Kinetic scheme describing the hysteretic behavior of BtuC2D2.
The model is written in terms of BtuC2D2 that cannot/hardly binds to BtuF (E1) and BtuC2D2 that binds BtuF with high affinity (E2). Arrows are bolded to reflect the relative reaction rates and the red arrows depict the population shift that is affected by hysteresis. (K1) and (K2) are effective equilibrium association constants for binding of BtuF (represented as F) to BtuC2D2. KS, K3 and KsHYST are effective equilibrium constants for the conformational change between E1 and E2 and are agnostic to whether the change proceeds in one step or through many intermediates. For simplicity we assume that E1 is transport-incompetent and that E2 is fully functional. The transport experiment shown in Fig.4C shows that “worked” BtuC2D2 is approximately 50% more active than the fresh protein. Therefore, since only E2 is transport-competent we suggest that in the initial “fresh” state of BtuC2D2 the ratio of E2–E1 is approximately 2:3 (Ks = 0.66). In the worked state, following the hysteretic conformational change, the E2/E1 ratio a is shifted to 3:2, i.e., KsHYST = 1.5.