The addition of phosphate to a sugar is a common reaction that occurs in a cell. By itself, this process is not very energetically favorable (that is, it needs an input of energy to occur). Cells overcome this energy obstacle by using ATP to “drive” the reaction. The energy needed to drive reactions is harvested in very controlled conditions in the confines of an enzyme. This involves a process called ‘coupling’. In coupled reactions, an enzyme binds both a high energy molecule (usually ATP) and the other molecule(s) involved in the reaction. Hydrolysis of ATP provides energy for the enzyme to stimulate the reaction on the other substance(s).
Figure 2.4.1: Citric acid cycle.
Hexokinase, for example, catalyzes the phosphorylation of glucose to form glucose-6-phosphate. In the absence of ATP, the reaction has a fairly positive ΔG°’ (described later), but hydrolysis of ATP provides excess energy, giving the coupled reaction a fairly negative ΔG°’ value.