Bioenergetics Catabolism involves more than the simple breakdown of compounds Combustion of glucose yields large amounts of energy G 2870 kJ mol for glucose conversion to carbon dioxide and water however most of this energy is released as heat Metabolic processes occur in many steps to allow both the diversion of energy in usable form and the diversion of catabolic pathway intermediates for biosynthetic processes One major method for diverting energy in usable form involves the use of highenergy phosphate compounds High energy phosphate compounds Phosphate containing compounds are considered high energy if they have large G for hydrolysis large meaning more negative than 20 to 25 kJ mol High energy phosphate compounds are not used for long term energy storage They are temporary forms of stored energy and are used to carry energy from one reaction to another High energy phosphate compounds are not necessarily unstable Remember that G does not indicate anything about the rate of a reaction most high energy compounds are quite stable because the hydrolysis reaction activation energy is quite large In most cases the conversion of a high energy compound to a lowenergy compound at detectable rates requires the intervention of an enzyme Examples of high energy compounds Organisms use a number of high energy compounds The following includes a few examples The G for the hydrolysis reaction are included for each reaction Note that in most cases the hydrolysis reaction as drawn rarely occurs in cells One of these compounds deserves your special attention ATP Anyone studying biochemistry will become very familiar with ATP ATP is an extremely useful molecule for exchanging energy between enzymes The hydrolysis of ATP has a G of 30 5 kJ mol note that the precise value is somewhat variable depending on the presence of magnesium and other nonreactant species that alter the energetics of the reaction In addition because in cells the concentration of ATP is typically considerably higher than that of ADP the G for the reaction is more negative than the G value Recall that G is a measure of the amount of energy available to do work ATP hydrolysis can therefore Copyright 2000 2003 Mark Brandt Ph D 7 donate energy to other systems to allow those systems to perform reactions that would otherwise be thermodynamically unfavorable Among phosphate containing molecules ATP has a central location Although it contains high energy phosphate bonds the energy in each of the ATP phosphoanhydride bonds is somewhat lower than the energy of a few other biological molecules For example the hydrolysis of phosphoenolpyruvate has a large negative G due to the fact that the covalent bond to the phosphate traps pyruvate in the energetic enol configuration Phosphoenolpyruvate has a much higher G than ATP In cells the direct hydrolysis of phosphoenolpyruvate does not occur instead the energy stored in this molecule is transferred to ATP in a reaction catalyzed by the enzyme pyruvate kinase In this case the two reactions i e the hydrolysis of phosphoenolpyruvate and the reverse of the hydrolysis of ATP are coupled The normally unfavorable reaction the conversion of ADP phosphate to ATP becomes a spontaneous process A second interesting reaction is Note that the G for phosphocreatine hydrolysis is also strongly negative In cells the reaction catalyzed by creatine kinase below is reversible this reaction is important in muscle as a method for rapidly generating ATP under conditions of rapid utilization resting muscles replenish their supply of phosphocreatine using the right to left reaction Copyright 2000 2003 Mark Brandt Ph D 8 The G for the reaction is 12 8 kJ mol Note that this is the sum of the hydrolysis G values 43 3 kJ mol 30 5 kJ mol For this calculation the ADP conversion to ATP is the reverse of the hydrolysis reaction and therefore has a G If you consider the moderately large G for the creatine kinase reaction it is somewhat surprising that this reaction is reversible Once again however it is necessary to remember that G does not determine whether a reaction is spontaneous Instead it is the G for the reaction that determines whether a reaction is spontaneous The normal concentration of ATP in the cell is much higher than the concentration of ADP In resting cells the relative concentrations of ADP and ATP are such that the G for hydrolysis of ATP is about 50 kJ mol The creatine kinase reaction thus illustrates two important points The first point is that the actual physiological value of G varies depending on the cellular conditions Reactions may therefore be reversible or irreversible under cellular conditions in spite of sizable G values The second point is that for multisubstrate reactions it is possible to couple a favorable reaction to an unfavorable one When this is done the overall G is the sum of the G for the individual reactions as shown above The coupled reactions shown thus far involve the transfer of phosphate from one molecule to another or the hydrolysis of phosphate compounds ATP is heavily involved in these phosphate transfer reactions and most kinase2 reactions use ATP as the phosphate donor However in some cases cleavage of the high energy bond is used entirely as a driving force and the reaction does result in hydrolysis of the bond Once again ATP is the most commonly used energy source for these reactions An example of this is the reaction catalyzed by the biotin dependent enzyme pyruvate carboxylase The product oxaloacetate does not contain a phosphate Instead the thermodynamically unfavorable addition of carbonate to the pyruvate is coupled to the thermodynamically favorable ATP hydrolysis by the enzyme ATP hydrolysis is also widely used by active transport pump proteins that move 2 A kinase is an enzyme that phosphorylates its substrate The name of the enzyme usually comes from the phosphate acceptor In some cases confusion as to the physiological substrate results in confusing nomenclature As an example the name pyruvate kinase implies that the enzyme phosphorylates pyruvate while in fact this physiologically irreversible enzyme produces pyruvate and ATP from phosphoenolpyruvate and ADP Copyright 2000 2003 Mark Brandt Ph D 9 molecules across membranes against electrochemical gradients Once again this process uses the G released by ATP hydrolysis to perform a process that would otherwise be thermodynamically unfavorable Representations of high energy phosphate bonds In some cases the actual