Benefits and constrains of covalency: the role of loop length in protein stability and ligand binding.

Protein folding is governed by non-covalent interactions under the benefits and constraints of the covalent linkage of the backbone chain. In the current work we investigate the influence of loop length variation on the free energies of folding and ligand binding in a small globular single-domain protein containing two EF-hand subdomains-calbindin D. We introduce a linker extension between the subdomains and vary its length between 1 to 16 glycine residues. We find a close to linear relationship between the linker length and the free energy of folding of the Ca-free protein. In contrast, the linker length has only a marginal effect on the Ca affinity and cooperativity. The variant with a single-glycine extension displays slightly increased Ca affinity, suggesting that the slightly extended linker allows optimized packing of the Ca-bound state. For the extreme case of disconnected subdomains, Ca binding becomes coupled to folding and assembly. Still, a high affinity between the EF-hands causes the non-covalent pair to retain a relatively high apparent Ca affinity. Our results imply that loop length variation could be an evolutionary option for modulating properties such as protein stability and turnover without compromising the energetics of the specific function of the protein.