Photoenergetics of octopus rhodopsin. Convergent evolution of biological photon counters

摘要: The enthalpy changes associated with each of the major steps in photoconversion octopus rhodopsin have been measured by direct photocalorimetry. Formation primary photoproduct (bathorhodopsin) involves energy uptake about 130 kJ/mol, corresponding to storage over 50% exciting photon energy, and is comparable previously observed bovine rhodopsin. Subsequent intermediates involve step-wise dissipation this give physiological end-product (acid metarhodopsin) at a level only slightly above parent No significant differences energetics are between microvilli membrane suspensions or detergent dispersions. Use different buffer systems calorimetric experiments shows that conversion acid metarhodopsin no light-induced protonation change, whereas alkali photoproduction occurs release one proton per molecule an additional increase 50 kJ/mol. Van't Hoff analysis effect temperature on reversible equilibrium gives for → transition consistent result, confirmed observation pH changes. Acid-base titration yields apparent pK 9.5 transition, though profile deviates from ideal behaviour. We suggest high obligatory feature efficient biological photodetectors, as opposed transducers, similarity stage cephalopod vertebrate rhodopsins represents either convergent evolution molecular strong conservation crucial functional characteristic.