Previously I described how enzymes can catalyse a reaction. If this were all enzymes could do then cells would be very limited - they could only ever move subsets of reactions towards equilibrium. Although enzymes are limited to moving reactions to equilibrium, they can expand the repertoire of reactions available by coupling together two or more reactions, effectively, creating a new reaction. For example, real life cells couple the synthesis of DNA to the hydrolysis of GTP. The addition of a nucleotide to a strand of DNA would normally be energetically unfavourable (the equilibrium favours removing a nucleotide), but the hydrolysis of GTP is more favourable. The coupled reaction therefore favours DNA synthesis.
In order for the simulated cells to drive energetically unfavourable reactions, such as DNA synthesis, they therefore require enzymes that can couple reactions. If we add two separate enzymes, ADase and EHase to a solution containing 1% AD and EH, and 0.5% A, D, E and H, then both AD synthesis and EH synthesis are unfavourable, so both are hydrolysed.
(The concentrations of D and H are equal to the concentrations of A and E respectively, so are not shown. And for reference, I used a 0.01% solution of each enzyme, and the km of each was 0.0625.)
However, if we add a single enzyme that couples the ADase reaction to the reverse EDase reaction we create, in effect, a new reaction:
AD + E + H ⇌ A + D + EH
Since the AD hydrolysis reaction is more favourable in our solution, it drives the reaction in the direction of EH synthesis. Our enzyme thus functions as an ADase-dependent EH synthase.