs of the Krebs (citric acid) cycle. His Nobel citation read in part,... 

A phosphate anhydride bond is hydrolyzed and a phosphate ester bond is formed with the C-3 of ribose. 

Because ATP can transfer a phosphate group, we say that ATP has a high "phosphoryl group transfer potential" rather than calling it a high energy compound. The phosphate anhydride bonds of ATP, ADP,... 

Chapter 3 introduces ATP as a nucleoside triphosphate building block involved in RNA synthesis. The hydrolysis of the terminal phosphate anhydride bond when ATP is converted into adenosine diphosphate (ADP) releases energy. Further hydrolysis of ADP is possible, giving rise to adenosine monophosphate (AMP), with the release of a slightly smaller amount of energy. 

Many metabolites, especially those involved in glycolysis (Sec. 10.6) and the Krebs cycle (Sec. 10.7), have high-energy phosphate-ester bonds. Compared with these metabolites, ATP is an intermediate with respect to the amount of energy stored in its phosphate anhydride bonds. This makes sense, because a denomination of currency would not be practical if it needed to be changed (the energy content split up into smaller packets) each time it was used. 

The answer is B. The indicated bond is best described as a phosphate anhydride bond. 

Other major sources of protons within living cells are the reactions involving cleavage of the phosphate anhydride bonds of ATP (or other high-energy compounds). Thus the phosphorylation of glucose generates a proton ... 

A primed numeral locates the position of the phosphate on the sugars of mononucleotides (eg, 3 -GMP, 5 -dCMP). Additional phosphoryi groups linked to the first by acid anhydride bonds form nucleoside diphosphates and triphosphates. 

Nucleotides are formed when one or more phosphate groups is attached to the 5 carbon of a nucleoside (Figure 1-1-6). Nucleoside di- and triphosphates are high-energy compounds because of the hydrolytic energy associated with the acid anhydride bonds (Figure 1-1-7). 

When two phosphate residues bond, they do not form an ester, but an energy-rich phosphoric acid anhydride bond, as... 

Phosphoric acid molecules can form acid-anhydride bonds with each other. It is therefore possible for two nucleotides to be linked via the phosphate residues. This gives rise to dinucleotides with a phosphoric acid-anhydride structure. This group includes the coenzymes NAD(P) " and CoA, as well as the flavin derivative FAD (1 see p. 104). 

In ATP, a chain of three phosphate residues is linked to the 5 -OH group of the nucleoside adenosine (see p. 80). These phosphate residues are termed a, p, and y. The a phosphate is bound to ribose by a phosphoric add ester bond. The linkages between the three phosphate residues, on the other hand, involve much more unstable phosphoric add anhydride bonds. The active coenzyme is in fact generally a complex of ATP with an Mg "" ion, which is coordinatively bound to the a and p phosphates (Mg "" ATP ). However, the term ATP is usually used for the sake of simplicity. 

In standard conditions, the change in free enthalpy AG° (see p. 18) that occurs in the hydrolysis of phosphoric acid anhydride bonds amounts to -30 to -35 kj mol at pH 7. The particular anhydride bond of ATP that is cleaved only has a minor influence on AG° (1-2). Even the hydrolysis of diphosphate (also known as pyrophosphate 4) still yields more than -30 kJ mol . By contrast, cleavage of the ester bond between ribose and phosphate only provides -9 kJ mol (3). 

RGURE 8-40 The phosphate ester and phosphoanhydride bonds of ATP. Hydrolysis of an anhydride bond yields more energy than hydrolysis of the ester. A carboxylic acid anhydride and carboxylic acid ester are shown for comparison. 

The energy released by hydrolysis of ATP and the other nucleoside triphosphates is accounted for by the structure of the triphosphate group. The bond between the ribose and the a phosphate is an ester linkage. The a, ft and ft,y linkages are phosphoanhydrides (Fig. 8-40). Hydrolysis of the ester linkage yields about 14 kJ/mol under standard conditions, whereas hydrolysis of each anhydride bond yields about 30 kJ/mol ATP hydrolysis often plays an important thermodynamic role in biosynthesis. When coupled to a reaction with a positive free-energy change, ATP hydrolysis shifts the equilibrium of the overall process to favor product forma-... 

Another three-carbon compound, 1,3-bisphospho-glycerate (Fig. 13—4), contains an anhydride bond between the carboxyl group at C-l and phosphoric acid. Hydrolysis of this acyl phosphate is accompanied by a large, negative, standard free-energy change (AG ° =... 

The AG° values for the hydrolysis of any P - O - P bond of ATP, inorganic pyrophosphate, or any acyl CoA thiolester are all about -34 kj / mole, while the corresponding figure for the hydrolysis of a mixed carboxylic phosphate anhydride is about -55 kj / mole. Calculate the value of AG° for the following reaction describing the activation of fatty acids to the fatty acyl adenylate. 

All synthase reactions proceed in two separate steps (fig. 29.9). In the first step the synthase recognizes its corresponding amino acid and its second substrate, ATP, and forms a mixed anhydride bond between the carboxyl group of the amino acid and the phosphate of AMP with the release of PPj. 

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. 

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