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. 2002 Nov;120(5):693–705. doi: 10.1085/jgp.20028645

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Copyright © 2002, The Rockefeller University Press

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Figure 7. — Interpreting the effect of MTS reagents on the transport properties of NaPi-IIa, based on an alternating access kinetic scheme. (A) Before Cys modification, transport activity assumes either leak mode (left) or cotransport mode (right), depending on the availability and concentration of external P_i. It is assumed that the internal concentrations of Na⁺ and P_i are such that the cycle proceeds in the direction indicated. (B) After Cys-modification, leak mode still operates (left) in the absence of external P_i. In the case of MTSES/MTSEA, a higher turnover rate is also predicted that could result from modification of rates associated with the transition 2–7. For external P_i > 0, the probability of occupancy of state 3 increases, thereby reducing the probability of transition 2–7 occurring, so that the leak activity is progressively suppressed in a dose dependent manner as the concentrations of external P_i and/or Na⁺ increase (see text). At saturating P_i, and Na⁺, the probability of occupying state 4 is high, but the translocation transition (4–5) is prevented by the modified Cys residue.