Vectorial catalysis Flashcards
What is vectorial catalysis?
Movement –> direction of movement, it deforms the gradient of diffusion. Coupling of a scalar process (ATP hydrolysis in the cytosol) to a vectorial process (Ion Flux across the membrane) requires a vectorial coupling mechanism. Directionality is achieved through the Splitting of desired movement & chemical reactions into SEQUENTIAL HALF REACTIONS.
What is the Curie-Prigogine Principle?
Coupling of a scalar process (ATP hydrolysis in the cytosol) to a vectorial process (Ion Flux across the membrane) requires a vectorial coupling mechanism (Curie-Prigogine Principle) or, an enzyme arranged asymmetrically in the membrane.
Explain how the calcium ATPase of the SR membrane works with vectorial catalysis.
–> 1 ATP is hydrolized, 2 Ca2+ in SR and 3 H+ out of SR.
–> ATP Hydrolysis and ca2+ transport can never occur alone. So there is always directed fluxes and compartmentalized substrates and products = vectorial catalysis
Give one example of biological photoreceptors (and its biological function), that convey the principles of vectorial catalysis.
–> lilac membrane from Halobacteria -Capture of energy from sunlight.
- transport of H+ against electrical and ionic gradients - H+ transport accumulates energy in the cell by increasing the electrochemical gradient, µH, : like charging a battery.
- the electrochemical gradient serves as an energy source for other transport processes, probably most importantly including production of the universal energy currency ATP via F0F1-ATP synthase. ➜ A simple conversion of light energy into chemical energy for cellular function
What is the chromophor in Bacteriorhodopsin?
RETINAL - Retinal is incorporated into the protein as a light absorber and linked to a Lysine residue via a retinal Schiff base (RSB) - Retinal is a vitamin A-aldehyde The distribution of the positive charge along the polyene chain determines the colour
What is bacteriorhodopsin and how does the light activation of the chromophore works?
–> Proton pump, it captures light energy and uses it to move protons across the membrane. The resulting proton gradient is subsequently converted into chemical energy.
- Absorption takes < 10–15 s;
- retinal isomerisation (change of shape): < 10–12 s
- Extinction coefficient > 50000 M–1cm–1,
- Quantum Efficiency: 0.3 - 0.6 (max would be 1)
–> the protein returns to the dark state through several thermal reactions (“thermal relaxation”). This relaxation necessarily occurs with a loss of energy –> H+ is transported in discontinuous steps, that sum up to the transport of 1 H+ across the membrane. These are the “half-reactions” with vector character.
Explain why does the Franck- Condon principle is relevant for the steps in the BR (Bacteriorhodopsin) Photocycle.
The Franck-Condon principle is relevant because electronic excitation by a photon happens much faster than any change in the vibrational states of retinal. Therefore, the state immediately after photon absorption is not the lowest energy excited state – it’s vibrational modes are aberrantly high. Later, atomic positions and water can rearrange to find the lowest energy.1 . photo excitation in the FCES 2.relaxation into two options. a) J/K 12-cis-Retinal or b) all-trans ( reverts back to the dark state. 3. if J/K/L/M path is skipped, this excitation of retinal does not yield a transported proton.
Describe the steps of the I,S,T Model for the BR Photocycle and its intermediate states.
I: retinal isomerization
S: structural change
T: transport- H+ uptake inside the cell, and release to the extracellular bulk phase
INTERMEDIATE STATES –> difference absorbance espectra
BR568 - ground/ resting state - all trans
BR* excited state - all trans
J620
K590
L550
M412 RSB deprotonated (yellow)
N530
O640 - all trans
Why does the colour of retinal changes during the photocycle?
–> Colour is based on electrostatic interaction with the environment -M intermediate ( yellow) nearly white
What drives the proton transport?
- light excitation of S0 to S1 results in a drastic pKa-change of Retinal-Schiff base (RSB) nitrogen and of surrounding amino acids. = LIGHT ABSORPTION
- the pKa-changes cause proton motion within the protein at the H+- donor side (internal) and H+- acceptor side (external) –> PKA DIFFERENCE
- note: charge redistribution drives RSB isomerization and pKa-changes drive the H+-transfer.
What are the steps in the proton- transport in Bacteriorhodopsin?
- Photonabsorption
- Retinalisomerisierung mit pka change der RSB um mehr als 7 log Einheiten
- H+ tranfer zum 1 akzeptor ASP85 (–> wasser 402)
- H+ Release von 2 akzeptor GLU204 Transport ins EC-Medium, nach rotation von ARG82 (–> wasser 401)
- Reprotonierung der RSB mitters H+ -Donor ASP96 –> Konversion in h+ in gebundener stadium
- Reprotonierung von Donor ASP96
- Transfer H+ von ASP86 zum GLU204.
Which are the cheminal & which are the vectorial half steps in the vectorial catalysis of Bacteriorhodopsin?
Chemical (Isomerisation & Reprotonation RSB)
- Retinalisomerisierung
- Reprotonierung der RSB mitters H+ -Donor ASP96 –> Konversion in h+ in gebundener stadium
Vectorial ( H+ discontinuos transport & conformational changes)
- Grundstadium: all trans retinal protoniert
- L-Intermediat: 13-cis Retinal protoniert
- Spätes M-Intermediat: 13-cis neutral
- N-Intermediat: 13-cis protoniert
What is “active water” and how it mediates the proton transfer in BR?
Vermittelt den Proton transfer –> Distanz zwischen Sensor 1 (RSB) und H+ Akzeptor (A1) ist zu groß für den Transport.
–> WASSER-402: Übergang von H+ zu A1- Asp85
–>WASSER-401 Transferiert H+ zu A2-Glu 204
What is the proton release Cluster ?
In the photocycle of bacteriorhodopsin at pH 7, a proton is ejected to the extracellular medium during the protonation of Asp-85 (H+ from RSB) upon formation of the M intermediate.
Proton release cluster
- Asp 204 & Asp 194 face the water molecules with its negatively charged sidechains.
- Neutral pH (7) is needed for proton release =M2, low pH (5) RSB can still interact with Asp= M1, and so no H+ release can be achieved.
What are the steps in the water movement mechanism in BR?
PHOTO
B.(1) Isomerization
Three water molecules, retinal Schiff base, Asp85, and Asp212 form a pentameric arrangement of strong hydrogen bonds in the BR ground state–> X after isomerization
C.(2) Arg Movement
protonation of Asp85, moves the protonated water molecules towards Glu194 and Glu204 in the M intermediate. The hydrogen bond between Glu204 and Ser193 is broken, and the Ser193 gate opens at the release site
A.(3) Transient water chain
After deprotonation, the retinal relaxation around the single bonds triggers a relocation of these water (401/402) molecules, which then form a transient linear chain between Asp96 and the Schiff base in the M2/N Intermediate.
