ABSTRACT
Two reactions are coupled if they share a common intermediate and if the overall free energy change is negative. Biological systems make extensive use of coupled reactions, as will be shown shortly. The reactions A → B and B → C are coupled if A → C is exergonic, B being the common intermediate, yielding A → B → C. It does not matter if either A → B or B → C is separately endergonic as long as A → C is exergonic overall because the path between A and C does not matter.
