Isotope-exchange fluxes in the aldolase-catalysed reaction. (A) shows the whole reaction cycle when the enzyme (E) forms a complex with fbp (E-fbp), releases g3p keeping dhap (E-dhap), and finally releases dhap returning to (E). Forward flux (green lines) through the whole reaction cycle brings isotopes originated from the fbp pool into the pools of dhap and g3p, and reverse flux (red lines) brings isotopes originated from the pools of dhap and g3p into fbp pool. (B) shows forward (green lines) and reverse (red lines) fluxes that only exchange isotopes of upper part of fbp molecule with g3p pool without releasing dhap. vi designate the respective rates of elementary steps of reaction mechanism.