The sarcoplasmic reticulum calcium pump. Localization of free energy transfer to discrete steps of the reaction cycle.

Free energy transduction in active transport resembles other protein-catalyzed processes, occurring by an ordered sequence of discrete bond-breaking and bond-making steps. The bonds that affect the transported ion directly are chelation bonds, which alter the chemical potential of the bound ion, but not its chemical identity. Available data for the sarcoplasmic reticulum Ca pump (admittedly incomplete) suggest that more than 50% of the free energy transfer may be localized to a single step of the reaction cycle.