Perhaps the most important structural bioinformatics result ever published… Chothia, C. & Lesk, A. M. (1986). The relation between the divergence of sequence and structure in proteins. EMBO J., 5(4):823-826. Starting now, the relationships identified by this simple graph will impact everything we do throughout the remainder of the class.
primary structure (set of covalent bonds within the structure) secondary structure (helices, strands, coils/loops) tertiary structure (3D packing of secondary structures) quaternary structure (spatial arrangements of multiple chains) LIRLFKSHPETLEKFDRFKHL… Levels of Protein Structures
Globular protein Fibrous protein Membrane protein The three most common classes of proteins
Formation of a Peptide Bond by Condensation Amino Acid 1 Amino Acid 2 Peptide bond Note: this chemistry will not work as drawn! Peptide bond is the amide linkage that is formed between two amino acids, which results in (net) release of a molecule of water (H 2 O). The four atoms in the yellow box form a rigid planar unit and, as we will see next, there is no rotation around the C-N bond.
The primary structure is the set of all covalent bonds within the protein, which is approximated by the sequence (-CSS). Note: Primary structure or sequence, but not primary sequence.
Peptides Q: why is the pentapeptide SGYAL different than LAYGS?
A Closer Look at the Peptide Bond The carbonyl group has a partial negative charge and the amide nitrogen has a partial positive charge, which set up a small electric dipole. The peptide C-N bond has a partial double-bond character (partial sharing of two pairs of electrons between O and N. The peptide bond is planar and rigid, cannot rotate freely (see next slide). Resonance: delocalization of bonding electrons over more than one chemical bond. Lehninger Principles of Biochemistry
A Closer Look at the Peptide Bond But, bonds N-C and C -C can rotate, which can be described by two torsion angles: (phi) and (psi). (phi): C-N-C -C (psi): N-C -C-N (omega): C -C-N- C In principle, and can have any value between -180 O and +180 O. But due to steric interference……
A Closer Look at the Peptide Bond Bond lengths : C -C C-N N-C The peptide bond is normally in the trans configuration (~99.6% of the time) = 180 O One exception is for proline. The fraction of X-Pro peptide bonds in the cis isomer under unstrained conditions ranges from 10-40%. The fraction depends slightly on the preceding amino acid X. Note the differences between C-N and N-C
An (a.) unstable vs. (b.) the most stable Ala-Ala dipeptide conformation (a.)(b.)
Rotation of phiRotation of psi The rotatable backbone torsion angles
The Ramachandran plot With a molecular modeling kit, prove to yourself that (0,0) is an unallowable due to a steric clash.
Liu et al. Nature Chemical Biology. 3, 619 - 629 (2007) Prolyl cis-trans isomerization as a molecular switch “The local environment of proline within a protein can influence the relative free energies of the cis and trans isomeric states, leading to wide variations in the ratio of cis:trans populations in different proteins. Although most structures require proline to adopt one or the other isomer in the context of native protein folds, several recent structures show the presence of both populations for specific proline residues. ”
--The primary structure is a complete description of the covalent bond network within a protein. --This is almost(!) completely described by the sequence of amino acids. --If you know that the protein is AVG…, you can look up the structures of A, V and G, plus what you know about peptide bonding allows you to complete the covalent bond structure. --So, when does the primary structure not fully describe the covalent bond network? Final thoughts on primary structure
Protein Secondary Structure: helices =-49 O, =-26 O Hydrogen bond pattern: C=O (i) and N-H (i+3) Residues per turn: 3 =-57 O, =-70 O Hydrogen bond pattern: C=O (i) and N-H (i+5) Residue per turn: 4.4 Other helical conformations 3 10 helix, helix
Image from “Protein Structure and Function” by Gregory A Petsko and Dagmar Ringe Amphipathic helix hydrophobic Hydrophilic protein packing and function
-sheet (Pleated Sheet) 7Å7Å 6.5Å = -139 O, = +135 O = -119 O, = +113 O Anti-parallel -sheet Parallel -sheet Lehninger Principles of Biochemistry
Amino acid propensity Used in the first generation of secondary structure prediction methods, e.g. Chou-Fasman
-turns the distance between the C atom of residue i and the C atom of residue i+3 is less than 7Å the central two residues are not helical on the basis of the phi, psi angles of residues i+1 and i+2
-turns Lehninger Principles of Biochemistry Turn propensities (Ft/Fb)
Ramachandran Plot Red: allowed regions Yellow: additionally allowed regions White: disallowed regions
Relating the Ramachandran plot to secondary structures
PROLINEGLYCINE THE 18 STANDARD AMINO ACIDS Not all Ramachandran plots are created equal.
Tertiary structure = the 3D shape of a single protein chain, which is stabilized by a large number of noncovalent interactions.
Ruminations on protein stability... Protein stability is a small difference of large numbers. Proteins are stable ( G < 0) only over a narrow environmental range. In fact, there are forces pushing the equilibrium between folded and unfolded in both directions. Stabilizing forces: Intraprotein salt bridges, hydrogen bonds, dipole-dipole interactions and VDW interactions (all of which are electrostatic in nature). Destabilizing forces: Primarily electrostatic interactions with solvent and conformational entropy reduction.