Figure 11.

Kinetic model of DraNramp activity. (A) Transport cycle diagram including all possible binding/unbinding events as well as all observed (solid lines) and hypothetical (dashed lines) transport events for DraNramp. While metal transport always requires bulk conformational change, proton uniport occurs through the outward-open state (Bozzi et al., 2019). Cd²⁺ uniport (horizontal cycles) could occur in the presence or absence of a bound proton. (B) Simplified, hypothetical free energy diagrams for DraNramp transport events in a typical physiological context of higher [H⁺] outside, higher [M²⁺] inside, and a moderate negative-inside ΔΨ. Proton cotransport may significantly reduce the barrier to Mn²⁺ transport, while Cd²⁺ uniport may instead have the lowest barrier under physiological conditions. Voltage affects both the magnitude of the energetic barriers for metal transport as well as the relative ΔG for transport, while ΔpH affects the energetic barrier and relative ΔG depending on the extent of thermodynamic coupling between metal and proton transport. The much faster rates seen for metal forward-transport over back-transport imply asymmetric kinetic barriers (and thus likely additional stable intermediate states), which are not shown in this model.