Light-driven chloride ion transport by halorhodopsin from Natronbacterium pharaonis: 2. Chloride release and uptake, protein conformation change, and thermodynamics

Journal Article · Tue Nov 07 04:00:00 EST 1995 · Biochemistry (Eaton)

DOI:https://doi.org/10.1021/bi00044a028· OSTI ID:415159

Varo, G; Needleman, R; Lanyi, J K ^[1]

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1. Univ. of California, Irvine, CA (United States)

The photocycle of the light-driven chloride pump, N. pharaonis halorhodopsin, is described by the scheme HR {yields} K {yields} {leftrightarrow} L {leftrightarrow} N {leftrightarrow} O {leftrightarrow} HR{prime} {yields} HR. From the chloride dependencies of the rate constants in this model we identify the N {yields} O and O {yields} HR{prime} reactions as the steps where chloride release and uptake occur, respectively, during the transport. The dependencies of the rate constants on temperature describe a thermodynamic cycle in which enthalpy-entropy conversion occurs in the O{yields}HR{prime} reaction. The dependencies of the rate constants on hydrostatic pressure indicate that a substantial volume decrease occurs at the L{yields}N reaction, a result of a large-scale conformational change. This is the opposite of the volume increase in the photocycle of the proton pump, bacteriorhodopsin, that is implicated in the access change of the active site during the transport and the passage of a proton from the cytoplasmic surface to the active site. The results together suggest a chloride transport mechanism, in which the equivalents of all the ion transfer steps in bacteriorhodopsin occur but in the reverse sense, so as to cause the extracellular-to-cytoplasmic translocation of a chloride ion instead of the cytoplasmic-to-extracellular transport of a proton. 65 refs., 12 figs.

• Sponsoring Organization: USDOE
• OSTI ID: 415159
• Journal Information: Biochemistry (Eaton), Journal Name: Biochemistry (Eaton) Journal Issue: 44 Vol. 34; ISSN 0006-2960; ISSN BICHAW
• Country of Publication: United States
• Language: English

Related Subjects

55 BIOLOGY AND MEDICINE
BASIC STUDIES
BACTERIA
BIOCHEMISTRY
CHLORIDES
ISOMERIZATION
MEMBRANE PROTEINS
MEMBRANE TRANSPORT
PHOTOCHEMICAL REACTIONS
PROTONS
RHODOPSIN
TEMPERATURE DEPENDENCE
THERMODYNAMICS
UPTAKE