Harpers

84 / CHAPTER 10(1) Glucose+P i → Glucose 6- phosphate+ H 2 O( ∆G 0′ = +13.8 kJ/ mol)To take place, the reaction must be coupled with another—moreexergonic—reaction such as the hydrolysisof the terminal phosphate of ATP.Figure 10–7. Transfer of high-energy phosphate betweenATP and creatine.comes from respiratory chain oxidation using molecularO 2 within mitochondria (Chapter 11).(2) Glycolysis: A net formation of two ∼P resultsfrom the formation of lactate from one molecule of glucose,generated in two reactions catalyzed by phosphoglyceratekinase and pyruvate kinase, respectively (Figure17–2).(3) The citric acid cycle: One ∼P is generated directlyin the cycle at the succinyl thiokinase step (Figure16–3).Phosphagens act as storage forms of high-energyphosphate and include creatine phosphate, occurring invertebrate skeletal muscle, heart, spermatozoa, andbrain; and arginine phosphate, occurring in invertebratemuscle. When ATP is rapidly being utilized as asource of energy for muscular contraction, phosphagenspermit its concentrations to be maintained, but whenthe ATP/ADP ratio is high, their concentration can increaseto act as a store of high-energy phosphate (Figure10–7).When ATP acts as a phosphate donor to form thosecompounds of lower free energy of hydrolysis (Table10–1), the phosphate group is invariably converted toone of low energy, eg,When (1) and (2) are coupled in a reaction catalyzed byhexokinase, phosphorylation of glucose readily proceedsin a highly exergonic reaction that under physiologicconditions is irreversible. Many “activation” reactionsfollow this pattern.Adenylyl Kinase (Myokinase)Interconverts Adenine NucleotidesThis enzyme is present in most cells. It catalyzes the followingreaction:This allows:(2) ATP→ ADP+P i (∆G 0′ =−30.5 kJ/mol)(1) High-energy phosphate in ADP to be used inthe synthesis of ATP.(2) AMP, formed as a consequence of several activatingreactions involving ATP, to be recovered byrephosphorylation to ADP.(3) AMP to increase in concentration when ATPbecomes depleted and act as a metabolic (allosteric) signalto increase the rate of catabolic reactions, which inturn lead to the generation of more ATP (Chapter 19).When ATP Forms AMP, InorganicPyrophosphate (PP i ) Is ProducedThis occurs, for example, in the activation of longchainfatty acids (Chapter 22):ATP Allows the Coupling ofThermodynamically UnfavorableReactions to Favorable OnesThe phosphorylation of glucose to glucose 6-phosphate,the first reaction of glycolysis (Figure 17–2), ishighly endergonic and cannot proceed under physiologicconditions.This reaction is accompanied by loss of free energyas heat, which ensures that the activation reaction willgo to the right; and is further aided by the hydrolyticsplitting of PP i , catalyzed by inorganic pyrophosphatase,a reaction that itself has a large ∆G 0′ of −27.6 kJ/