膜蛋白的结构与功能三.ppt

膜蛋白的结构与功能三
第1页，共47页，编辑于2022年，星期三
Bacteriorhodopsin
Introduction to bacteriorhodopsin
Archaebacteria Halobacteriacrystals.
Lipid is 25 % of purple membrane & bR is the other 75 %.
First membrane protein structure: Henderson & Unwin, Nature 257, 28-32 (1975).
Electron micrographs from 1 mm2 × 45 Å samples (120,000 protein molecules).
Diffraction pattern is not a three dimensional lattice of points, but a two dimensional lattice of continuous lines.
Electron microscopy images give phases directly, but at lower resolution than a diffraction pattern.
Need to tilt the sample to get information perpendicular to the plane of the membrane → technical complications.
Structure showed seven a helices, and that the protein packed as a trimer within the membrane.
第7页，共47页，编辑于2022年，星期三
Bacteriorhodopsin
7 TM helices
Forms a homotrimer
Homotrimers aggregate to form the purple membrane
Stability of trimer by:
G113, I117, L48
Most stability comes from surrounding lipids
第8页，共47页，编辑于2022年，星期三
Bacteriorhodopsin
Proton pumping channel & water molecules
Ground state reveals a proton pumping channel:
- Charged residues line up like a relay team.
- Schiff base linkage at the centre of this channel.
Waters have a functional role in proton-transport.
- An extensive hydrogen-bond network links the Schiff base to the extracellular medium.
- No such waters are visible to the cytoplasmic side.
第9页，共47页，编辑于2022年，星期三
Bacteriorhodopsin
The complex-counter ion
A Schiff base is formed by a covalent double bond to a nitrogen.
- Archaeal rhodopsins have a well conserved lysine in helix G.
A protonated Schiff base is positively charged.
- Buried charges usually energetically unfavoured.
- A `counter-ion' must compensate the Schiff base charge.
Bacteriorhodopsin has several charged groups near the Schiff base.
- Water molecules stabilise the positively charged Schiff base & Arg82, and the negatively charged Asp85 & Asp212.
- Whole arrangement is called the complex count