Phosphate esters high energy bonds

Two and twelve moles of ATP are produced, respectively, per mole of glucose consumed in the glycolytic pathway and each turn of the Krebs (citrate) cycle. In fat metaboHsm, many high energy bonds are produced per mole of fatty ester oxidized. Eor example, 129 high energy phosphate bonds are produced per mole of palmitate. Oxidative phosphorylation has a remarkable 75% efficiency. Three moles of ATP are utilized per transfer of two electrons, compared to the theoretical four. The process occurs via a series of reactions involving flavoproteins, quinones such as coenzyme Q, and cytochromes. 

The insensitivity of the ester dianion reaction to the nature of nucleophile can be used for trapping of unreactive nucleophiles in high energy bonds such as P-O-P in ATP. For these reactions, aU one needs to do is to stabilize the departing group This stabilization provides the key mechanistic insights into the key phosphate transfer in biology, the ATP synthase reaction. This reaction produces ATP in the human body from... 

High energy bonds, energy-rich bonds chemical bonds which release more than 25 kJ/mol on hydrolysis. They are usually esters (enol, thio and phosphate esters), acid anhydrides, or amidine phosphates. 

PhenylPhosphorothioa.te Esters. These are the most widely used OP iasecticides and iacorporate pseudoanhydride high energy phosphate bonds between phosphoric acid and phenols that are present ia the activated P=0 state. 

Phosphoenolpyruvate (Fig. 13-3) contains a phosphate ester bond that undergoes hydrolysis to yield the enol form of pyruvate, and this direct product can immediately tautomerize to the more stable keto form of pyruvate. Because the reactant (phosphoenolpyruvate) has only one form (enol) and the product (pyruvate) has two possible forms, the product is stabilized relative to the reactant. This is the greatest contributing factor to the high standard free energy of hydrolysis of phosphoenolpyruvate AG ° = -61.9 kJ/mol. 

Enolase catalyzes the dehydration of 2-phosphoglycerate to form phospho-enolpyruvate (PEP). This reaction converts the low-energy phosphate ester bond of 2-phosphoglycerate into the high-energy phosphate bond of PEP. 

Transcription, the synthesis of RNA from a DNA template, is carried out by RNA polymerases (Fig. 14.2). Like DNA polyma-ases, RNA polymerases catalyze the formation of ester bonds between nucleotides that base-pair with the complementary nucleotides on the DNA template. Unlike DNA polymerases, RNA polymerases can initiate the synthesis of new chains in the absence of primers. They also lack the 3 to 5 exonuclease activity found in DNA polymerases. A strand of DNA serves as the template for RNA synthesis and is copied in the 3 to 5 direction. Synthesis of the new RNA molecule occurs in the 5 to 3 direction. The ribonucleoside triphosphates ATP, GTP, CTP, and UTP serve as the precursors. Each nucleotide base sequentially pairs with the complementary deoxyribonucleotide base on the DNA template (A, G, C, and U pair with T, C, G and A, respectively). The polymerase forms an ester bond between the a-phos-phate on the ribose 5 -hydroxyl of the nucleotide precursor and the ribose 3 -hydroxyl at the end of the growing RNA chain. The cleavage of a high-energy phosphate bond in the nucleotide triphosphate and release of pyrophosphate (from the (3 and y phosphates) provides the energy for this polymerization reaction. Subsequent cleavage of the pyrophosphate by a pyrophosphatase also helps to drive the polymerization reaction forward by removing a product. 

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 effects of radiation on phosphorus compounds may be chemical, in which case bonds are broken, and ions, radicals or new molecules are formed, or they may be physical, such as the creation of holes (vacancies) or other defects in the crystal lattice. The effects of high-energy radiation on phosphorus compounds are important since x-rays and y-rays are used in food preservation, and organophospho-rus esters are used in the complexing of metals in the purification of nuclear fuels. The importance of the effects on phosphate esters present in living systems has long been recognised. 

Phosphorylated compounds, e.g., the esters, amides, and anhydrides of phosphoric acid, participate in many secondary reactions. High-energy and low-energy phosphates may be distinguished. The former liberate up to 13,000 cal/mol if the bond between the phosphate residue and the acceptor molecule is hydrolyzed, the latter set free about 3,000 cal/mol (Table 13). 

The energy for the reaction (two high-energy phosphate bonds) is provided by ATP and stored in the ester bond of the aminoacyl-tRNA to be used subsequently for peptide bond synthesis. 

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