The reaction catalyzed by the enzyme hexokinase (Reaction 3 below) can be thought of as the sum of Reactions 1 and 2. Reaction 1 is the breakdown of ATP to ADP, which releases energy. Reaction 2, in which glucose is phosphorylated, requires energy.
Reaction 1: ATP + H2O ⇌ ADP + Pi
Reaction 2: Pi + glucose ⇌ glucose 6-P + H2O
Reaction 3: ATP + glucose ⇌ ADP + glucose-6-P
When Reactions 1 and 2 are coupled in the enzyme active site, Reaction 3 will occur spontaneously because:
both Reactions 1 and 2 can occur spontaneously.
the energy required to form glucose-6-P in Reaction 2 is less than the energy released by ATP breakdown in Reaction 1.
neither Reaction 1 or 2 can occur spontaneously alone, but both can occur spontaneously when coupled in the active site of the enzyme.
the energy required to form glucose-6-P in Reaction 2 is greater than the energy released by ATP breakdown in Reaction 1.
Reaction 2 has a negative delta Go