Electrostatic Effects on the Free-Energy Balance in Folding a Ribosome-Inactivating Protein
ARMY MEDICAL RESEARCH INST OF INFECTIOUS DISEASES FORT DETRICK MD
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Electrostatics of globular proteins provides structural integrity as well as specificity of biological function. This dual role is particularly striking for ricin A-chain RTA, an N-glycosidase which hydrolyzes a single adenine base from a conserved region of rRNA. The reported X-ray crystallographic structure of the RTA mutant E177A demonstrated a remarkable rescue of charge balance in the active site achieved by the rotation of a second glutamic acid Glu-208 into the vacated space. To understand this conformational reorganization, molecular-dynamics simulations were applied to estimate relative free energies that govern the thermodynamic stability of E177A together with mutants E177Q and E177D. The simulations anticipate that while E177A is a non-conservative substitution, the protein is more stable than the other two mutants. However, the structural plasticity of the RTA active site is not obtained penalty-free, rather E177A among the mutants shows the largest unfavorable net change in the electrostatic contribution to folding. Of the E177A folded state, reorganization of Glu-208 lowers the electrostatic cost of the free-energy change, yet interestingly, protein interactions oppose the rotational shift, while solvent effects favor the transition.
- Organic Chemistry