Multisubstrate Allostery with a Single Helical Switch in Cytochrome P450cam

Cytochrome P450cam exhibits striking conformational heterogeneity and substrate-dependent allostery, yet a unifying mechanism has remained elusive. Here, we reveal the I-helix (αI) as the master regulator of the P450 function. Multimicrosecond MD simulations, NMR pseudocontact shifts, and ensemble analysis of more than 100 crystal structures show that glycines G248 and G249 confer intrinsic flexibility, enabling reversible transitions between <i>straight</i> and <i>kinked α</i>I. Substrate binding stabilizes the <i>straight</i> conformation, closing solvent channels 1 and 2 while enlarging the distal allosteric site, thereby establishing reciprocal allosteric coupling. This I-helix-centric mechanism resolves decades of structural discrepancies and explains cooperativity, with G249 conserved across human P450s. Proof-of-concept mutants locked in constitutively open or closed states validate αI as a tunable allosteric switch. By overturning the conventional FG-lid focus, this work provides an immediate structural blueprint for the rational engineering of P450 reactivity and selectivity.