A model of dynamic side-chain-side-chain interactions in the α-lactalbumin molten globule

Proteins in the molten globule state contain high levels of secondary structure, as well as a rudimentary, nativelike tertiary topology, Thus, the structural similarity between the molten globule and native proteins may have a significant bearing in understanding the protein-folding problem. To explore the nature of side-chain-side-chain interactions in the alpha -lactalbumin (alpha -LA) molten globule, we determined the effective concentration for formation of the 28-111 disulfide bond in 14 double-mutant proteins, each containing two hydrophobic core residues replaced by alanine. We compared our results with those of single-alanine substitutions using the framework of double-mutant cycle analysis and found that, in the majority of cases, the effects of two alanine substitutions are additive. Based on these results, we propose a model of side-chain-side-chain interactions in the alpha -LA molten globule, which takes into consideration the dynamic nature of this partially folded species.