The free energy of thiol ester hydrolysis.

Thiol esters, such as aeetyl coenzyme A, the principal biological carrier of activated acetyl groups, have generally been supposed to be “energy-rich” compounds; i.e. to have a free energy of hydrolysis at pH 7.0 in the range of -7000 to -14,000 calories per mole. The reported numerical values for the free energy of thiol ester hydrolysis have varied over a wide range (for recent estimates see references 1 and 2) because of changes in the estimates for the equilibria and free energies of formation on which they were based, and Wadso, who has recently measured their heats of hydrolysis, has questioned whether thiol esters are “energy-rich” compounds at all (3, 4). Thiol esters which contain a hydroxyl group in a suitable position for intramolecular attack on the acyl group have been shown by Harding and Owen to isomerize to the oxygen ester in dilute alkali at a faster rate than they undergo hydrolysis (5, 6) :