Jul 13, 1976

Energetics of primary processes in visula escitation: photocalorimetry of rhodopsin in rod outer segment membranes

Biochemistry
A Cooper, C A Converse

Abstract

A sensitive technique for the direct calorimetric determination of the energetics of photochemical reactions under low levels of illumination, and its application to the study of primary processes in visula excitation, are described. Enthlpies are reported for various steps in the bleaching of rhodopsin in intact rod outer segment membranes, together with the heats of appropriate model reactions. Protonation changes are also determined calorimetrically by use of buffers with differing heats of proton ionization. Bleaching of rhodopsin is accompanied by significant uptake of heat energy, vastly in excess of the energy required for simple isomerization of the retinal chromophore. Metarhodopsin I formation involves the uptake of about 17 kcal/mol and no net change in proton ionization of the system. Formation of metarhodopsin II requires an additional energy of about 10 kcal/mol and involves the uptake on one hydrogen ion from solution. The energetics of the overall photolysis reaction, rhodopsin leads to opsin + all-trans-retinal, are pH dependent and involve the exposure of an additional titrating group on opsin. This group has a heat of proton ionization of about 12 kcal/mal, characteristic of a primary amine, but a pKa in the ...Continue Reading

  • References17
  • Citations21

References

  • References17
  • Citations21

Citations

Mentioned in this Paper

Buffers
Thermodynamics
RHO gene
Tissue Membrane
Amines
Rhodopsin
Opsins
Rod Opsins
Vision
Uptake

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