Effect of temperature, pH, and metal ion binding on the secondary structure of bacteriorhodopsin: FT-IR study of the melting and premelting transition temperatures.
We have measured the temperature dependence of the FT-IR spectra of bacteriorhodopsin (bR) as a function of the pH and of the divalent cation regeneration with Ca(2+) and Mg(2+). It has been found that although the irreversible melting transition shows a strong dependence on the pH of the native bR, the premelting reversible transition at 78-80 degrees C shows very little variation over the pH range studied. It is further shown that the acid blue bR shows a red-shifted amide I spectrum at physiological temperature, which shows a more typical alpha-helical frequency component at 1652 cm(-)(1) and could be the reason for the observed reduction of its melting temperature and lack of an observed premelting transition. Furthermore, the thermal transitions for Ca(2+)- and Mg(2+)-regenerated bR (Ca-bR and Mg-bR, respectively) each show a premelting transition at the same 78-80 degrees C temperature as the native purple membrane, but the irreversible melting transition has a slight dependence on the cation identity. The pH dependence of the Ca(2+)-regenerated bR is studied, and neither transition varies over the pH range studied. These results are discussed in terms of the cation contribution to the secondary structural stability in bR.
Heyes
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Laser Dynamics Laboratory, School of Chemistry and Biochemistry, Georgia Institute of Technology, Atlanta, Georgia 30332-0400, USA.
El-Sayed
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MA
D001436:Bacteriorhodopsins / Q000737:chemistry / Q000378:metabolism*; D002118:Calcium / Q000378:metabolism*; D006863:Hydrogen-Ion Concentration; D007477:Ions; D008274:Magnesium / Q000378:metabolism*; D011485:Protein Binding; D017433:Protein Structure, Secondary; D017550:Spectroscopy, Fourier Transform Infrared; D013696:Temperature
D007477:Ions; D001436:Bacteriorhodopsins; D008274:Magnesium; D002118:Calcium