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J Tittor

Publications and source records attributed to J Tittor.

24 records · Page 2Linked to original sources
PubMed1989

A defective proton pump, point-mutated bacteriorhodopsin Asp96----Asn is fully reactivated by azide.

Addition of azide fully restored the proton pump activity of defective bacteriorhodopsin (BR) mutant protein Asp96----Asn. The decay time of M of BR Asp96----Asn, the longest living intermediate, was decreased from 500 ms at pH 7.0 to approximately 1 ms under conditions of saturating azide concentrations. This decay was faster than the decay of M in the wild-type, where no such azide effect was detectable. Stationary photocurrents, measured with purple membranes immobilized and oriented in a polyacrylamide gel, increased upon addition of azide up to the level of the wild-type. Different small anions of weak acids restored the pump activity with decreasing affinity in the order: cyanate greater than azide greater than nitrite greater than formiate greater than acetate. The activation energy of the M decay in the mutant was higher in the presence (48 kJ/mol) than in the absence (27 kJ/mol) of 100 mM azide even though the absolute rate was dramatically increased by azide. This effect of azide is due to the substitution of a carboxamido group for a carboxylic group at position 96 which removes the internal proton donor and causes an increase in the entropy change of activation for proton transfer which is reversed by azide.

Anions↗
PubMed1989

Two pumps, one principle: light-driven ion transport in halobacteria.

Comparison of the primary structure of the chloride pump halorhodopsin with that of the proton pump bacteriorhodopsin provides insight into light-driven ion transport by retinal proteins. Several conserved amino acid residues in the membrane-spanning region of both proteins and their interaction with different isomerization states of retinal are suggested to be the key element for ion transport in both proteins.

Amino Acid Sequence↗
PubMed1985

The photocycle of the chloride pump halorhodopsin. II: Quantum yields and a kinetic model.

The quantum yield of the primary photoreaction of HR478 was determined as phi(green) = 0.34 +/- 0.02 and that of the photochemical reconversion of HRL410 into HR578 as phi(blue) = 0.01 +/- 0.004. From steady-state illumination and flash-photolysis experiments, a quantitative description of HRL410 formation and decay was made leading to a model of the photocycle in isolated, light-adapted HR. This model satisfies all known facts about HR including its photochronic behaviour.

Chloride Channels↗
PubMed1982

Electrogenic Cl- pump in Acetabularia.

Measurements of this transmembrane potential difference (V) under various conditions have demonstrated the operation of an electrogenic Cl- pump in the outer plasma membrane (plasmalemma) of the unicellular marine alga Acetabularia. In preparations of partly purified membranes (containing plasmalemma), there is Cl- stimulated, N,N'-dicyclohexylcarbodiimide-insensitive, vanadate-sensitive ATPase activity with a pH optimum around pH 6.5. These properties are consistent with the assumption that the electrogenic Cl- pump is an ATPase. In order to investigate electrical details of the "Mitchellian" type of charge-translocating enzyme, steady-state current-voltage curves of the electrogenic pump (Ip(V)) were measured in vivo under dark and light conditions and analysed by two-state reaction kinetic model. This model with the resulting parameters predicts V-sensitive, undirectional Cl- effluxes through the pump. The predictions of this model agree with the experimental results. Green light causes a fast decrease of V, which is explained as a disturbance of the pump cycle. Relaxation studies on this effect and reaction kinetic analysis of Ip(V) under different external Cl- concentrations are used to develop a consistent three-state model of the pump that includes the order of and absolute rate constants of individual reactions, states of charge, stoichiometry, voltage-sensitivity and density of the pump molecules in the membrane.

Adenosine Triphosphatases↗
PubMedDate not supplied

Bacteriorhodopsin mutants D85N, D85T and D85,96N as proton pumps.

Proton translocation in the BR mutants D85N, D85T and D85,96N was studied by attachment of purple membranes to planar lipid bilayers. Pump currents in these mutants were measured via capacitive coupling and by use of the appropriate ionophores. All mutants have a reduced pK of their Schiff bases around 8-8.5 in common. At physiological pH, a mixture of chromophores absorbing at 410 nm (deprotonated form) and around 600 nm (protonated form) coexists. Excitation with continuous blue light induces in all three mutants an outwardly directed stationary pump current. These currents are enhanced upon addition of azide in D85N and D85,96N by a factor of 50, but no azide enhancement is observed in D85T. Yellow light alone induces transient inwardly directed currents in the mutants but additional blue light leads to a stationary current with the same direction. All the observed currents are carried by protons, so that the consecutive absorption of a yellow and a blue photon leads to inverted stationary photocurrents by the mutants, as observed with halorhodopsin (HR). A mechanistic model describing the inversion of proton pumping is discussed by the cis-trans, trans-cis isomerization of the retinal and the different proton accessibility of the Schiff base from the extracellular or the cytoplasmic side of the membrane.

Journal Article↗