Transcript Document
Principles of Protein Structure
primary structure
ACD EFGH I K LM NPQ RST VW Y
Different Levels of Protein Structure
NH 2 Lysine Histidine Valine Arginine Alanine COOH
Common Secondary Structure Elements
• The Alpha Helix
Properties of alpha helix
• • • • • • •
3.6 residues per turn, 13 atoms between H-bond donor and acceptor
approx. 60º;
approx. 40º
H- bond between C=O of i th residue & -NH of (i+4) th residue First -NH and last C=O groups at the ends of helices do not participate in H bond Ends of helices are polar, and almost always at surfaces of proteins Always right- handed Macro- dipole
Alpha Helix
Helical wheel
Residues i, i+4, i+7 occur on one face of helices, and hence show definite pattern of hydrophobicity/ hydrophilicity
Association of
helices: coiled coils Introduction to Molecular Biophysics
These coiled coils have a
heptad repeat
abcdefg with nonpolar residues at position a and d and an electrostatic interaction between residues e and g.
Isolated alpha helices are unstable in solution but are very stable in coiled coil structures because of the interactions between them The chains in a coiled-coil have the polypeptide chains aligned parallel and in exact axial register. This maximizes coil formation between chains.
The coiled coil is a protein motif that is often used to control oligomerization.
They involve a number of alpha-helices wound around each other in a highly organised manner, similar to the strands of a rope.
Initially identified as a structural motif in proteins involved in eukaryotic transcription. (Landschultz et al., Science 240: 1759-1763 (1988).
Originally identified in the liver transcription factor C/EBP which has a Leu at every seventh position in a 28 residue segment.
Association of
helices: coiled coils
The helices do not have to run in the same direction for this type of interaction to occur, although parallel conformation is more common.
Antiparallel conformation is very rare in trimers and unknown in pentamers, but more common in intramolecular dimers, where the two helices are often connected by a short loop.
Chan et al., Cell 89, Pages 263-273.
Basis for the helical dipole In an alpha helix all of the peptide dipoles are oriented along the same direction.
Consequently, the alpha helix has a net dipole moment.
Since the dipole moment of a peptide bond is 3.5 Debye units, the alpha helix has a net macrodipole of: n X 3.5 Debye units (where n= number of residues) This is equivalent to 0.5 – 0.7 unit charge at the end of the helix. The amino terminus of an alpha helix is positive and the carboxy terminus is negative.
Structure of human TIM Two helix dipoles are seen to play important roles: 1.
2.
Stabilization of inhibitor 2-PG Modulation of pKa of active site His-95.
Helical Propensities
Ala Arg Lys Leu Met Trp Phe Ser Gln Glu Cys Ile Tyr Asp Val Thr Asn His Gly Pro -0.77
-0.68
-0.65
-0.62
-0.50
-0.45
-0.41
-0.35
-0.33
-0.27
-0.23
-0.23
-0.17
-0.15
-0.14
-0.11
-0.07
-0.06
0 ~3
Common Secondary Structure Elements
• The Beta Sheet
Secondary structure: reverse turns
Secondary Structure: Phi & Psi Angles Defined
• Rotational constraints emerge from interactions with bulky groups (ie. side chains).
• Phi & Psi angles define the secondary structure adopted by a protein.
The dihedral angles at C
atom of every residue provide polypeptides requisite conformational diversity, whereby the polypeptide chain can fold into a globular shape
Ramachandran Plot
Secondary Structure
Table 10
Phi & Psi angles for Regular Secondary Structure Conformations Structure Antiparallel b -sheet Parallel b -Sheet Right-handed -helix 3 10 helix p helix Polyproline I Polyproline II Polyglycine II Phi ( F ) -139 -119 +64 -49 -57 -83 -78 -80 Psi( Y ) +135 +113 +40 -26 -70 +158 +149 +150
Beyond Secondary Structure
Supersecondary structure (motifs) : small, discrete, commonly observed aggregates of secondary structures b sheet helix-loop-helix bb Domains : independent units of structure b barrel four-helix bundle *Domains and motifs sometimes interchanged*
Common motifs
Left handed
Supersecondary structure: Crossovers in b b -motifs
Right handed
EF Hand
• Consists of two perpendicular 10 to 12 residue alpha helices with a 12-residue loop region between • Form a single calcium-binding site (helix-loop-helix). • Calcium ions interact with residues contained within the loop region. • Each of the 12 residues in the loop region is important for calcium coordination. • In most EF-hand proteins the residue at position 12 is a glutamate. The glutamate contributes both its side-chain oxygens for calcium coordination. Calmodulin, recoverin : Regulatory proteins Calbindin, parvalbumin: Structural proteins
EF Fold
Found in Calcium binding proteins such as Calmodulin
Helix Turn Helix Motif
•Consists of two helices and a short extended amino acid chain between them.
•Carboxyl-terminal helix fits into the major groove of DNA.
•This motif is found in DNA-binding proteins, including l repressor, tryptophan repressor, catabolite activator protein (CAP)
Leucine Zipper
Rossman Fold
•The beta-alpha-beta-alpha-beta subunit •Often present in nucleotide-binding proteins
What is a Protein Fold?
Compact, globular folding arrangement of the polypeptide chain Chain folds to optimise packing of the hydrophobic residues in the interior core of the protein
Common folds
Tertiary structure examples: All-
Alamethicin The lone helix Rop helix-turn-helix Cytochrome C four-helix bundle
Tertiary structure examples: All-
b b sandwich b barrel
Tertiary structure examples:
/b placental ribonuclease inhibitor /b horseshoe triose phosphate isomerase /b barrel
Four helix bundle
•24 amino acid peptide with a hydrophobic surface •Assembles into 4 helix bundle through hydrophobic regions •Maintains solubility of membrane proteins
Oligonucleotide Binding (OB) fold
TIM Barrel
•The eight-stranded / b barrel (TIM barrel) •The most common tertiary fold observed in high resolution protein crystal structures •10% of all known enzymes have this domain
Zinc Finger Motif
Domains are independently folding structural units. Often, but not necessarily, they are contiguous on the peptide chain. Often domain boundaries are also intron boundaries.
Domain swapping: Parts of a peptide chain can reach into neighboring structural elements: helices/strands in other domains or whole domains in other subunits.
Domain swapped diphteria toxin:
Transmembrane Motifs
• Helix bundles Long stretches of apolar amino acids Fold into transmembrane alpha-helices “Positive-inside rule”
Cell surface receptors Ion channels Active and passive transporters
• Beta-barrel Anti-parallel sheets rolled into cylinder
Outer membrane of Gram-negative bacteria Porins (passive, selective diffusion)
Quaternary Structure
• Refers to the organization of subunits in a protein with multiple subunits • Subunits may be identical or different • Subunits have a defined stoichiometry and arrangement • Subunits held together by weak, noncovalent interactions (hydrophobic, electrostatic) • Associate to form dimers, trimers, tetramers etc. (oligomer) • Typical K d for two subunits: 10 -8 to 10 -16 M (tight association) –Entropy loss due to association - unfavorable –Entropy gain due to burying of hydrophobic groups - very favourable
Structural and functional advantages of quaternary structure
• Stability: reduction of surface to volume ratio • Genetic economy and efficiency • Bringing catalytic sites together • Cooperativity (allostery)