Electrostatic effects on the free-energy balance in folding a ribosome-inactivating protein.
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.
Olson
M A
MA
Department of Cell Biology and Biochemistry, USAMRIID, 1425 Porter Street, Frederick, MD 21702, USA. molson@ncifcrf.gov
D055598:Chemical Phenomena; D002627:Chemistry, Physical; D008958:Models, Molecular; D009154:Mutation; D011487:Protein Conformation; D017510:Protein Folding; D012270:Ribosomes / Q000187:drug effects*; D012276:Ricin / Q000737:chemistry* / Q000235:genetics / Q000494:pharmacology; D055672:Static Electricity; D013816:Thermodynamics
D012276:Ricin