Bacteriorhodopsin Essay

2616 Words 11 Pages
Bacteriorhodopsin is located in the plasma membrane of Halobacterium halobium, where it acts as a proton-pump, transporting protons out of the cell. Since its discovery in 1979, much research has been done on Bacteriorhodopsin and there have been many advancements in our knowledge. Firstly, on its structure and function and secondly, on the conformational changes it undergoes during retinal isomerisation.

Bacteriorhodopsin is a globular protein, (Henderson, 1975) which acts as a light driven proton- pump located in the native purple membrane of Halobacterium halobium. It was first discovered by Stoechenius and his lab in 1979 (Stoeckenius, 1979). Although Henderson and Unwin had already made a three-dimensional map of
…show more content…
For each proton absorbed one H+ ion is pumped out of the cell in a 1:1 ration (Tsutomu Kouyama, 1988). The energy for this transportation is provided by absorbing light energy (Lesk, 2010). Light induced, retinal isomerization is thought to be the main factor in proton transportation as it affects the pka values of charged amino acids, which are close by (White, 1994). These residue form a channel which pumps the protons from the cytoplasm to the extracellular space. During retinal isomerization the retinal molecule changes from the all-trans to its cis isomer as shown in figure …show more content…
This is known as the deprotonated form and from this state the retinal has the ability to return to the all-trans state, while another proton transfer occurs (Bonting, 1981). The second proton transfer also occurs in the extracellular half-channel (Hans-Thomas Richter, 1996). Asp-85 is deprotonated by protonating the empty proton release site (Lanyi, 1998).The proton is transferred from the protonated residue to the Schiff base, causing the salt-bridge to reform (Bonting, 1981). Proton uptake from the cytoplasm uses Asp-96 as the proton donor. This uptake is regulated by protein conformation and depends on this region being hydrated (Lanyi, 1998) and reprotonation of Asp-96 coincides with the retinal returning to the all-trans state (Lanyi, 2005).
The translocation of the proton must be close to the two aqueous interfaces as bound water has a very important role to play in the movement of the proton in the mechanism of proton translocation as proton transfer occurs via a bridging water molecule (Lanyi, 1998). This proton translocation is unidirectional and the proton is always moved from the cytoplasm to the extracellular surface. This is because the Schiff bas is not always accessible from both half- channels. The proton affinity for Asp-85 and Asp-96 also changes, depending on which stage of the photocycle is occurring (Lanyi,

Related Documents