Energetics of several various steps of the metabolisation of arachidonate anion into prostacyclins are been calculated at the B3LYP/6-311++G(3df,3pd) level of theory in ideal gas and in THF or water as solvents.Results clearly showed that prostaglandin G(2), resulting from enzymatic reactions of arachidonate anion with two molecules of dioxygen, is obtained after moderately exergonic processes (similar to 20 kcal mol(-1) overall) and therefore is a very reactive compound. After the action of the peroxidase active site of the cyclooxygenase, prostaglandin H-2 retains this high reactivity and the following isomerization steps giving rise to prostaglandin E-2, thromboxane A(2) and prostacyclin I-2, are highly exergonic reactions (>35 kcal mol(-1)). (C) 2015 Elsevier Ltd. All rights reserved.