Abstract
Proteins of halophilic organisms, which accumulate molar concentrations of KCl in their cytoplasm, have a much higher content in acidic amino acids than proteins of mesophilic organisms. It has been proposed that this excess is necessary to maintain proteins hydrated in an environment with low water activity, either via direct interactions between water and the carboxylate groups of acidic amino acids or via cooperative interactions between acidic amino acids and hydrated cations. Our simulation study of five halophilic proteins and five mesophilic counterparts does not support either possibility. The simulations use the AMBER ff14SB force field with newly optimized Lennard-Jones parameters for the interactions between carboxylate groups and potassium ions. We find that proteins with a larger fraction of acidic amino acids indeed have higher hydration levels, as measured by the concentration of water in their hydration shell and the number of water/protein hydrogen bonds. However, the hydration level of each protein is identical at low (bKCl = 0.15 mol/kg) and high (bKCl = 2 mol/kg) KCl concentrations; excess acidic amino acids are clearly not necessary to maintain proteins hydrated at high salt concentration. It has also been proposed that cooperative interactions between acidic amino acids in halophilic proteins and hydrated cations stabilize the folded protein structure and would lead to slower dynamics of the solvation shell. We find that the translational dynamics of the solvation shell is barely distinguishable between halophilic and mesophilic proteins; if such a cooperative effect exists, it does not have that entropic signature.
Highlights
Halophilic organisms, unlike most life on Earth, have the uncanny ability to survive at molar external NaCl concentrations
Our results do not support this scenario for the case of concentrated KCl solutions, typical of the cytoplasm of some halophilic organisms; the 10 proteins we investigate keep their hydration level constant under widely different external KCl concentrations despite having very different content in acidic amino acids
FRmin;KþO 1⁄4 2:0 does not result in the correct solution structure at bKCH3COO 1⁄4 0:5 mol/kg: the corresponding anion-cation radial distribution function (RDF), shown in Fig. 3 d, would suggest that only solventseparated ion pairs exist in potassium acetate solutions and that neither contact-shared (CIP) nor solvent-shared (SIP) occur (CIP, the two ions are in direct contact; SIP, the ions share one hydration layer; and solvent-separated, each ion retains its first hydration layer)
Summary
Halophilic organisms, unlike most life on Earth, have the uncanny ability to survive at molar external NaCl concentrations. 2746 Biophysical Journal 120, 2746–2762, July 6, 2021 induced by high external NaCl concentrations, some halophiles accumulate equivalent concentrations of KCl in their cytoplasm [5] At such high salt concentrations, the interactions that dictate the structure and structural stability of proteins differ markedly from those at the much lower salt concentrations found in most organisms. Proteins of halophilic organisms (here, termed halophilic proteins) are structurally stable and functioning at high salt concentrations, but often show lower (or no) stability and activity at KCl concentrations typical of the cytoplasm of mesophilic organisms [5]. Their resilience is Protein hydration is robust to [KCl]
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