Divalent cation biding to reduced and octanoyl acyl‐carrier protein

Mn(II) EPR binding studies with reduced acyl‐carrier protein (ACP‐SH) strongly suggest the presence of two relatively high‐affinity manganese‐binding sites (average K_d/site ∼ 80 μM) at physiological pH. Lowering the pH or titrating with sodium chloride reduces the average number of bound divalent cations and decreases the binding affinity. This is consistent with the idea that anionic ligand(s), e.g. the carboxylate of glutamic or aspartic acid, on the protein are involved in manganese ion coordination. At pH values above 8.0, binding affinity is also reduced, whereas the average number of bound metal ions increases to about five at pH 8.5. By interacting weakly with divalent cations (average K_d/site ∼ 1 mM), octanoyl acyl‐carrier protein (OcoACP) exhibits dramatically different metal‐ion‐binding properties compared to ACP‐SH. Calcium and magnesium can compete in either ACP species for manganese binding. Photochemically‐induced dynamic nuclear polarisation ¹H‐NMR experments strongly suggest that ACP‐SH and OcoACP undergo at pH‐induced conformational change between pH 5.5 and pH 7.0, and that divalent cations stabilize the protein against such pH‐induced structural perturbations.