FIGURE 8.

Comparison of c(s) and dc/dv for the sedimentation of a two-site reaction A + 2B ↔ AB + B ↔ ABB with equivalent noncooperative sites. Sedimentation profiles were calculated on the basis of Lamm equation solutions with explicit reaction terms for instantaneous local equilibrium. The parameters were based on a molecule A (100 kDa, 6 S) with two identical noncooperative sites available for binding of a smaller ligand molecule B (50 kDa, 4 S), resulting in 8-S and 10-S complexes. Simulated concentrations were for A: 0.1-fold K_D,1 (dark blue), 0.3-fold K_D,1 (light blue), K_D,1 (red), threefold K_D,1 (green), and 10-fold K_D,1 (gray), and B in twofold molar excess of A, respectively. (A) Sedimentation coefficient distributions c(s) (solid lines, in units of fringes/S) for P = 0.9, and asymptotic reaction boundaries dc/dv calculated for the same parameters (solid bars, in units of fringes/S), with the corresponding undisturbed boundary indicated as solid circles (shown in fringe units). For comparison, the short-dashed lines indicate c(s) profiles calculated for the three highest concentrations with a frictional ratio fixed at 1.3. (B) Isotherms of s_fast, as derived from integration of the fast c(s) peak only (black circles), and s_w of the total sedimenting system derived from integration of the complete c(s) distribution (gray squares). The solid lines are the theoretically expected isotherms for s_fast from GJT (Eq. 4) and for s_w from mass action law at rest (Eq. 5), respectively. (C) Signal amplitudes c_fast (○) and c_slow (•) determined by integration of the c(s) peaks of the boundary components, and corresponding isotherms determined by GJT (solid lines).