Computational Investigation of Calcium Binding Proteins Annexin A1 and Cardiac Troponin C

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Lewis, Kimberly A

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East Carolina University


Calcium binding proteins are vital for many biological functions. Many undergo calcium induced conformation changes through sensing motifs, resulting in functional changes for the proteins. In this study computational investigations were performed for annexin A1 and cardiac troponin C (cTnC). Annexin A1 is known to induce membrane aggregation, and prior to interaction with membrane or proteins, must undergo specific calcium induced conformation change. This study focuses on investigating the conformation change pathway for annexin A1. Cardiac troponin C is involved in the calcium induced regulation of cardiac muscle contraction. This study investigates the impact of N-terminal mutations on the regulatory binding site for cTnC. The goal of this study is to use knowledge of these calcium binding proteins to determine the conformation pathway of annexin A1 and mutational effects of cTnC linked to cardiac diseases. Multiple nudged elastic band (NEB) method simulations were performed for annexin A1. The trajectories for the conformation change were generated and examined for the full length annexin A1 protein. Our results suggest that the N-terminal domain of annexin A1 is removed from repeat III of the core domain in a sliding motion. Previously, it has been unclear how the N-terminal removes itself from the core. The loop region of repeat III covering the N-terminal helix in the apo structure does not lift up allowing the N-terminal to swing out of the pocket. The process resembles a sliding motion, where the N-terminal pulls out from the bottom of the core domain. Our results also indicate that the folding of helix D in repeat III of the core domain folds in a two-step process, and during the conformation change calcium binding sites undergo secondary structure change. The results obtained using the NEB method provides an atomistic explanation for the complete conformation change pathway of annexin A1. Molecular dynamics simulations totaling 1,425 ns were performed for wild type, D65A, S69C, A8V, L29Q, and C84Y mutations of cTnC. These mutations included five point mutations in the N-terminal domain, two of which are located at calcium binding site II. The simulation trajectories were analyzed using MMPBSA/MMGBSA, RMSD, and distance analysis. Our results showed that D65A, S69C, and L29Q have a decrease in calcium binding affinity. The A8V and C84Y mutations had an increase in calcium binding affinity. The loss of calcium binding affinity is detrimental when mutation of anchoring residues in site II occur, such as D65 and E76. Overall, trends in the effect of calcium binding affinity for these mutations and calcium binding to site II show consistencies with experimental studies. Mutations in the N-terminal domain of cTnC did not have as strong as an effect on calcium binding site III and IV of the C-terminal domain. These binding sites have higher calcium binding affinity than site II and are not affected much by the mutations tested in this work. The mutational studies of cTnC are important to investigate due to their link with cardiac diseases. cTnC's involvement in the muscle contracting process is vital, and many of these mutations lead to cardiac diseases that need to be studied in more detail to understand the mechanistic effect they have on muscle contraction.