A simple statistical mechanical model of lipid bilayers is proposed to account for the non-equilibrium action of the enzyme phospholipase A₂. The enzyme hydrolyses lipid-bilayer substrates and produces product molecules that lead to local variations in the bilayer interracial pressure. Computer simulation of the model shows, in quantitative agreement with experimental data, that the enzyme activity is modulated by nano-scale lipid-domain formation in the lipid bilayer leading to a characteristic lag-burst behavior.