Hg(II) Binding to a Weakly Associated Coiled Coil Nucleates an Encoded Metalloprotein Fold: A Kinetic Analysis

A detailed kinetic analysis of metal encapsulation by a de novo-designed protein is described. The kinetic mechanism of Hg(II) encapsulation in the three-stranded coiled coil formed by the peptide CH3CO-G LKALEEK CKALEEK LKALEEK G-NH2 (Baby L9C) is derived by global analysis. The mechanism involves rapid initial collapse of two peptides by Hg(II) forming $Hg(Baby\>L9C_{-H})_2$ with a linear thiolato Hg(II) bound to the cysteine sulfur atoms. Here, Baby L9CH denotes Baby L9C with the cysteine thiol deprotonated. Addition of the third peptide, forming the three-stranded coiled coil, is the rate-determining step and results in an intermediate state involving two separate species. One of the species, termed the properly folded intermediate, undergoes rapid deprotonation of the third cysteine thiol, yielding the desired three-stranded coiled coil with an encapsulated trigonal thiolato Hg(II). The other species, termed the misfolded intermediate, rearranges in an experimentally distinguishable step to the properly folded intermediate. The order of the reaction involving the addition of the third peptide with respect to the concentration of Baby L9C indicates that addition of the third helix only occurs through reaction of $Hg(Baby\>L9C_{-H})_2$ and Baby L9C that is unassociated with a coiled coil. Temperature dependence of the reaction afforded activation parameters for both the addition of the third helix $(\Delta H^\ddagger = 20(2)\>kcal/mol;\>\Delta S^\ddagger = 40(5)\>cal/mol\>K)$ and the rearrangement of the misfolded intermediate steps $(\Delta H^\ddagger = 23(2)\>kcal/mol;\>\Delta S^\ddagger = 27(5)\>cal/mol\>K)$. The mechanism is discussed with regard to metalloprotein folding and metalloprotein design.