The free-energy changes associated with the individual steps of the tricarboxylic acid cycle, glycolysis and alcoholic fermentation and with the hydrolysis of the pyrophosphate groups of adenosinetriphosphate

Abstract

The free-energy changes for the individual stages of the main biological energy-giving reactions were re-evaluated with improved accuracy. Data for the heats of combustion and entropies of compounds of biological interest were critically selected and used to evaluate the free energies of formation (AG[degree]f) as follows: acetaldehyde (gas), 33.71; acetic acid (liq.), 95.48; acetate, 88.99; acetone (liq.), 38.52; DL-alanine, 89.24; L-alanine, 88.75; NH3 (gas) 6.37; NH4+, 19.00; L-aspartic acid, 172.43; L-aspartate+2-, 167.11; n-butanol, 41.07; butyric acid (liq.), 91.2; butyrate, 846; CO2(gas), 92.26; HCO3-, 140.29; ethanol (liq.), 43.39; fumaric acid, 154.67; fumarate 2-, 144.41; alpha-D-glucose, 219.38; L-glutamic acid, 172.00; L-glutamate+2-, 166.11; glycerol (liq.), 116.76; succinic acid, 178.39; succinate2-, 164.97; kcals in aqueous soln. From these values and equilibrium data for reactions associated with the tricarboxylic acid cycle, values of [DELTA]G[degree]f were obtained for several compounds which are intermediates of, or associated with, the cycle. The free-energy changes for the stages of the tricarboxylic acid cycle and for certain associated reactions and for the stages of glycolysis and alcoholic fermentation are tabulated. The free-energy change associated with the hydrolysis of adenosinetriphosphate (ATP) to adenosinediphosphate (ADP) and inorganic phosphate was evaluated from the values of [DELTA]G[degree]f for glycerol and pyruvate and from the equilibrium data for the enzymic stages of the conversion of glycerol to triosephbsphate, and subsequently to pyruvate, coupled with the phosphorylation of ADP. The value obtained is -9.4 kcal. for pH 7.5 and standard concns. ([image] activity) of ATP, ADP and inorganic phosphate; for the concns. in animal tissues it is 13-16 kcal.