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Abstract

of 55 ± 0.5 °C and bound Ca(II) with a Kd = 7.9 ± 3.4 μM. The grafting approach was used to incorporate Ca(II)-binding EF-hand domains into the loops and N-termini of miniprotein scaffolds. [6, 9] EF-hand 2 of Calmodulin was grafted onto the N-terminus of the Trp-cage designed by Neidigh et al. in the Ca(II)-binding model Calmcage. [10] The fusion miniprotein folded into a similar structure to its parent peptide with an increased thermostability Tm = 50 ± 3 °C. Calmcage bound Tb(III) with a Kd = 200 ± 40 μM and Ca(II) with a Kd = 64 ± 26 mM. Another successfully grafted design was created from the EF-hand 3 of Calmodulin and the β-hairpin system C4 by Anderson et al. in the model Calmzip3. [11] It folded into a similar-to-native structure and changed conformation upon Ca(II)-binding with a thermostability of Tm = 47.5 ± 1.0 °C for the holo- and Tm = 47 ± 3 °C for the apo-protein. Calmzip3 revealed strong Tb(III) binding with a Kd = 0.96 ± 0.15 μM and Ca(II) binding in the same order of magnitude as the weakest value for Calmodulin and the strongest value for Laminin with Kd = 3.9 ± 1.4 μM. The computational design of a Superoxide Dismutase (SOD) site on the scaffold of the SH3 domain resulted in the model SO1. It folded into a β-sheet structure with high thermostability that increased in the presence of bivalent transition metals from Tm = 47 ± 0.5 °C for the holo- to Tm = 74 ± 1 °C for the Ni(II)-bound state. It showed SOD activity in the presence of Cu(II) in the same order of magnitude as small organic Cu(II)-complex mimics, outcompeting all miniprotein SOD enzymes to date with an apparent rate constant kSOD = 6.14 ± 0.58 × 107 M-1 s-1. [12-16] It therefore presented itself not only as a proof of concept for computational design but also as a potential model for studying SOD enzymes. In conclusion, all three approaches delivered functional miniproteins with Ca(II)-binding miniproteins that range from millimolar to low micromolar Kd-values. With binding affinities in the same range, they serve as ideal candidates for Ca(II)-dependent carbohydrate recognition after the example of D-type lectins.