structure and function of proteins ora schueler-furman 2009-2010 1
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Structure and Function Structure and Function of Proteinsof Proteins
Ora Schueler-Furman2009-2010
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Contact us: – Ora Schueler-Furman 87094 (02-6757094)
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PROTEINSPROTEINS
From the Greek word From the Greek word
“Proteios”“Proteios”- first rank, most important- first rank, most important
Play central roles in all biological processesPlay central roles in all biological processes
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Introduction into Introduction into Protein StructureProtein Structure
• The chemical nature of polypeptides
• Forces that determine protein structure
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The 4 Hierarchical Levels of Protein Structure
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Primary Structure: Sequence
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Formation of a Peptide Bond
O - oxygen
N - nitrogen
O-+H3N
R
H
CO
C||
H - hydrogen
C - carbon
cpk colors
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Dihedral Angles and define Backbone Geometry
The peptide bond is planar and polar9
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Basic Facts
Polypeptide chain: 50-10’000 aa But also >20000 aa: 34’350 aa (Titin)
Average MW of aa : 110 daltonsAverage MW of a protein of length n aa:
n X110Distance (Ci – Ci+1) <=3.63Ǻaa – amino acid
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Basic Terms
Main chain, backbone, side chain, residue bb scPeptide: a small # of connected aa
Polypeptide: a longer chain of aa
Protein: polypeptide chain with defined aa sequence & conformation
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The Protein Alphabet: 20 lettersA Ala AlanineC Cys CysteineD Asp AspartateE Glu GlutamateF Phe PhenylalanineG Gly GlycineH His HistidineI Ile IsoleucineK Lys LysineL Leu Leucine
M Met MethionineN Asn AspargineP Pro ProlineQ Gln GlutamineR Arg ArginineS Ser SerineT Thr ThreonineV Val ValineW Trp TryptophaneY Tyr Tyrosine
amino acids vary in: volume, shape, chemical nature(charge, hydrogen bonding capability, etc.)
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• The simplest aa• No sc• Very flexible bb
Special Amino Acids
• Cyclic aa• sc Connects bb N• Very constrained bb
N
CO
C H
HH
N
CO
C H
CH2
CH2H2C
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Aliphatic Amino Acids
• sc contains only carbon and hydrogen atoms• hate water 14
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Amino Acids with Hydroxyl Group
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Negatively Charged Amino Acids
different size → different tendency for 2. structure
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Amide Amino Acids
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Positively Charged Amino Acids
• large sc
• pKa 11.1 • pKa 12
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Aromatic Amino Acids
• sc contains aromatic ring
• pKa 7
• benzene ring
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Amino Acids with Sulfur
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Cystine
Oxidation of Sulfur atoms creates covalent disulfide bond (S-S bond)between two cysteines
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S-S Bonds Stabilize the Protein
A chain
G I V E Q C C A S V C S L Y Q L E N E N Y C N
s
s
s
s
B chainF V N Q H L C G S H L V E A L Y L V C G E R G F..
s
s
InsulinA chain
NN
CC
B chain22
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Post-Translational Post-Translational Modifications Modifications
• Processing (pro-insulin/insulin)– control of protein activity
• Glycosylation– protein trafficking
• Phosphorylation (Tyr, Ser, Thr) – regulation of signaling
• Methylation, Acetylation – histone tagging
• ….
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Metal Binding ProteinsMetal Binding Proteins
• aa: HCDE• Fe, Zn, Mg, Ca• Fe
– blood: red hemoglobin– electro-transfer: cytochrome c
• Zn – in DNA-binding “Zn-finger” proteins– Alcohol dehydrogenase: oxidation of alcohol
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Forces that Determine Protein
Structure
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Non-Covalent Forces Add up
Each bond is weak
Large number of bonds
From: the Molecular Biology of the Cell, 4th ed.
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Attractions between moleculesAttractions between molecules
E(r) = K/rp
p=1: Coulomb interactionbetween two charges
p>1: delocalized charges – weaker interactionsp=6: interaction between neutral molecules
Short-range interactions: p>=3
p=1
p=6
r – distance between molecules
E(r
) –
ener
gy o
f at
trac
tion
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1. Van der Waals Interactions (Lennard Jones Potential)
From: the Molecular Biology of the Cell, 4th ed.
rij
Attractive: weak, due to transient dipoles
Repulsive:Atoms do not penetrate each others →
spheres (VdW-radii)
0.5-1kcal/mol1Ǻ = 0.1nm
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2. Hydrogen Bonds
Hydrogen “shared” between two electronegative atoms
Important for 2nd struct.Interaction with water
1-3kcal/mol
NH
O Cd
Acceptor
Donor
From: the Molecular Biology of the Cell, 4th ed. 29
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3. Electrostatic Forces (Salt Bridges)
Coulomb’s law:
E = kqAqB/Dr
qA, qB : point chargesr: distance k=332 (for units of kcal/mol) D: dielectric constant
(water:80; protein: ~4)Solvent screening: D’ = DrAB
qBqA
r
3Ǻ: ~1.4 kcal/mol
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4. Aromatic Rings: and Cation- Interactions
Aromatic ring: cloud of electrons: negative charge
Can interact with • positive charge of Lys, Arg, His• edge of other aromatic ring
From: wikipedia.
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5. Water
• polar• cohesive
•competes with interactions in the protein (Hb, SB)• high D – reduces electrostatic forces
• hydrophobic effect• explicit modeling difficult
From: the Molecular Biology of the Cell, 4th ed. 32
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Accessible Surface Area
• Roll ball (radius = 1.4Ǻ; H2O) over molecule
• polar vs apolar: well solvated protein – polar atoms at surface, apolar atoms in the core
Van der Waals surface
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Non-Covalent Forces: Summary
• VdW: Many small contributions sum up to significant energy• Salt bridges and hydrogen bonds: polar interactions are reduced by competing water at the surface• Surface of protein: hydrophobic atoms should be all buried
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