Fig. 5: Mechanism for the reversible formation of the O2-resistant Hinact state in CbA5HWT.
a, b Structural alignment of CpI (state A1, blue) and CbA5Hair (Hinact; red) focusing on the TSC-loop. Residues labeled in red determine the structural rearrangement from A1 to Hinact. Corresponding CpI positions are shown in blue (yellow glow indicates clashes). c Illustration of the AAI mechanism. Main steps (1–3) in the transition from A1 (blue) to Hinact (red) via A2 (blue-red) are marked in green, corresponding to structural rearrangements (1-3) shown in a-b. 1: Partial shift of the TSC-loop, including T365, initiating the A1 to A2 transition. 2: W371-translocation and α-helix uplift, dragging along C367 closer to Fed (green arrows) 3: Binding of C367 to Fed (dotted green line) and oxidation of the H-cluster yields the inactive but O2-resistant Hinact state. Arrow and font sizes reflect relative rates of k1 and k-1 or kinact and kreact, in CbA5H, which define the dynamic equilibrium between A1, A2, and Hinact. Under reductive conditions (+e−/H+) the kinact/kreact ratio favors A2, whereas Hinact accumulates under oxidative conditions (−e−/H+).
