Reply to “Comment on ‘An Apparent General Solution for the Kinetic Models of the Bacteriorhodopsin Photocycle' ”

Hendler, Richard W.

doi:10.1021/jp056537m

Cited by 1 publication

(1 citation statement)

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“…Some kinetic models proposed that parallel or branched photocycles of both proton pumping and non-proton pumping sequences could explain the experimental results as well as the model including only a single cycle but with reversible and irreversible steps , . Hendler has made elaborate calculated fits to available data and conducted a thorough review of all models . The conclusion is that no one model fits all data.…”

Section: Resultsmentioning

confidence: 99%

Volume and Enthalpy Changes of Proton Transfers in the Bacteriorhodopsin Photocycle Studied by Millisecond Time-Resolved Photopressure Measurements

Liu

Edens²,

Grzymski

et al. 2008

Biochemistry

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The volume and enthalpy changes associated with proton translocation steps during the bacteriorhodopsin (BR) photocycle were determined by time-resolved photopressure measurements. The data at 25 degrees C show a prompt increase in volume followed by two further increases and one decrease to the original state to complete the cycle. These volume changes are decomposed into enthalpy and inherent volume changes. The positive enthalpy changes support the argument for inherent entropy-driven late steps in the BR photocycle [Ort, D. R., and Parson, W. M. (1979) Enthalpy changes during the photochemical cycle of bacteriorhodopsin. Biophys. J. 25, 355-364]. The volume change data can be interpreted by the electrostriction effect as charges are canceled and formed during the proton transfers. A simple glutamic acid-glutamate ion model or a diglutamate-arginine-protonated water charge-delocalized model for the proton-release complex (PRC) fit the data. A conformational change with a large positive volume change is required in the slower rise (M --> N of the optical cycle) step and is reversed in the decay (N --> O --> BR) steps. The large variation in the published values for both the volume and enthalpy changes is greatly ameliorated if the values are presented per absorbed photon instead of per mole of BR. Thus, it is the highly differing assumptions about the quantum or reaction yields that cause the variations in the published results.

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Section: Resultsmentioning

confidence: 99%