Journal article
The Effect of Salts on the Activity and Stability of Escherichia coli and Haloferax volcanii Dihydrofolate Reductases
Journal of molecular biology, Vol.323(2), pp.327-344
2002
Handle:
https://hdl.handle.net/2376/106949
PMID: 12381324
Abstract
The extremely halophilic Archae require near-saturating concentrations of salt in the external environment and in their cytoplasm, potassium being the predominant intracellular cation. The proteins of these organisms have evolved to function in concentrations of salt that inactivate or precipitate homologous proteins from non-halophilic species. It has been proposed that haloadaptation is primarily due to clustering of acidic residues on the surface of the protein, and that these clusters bind networks of hydrated ions. The dihydrofolate reductases from
Escherichia coli (ecDHFR) and two DHFR isozymes from
Haloferax volcanii (hvDHFR1 and hvDHFR2) have been used as a model system to compare the effect of salts on a mesophilic and halophilic enzyme. The KCl-dependence of the activity and substrate affinity was investigated. ecDHFR is largely inactivated above 1
M KCl, with no major effect on substrate affinity. hvDHFR1 and hvDHFR2 unfold at KCl concentrations below ∼0.5
M. Above ∼1
M, the KCl dependence of the hvDHFR activities can be attributed to the effect of salt on substrate affinity. The abilities of NaCl, KCl, and CsCl to enhance the stability to urea denaturation were determined, and similar efficacies of stabilization were observed for all three DHFR variants. The Δ
G°(H
2O) values increased linearly with increasing KCl and CsCl concentrations. The increase of Δ
G°(H
2O) as a function of the smallest cation, NaCl, is slightly curved, suggesting a minor stabilization from cation binding or screening of electrostatic repulsion. At their respective physiological ionic strengths, the DHFR variants exhibit similar stabilities. Salts stabilize ecDHFR and the hvDHFRs by a common mechanism, not a halophile-specific mechanism, such as the binding of hydrated salt networks. The primary mode of salt stabilization of the mesophilic and halophilic DHFRs appears to be through preferential hydration and the Hofmeister effect of salt on the activity and entropy of the aqueous solvent. In support of this conclusion, all three DHFRs are similarly stabilized by the non-ionic cosolute, sucrose.
Metrics
9 Record Views
Details
- Title
- The Effect of Salts on the Activity and Stability of Escherichia coli and Haloferax volcanii Dihydrofolate Reductases
- Creators
- Donna B WrightDouglas D BanksJeremy R LohmanJacqueline L HilsenbeckLisa M Gloss
- Publication Details
- Journal of molecular biology, Vol.323(2), pp.327-344
- Academic Unit
- Graduate School
- Publisher
- Elsevier Ltd
- Identifiers
- 99900546843701842
- Language
- English
- Resource Type
- Journal article