Fig. 1.

Kinetic scheme for lactose/H⁺ transport cycle. Six steps include protonation/deprotonation of LacY (1 and 5), substrate binding/release (2 and 4), and conformational changes (3 and 6). Steps 2, 3, and 4 are involved in equilibrium exchange and entrance counterflow that proceed without deprotonation (5, 7).