Low-energy phosphates

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. 

The enzyme fructose 1,6-bisphosphatase releases inorganic phosphate from fructose 1,6-bisphosphate to form fructose 6-phosphate. This is not a reversal of the PFK-1 reaction ATP is not produced when the phosphate is removed from the 1 position of fructose 1,6-bisphosphate, because that is a low-energy phosphate bond. Rather, inorganic phosphate is released in this hydrolysis reaction. In the next reaction of gluconeogenesis, fructose 6-phosphate is converted to glucose 6-phosphate by the same isomerase used in glycolysis (phosphoglucoisomerase). 

In 1954 Chargaff (63) and associates discovered an enzyme (or group of enzymes) which was widely distributed in nature and which would transfer phosphate from a low-energy phosphate ester to a nucleoside. 

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 following abbreviations will be used in this article. ATP = adenosine triphosphate ADP — adenosine diphosphate DPNox oxidized diphosphopyri-dine nucleotide DPNred = reduced diphosphopyridine nucleotide CoA or CoA—SH = coenzyme A CoA—S—COCH3 = acetyl-coenzyme A FAD = flavin adenine dinucleotide R—P = low-energy phosphate bond R P = high-energy bond Pi = inorganic orthophosphate. 

ADP, ATP FAD, DPN, TPN Thiamine pyrophosphate Inorganic pyrophosphate II. Low-Energy Phosphate Esters... 

The hydrolysis of low-energy phosphate compounds has been studied by equilibrium methods, and the energy is found to be in the range of —2000 to —4000 cal. per mole. The high concentration of water as compared to phosphate is largely responsible for the negative value of this AF . 

Critical CPP. As stated above, flow is related to the pressure gradient. However, in stenotic vessels, flow will remain disproportionally low, as autoregulation cannot cause maximal vasodilation as in the healthier vessels. This may exacerbate regional hypoxia, the buildup of low-energy phosphates, and acidosis (Idris 1996). 

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