1 Bi 1 Lecture 3 Thursday, March 30, 2006 What is a Receptor? Receptors and Ion Channels as Examples of Proteins.
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Bi 1 Lecture 3
Thursday, March 30, 2006
What is a Receptor?
Receptors and Ion Channels as Examples of Proteins
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receptor
Most drug receptors are proteins.
a molecule on the cell surface or in the cell interior that has an affinity for a specific molecule (the ligand).
Latin, “to tie”
Greek, “first”
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side chains
“peptide”or
amide bonds
link the
“backbone”or
“main chain”or
“-carbons”
Little Alberts Figure 2-22© Garland publishing
shortest: 9longest: 5500
20 types
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helices sheets
http://www.its.caltech.edu/~lester/Bi-1/alpha-helix-alphabetical.pdb
http://www.its.caltech.edu/~lester/Bi-1/beta-sheet-antiparallel.pdb
Proteins contain a few structural motifs:
Hide side chainsShow H-bonds and distancesShow ribbons & arrowsShow side chainsShow Van der Waals radii
(Swiss-prot viewer must be installed on your computer)
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nicotinic acetylcholine
receptor
Most drug receptors are membrane proteins
Outside the cell
Inside the cell = cytosol
(view in ~1995)
natural ligand(agonist)
nicotine, another agonist
Membrane = lipid bilayer
~ 100 Å= 10 nm
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Overall topology of the nicotinic acetylcholine receptor(view in ~2000)
outside the cell:
5 subunitseach subunit has 4 -helices
in the membrane (20 membrane helices total)
Little Alberts figure 12-42© Garland publishing
Binding Region
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The acetylcholine binding protein (AChBP) from a snail, discovered in 2001, strongly resembles the binding region
(Swiss-prot viewer must be installed on your computer)
Color by chainShow 2 subunits,Chains,Ribbons
5 subunits
Little Alberts figure 12-42© Garland publishing
http://www.its.caltech.edu/~lester/Bi-1/AChBP+Carb-5mer.pdb
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http://www.its.caltech.edu/~lester/Bi-1-2004/AChBP-2004-BindingSite.pdb
The AChBP binding site occupied by an acetylcholine analog (2004)
http://www.its.caltech.edu/~lester/Bi-1/AChBP-2004-BindingSite.pdb
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Binding region
Membrane region
Cytosolicregion
Colored by secondary
structure
Colored by subunit(chain)
Nearly Complete Nicotinic Acetylcholine Receptor (February, 2005)
http://pdbbeta.rcsb.org/pdb/downloadFile.do?fileFormat=PDB&compression=NO&structureId=2BG9
~ 2200 amino acids in 5 chains
(“subunits”),
MW ~ 2.5 x 106
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How the binding of agonist (acetylcholine or nicotine) might open the channel: June 2003 view
M2
M1
M3
M4
Ligand-bindingregion
11Nicotinic acetylcholine receptor
Most drug receptors are membrane proteins
Some drugs bind on the axis
Some drugs compete with nicotine or acetylcholine
membraneregion
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Protein Lecture #
Ligand-gated Ion Channels 3 (today)
Pumps and transporters 5, 13
Motors 10
G protein-coupled receptors and G proteins 12
Enzymes 13, 15
DNA-binding proteins 18
RNA polymerase, ribosome 18
Cystic Fibrosis Transmembrane Regulator 20
Rhodopsin 26
All I really need to know about lifeI learned in Bi 1
1. If you want a job done right, get a protein
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Protein structure prediction:
An important 21st-century problem
Want to test your own skill at predicting protein structure?
Then enter “Critical Assessment of Techniques for Structure Prediction”or CASP 7
http://predictioncenter.org/
Winners earn an automatic “A+” in Bi1 (retroactively, if appropriate)
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Protein Folding vs. “Inverse Folding” = Computational Protein Design
Protein Folding(no degeneracy)
Inverse Folding(large degeneracy)Set of All
StructuresSet of All
Sequences
Individualamino acids
Several ways to make an arch
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X-ray Crystallography
Crystal Growth
X-ray Data
Electron Density
Protein Model
http://www.search.caltech.edu/CIT_People/action.lasso?-database=CIT_People&-response=Detail_Person.html&-layout=all_fields&person_id=29067&-search
Bi 1 Cameo by Professor Pamela J. Bjorkman
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X-ray crystallography
• Why X-rays?Right wavelength to resolve atoms
• Why crystals?Immobilize protein, enhance weak signal from scattering
• What is a protein crystal?Large solvent pathways (20-80% solvent)Same density as cytoplasmEnzymes active in crystals
• Are crystal structures valid compared with solution structures?Usually -- Compare NMR and X-ray structuresStructures correlate with biological functionMultiple crystal forms look same -- small effects of
packing
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Overview of imaging
No lens to refocus X-rays, so must
understand reciprocal space and
diffraction
Diffraction:Scattering followed
by interference
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Bragg’s lawConsider simultaneous reflection of a large number of x-rays.
See diffraction maximum in direction only if diffracted waves are in phase.
Path difference (2dsin) must represent an integral number of wavelengths to get
constructive interference.
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Learned two things from Bragg’s Law
• sin = n/2 x 1/d
Low angle: large interplanar spacing
High angle: small interplanar spacing
Since sin 1/d, structures with large interplanar
spacings (d) will have diffraction patterns with small
spacings and vice-versa.
• Repeating unit in real space (crystal) --> diffraction
maxima and minima
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Same molecular transform sampled by different lattices
Modified from Lipson & Taylor, 1964
a) Molecular transform b) Lattice
d - f ) Same molecular transform sampled by different lattices
c) Convolution of lattice and transform
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Resolution
From Harburn, Taylor, Wellbery, An Atlas of Optical Transforms
An inverse Fourier transform (FT) including all of the high angle information gives back the original image.
An inverse FT including only the low angle information gives back a low resolution view of Mickey.
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The electron density equation and the phase problem
• There are experimental methods for
determining the phase for each reflection hkl.
(xyz ) 1
V h
k | F(hkl) | exp[( 2i(hk ky lz ) ihkl]
l
Can measure this|F(hkl)|=I1/2
Can’t measure this
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X-ray detection
• Film (relic of the past)
• Diffractometers (almost relic
of past, but used for small
molecules)
• Multiwire detectors (almost
relic of past)
• Phosphorimager detectors
(R-AXIS, MAR)
• CCD detectors
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Synchrotron x-ray sources
• High-intensity x-ray emitted by charged particles accelerated in a curved path
• X-ray wavelength in range of 0.5 - 2 Å (from E=h=hc/)
• Is tunable!!
Radiation emitted by accelerating charged particle
tangent to path of circle
e-
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