Nonspecific hydrophobic interactions stabilize an equilibrium intermediate of apomyoglobin at a key position within the AGH region

Acid-induced unfolding of apomyoglobin (apoMb) proceeds in a multistate process involving at least one equilibrium intermediate (I) at pH 4.2. The structure of the I form has been investigated thoroughly, with significant effort devoted to identifying potentially stabilizing native contacts. Here, we test whether rigid side-chain packing interactions like those in holomyoglobin persist at a buried position, Met-131, within the low-pH apoMb intermediate. We have measured the urea-induced unfolding transitions of overpacking, underpacking, and polar substitutions of Met-131 to determine the effect on the stability of the native and intermediate states of apoMb. Whereas underpacking substitutions should destabilize the I form irrespective of the degree of native side-chain-packing interactions, we anticipate that overpacking replacements might show opposite effects in a tightly packed environment, compared with a region lacking native side-chain packing interactions. We observe that, whereas underpacking and polar substitutions destabilize the I form, overpacking substitutions are stabilizing, implying that I is structurally plastic. We also report a strong correlation between the I state unfolding free energies and side-chain transfer free energies from water to octanol. Our results suggest that, whereas side-chain hydrophobicity is important for the stability of the I form, specific side-chain packing interactions are not.