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

Abstract

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-chainpacking 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.

Thermal Denaturation Profiles of Tuna Myoglobin

“…The results of DSC suggest that structures other than helical segments are important for the stability of Mb. Bertagna and Barrick 27) found that the structure of apoMb is stabilized not only by buried hydrophobic residues in the interfaces of helices but also by non-specific hydrophobic interactions.…”

How strong are side chain interactions in the folding intermediate?

“…For example, NMR studies demonstrated that the equilibrium and early (<10 À3 s, formation time) kinetic intermediates have a virtually complete secondary structure, and the side chains of the A, G and H helices, and in part, of the B and E helices too, are somewhat screened from the solvent. The aforementioned articles and Bertagna and Barrick 28 offer evidence both for and against involvement of the apoMb side chains in stabilization of the intermediate state.…”

Separation of flavonoid and alkaloid using collagen fiber adsorbent

“…As a result, although hydrogen bonds still have an effect on the adsorption, the dominant adsorption force of rutin, baicalin, matrine and caffeine on CFA at lower ethanol concentration (o30%) is hydrophobic interaction between the hydrophobic domain of adsorbates and polypeptide chains of CFA. It is known that hydrophobic interaction is a non-specific interaction [42,43], because this interaction is the tendency of any hydrocarbons or lipophilic hydrocarbon-like groups in solutes to form intermolecular aggregates in an aqueous medium [44]. Therefore, CFA exhibits a lower adsorption selectivity at lower ethanol concentration, as shown in Fig.…”