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									   Protein Structure


Nimrod Rubinstein
Bioinformatics Seminar
Protein Synthesis
       1.   Attachment of correct
            amino acids (AAs) to their
            corresponding tRNAs.
       2.   Initiation: forming the
            initiation complex.
       3.   Elongation: sequentially
            forming peptide bonds.
       4.   Termination: synthesis is
            terminated and the
            polypeptide is released.
         From Sequence to Structure
    Structure Hierarchies:
       Primary structure: the sequence of AAs covalently
       bound along the backbone of the polypeptide chain.

                       Ala                  Gly                  Cys

                                O
                                                Cα
                   N            ψ   C                ψ                        O
                                        N   ф                N   ф        C
                       ф   Cα                            C
                                                                     Cα   ψ
                                                     O




-1800 ≤ ф ≤ 1800
-1800 ≤ ψ ≤ 1800
          From Sequence to Structure
   Structure Hierarchies:
          Secondary structure: local conformation of some
          part of the polypeptide.

α Helix                                     β Sheet




                            Anti Parallel        Parallel
   From Sequence to Structure
Structure Hierarchies:
 Tertiary structure: the overall
 3-dimensional arrangement of all the
 atoms in the protein.
   From Sequence to Structure
Structure Hierarchies:
 Quaternary structure: some proteins contain two or
 more separate polypeptide chains, which may be
 identical or different.
        Globular                       Fibrous
   From Sequence to Structure
Additional Parameters:
 Surface accessibility:   The surface area of the molecule that is
                          exposed to the solvent, derived from
                          the complete structure.
                          •VDW surface: the surface area of an
                          atom.
                          •Connolly surface: the interface between
                          the molecule and the solvent sphere
                          (conventionally with r = 1.4Å) .

                          •Solvent accessible surface: the path of
                          the center of the solvent sphere rolled ove
                          the VDW surface.
                          •Relative accessibility = (SAS)/(maxSAS)
                                •maxSAS = SAS(Gly-X-Gly)
   From Sequence to Structure
Additional Parameters:
 Coordination number:
                    •The number of structure stabilizing
                    contacts each residue in the structure
                    makes.
                    •Computation: encapsulating an AA with
                    a sphere, centered at the residue’s
                    center of mass, and counting the
                    number of residues falling inside this
                    sphere.
                    •Usually done with different cutoff radii.
   From Sequence to Structure
Protein Folding:
 The Levinthal paradox: [Levinthal C.; J. Chym. Phys. (1968)]
 Assume a protein is comprised of 100 AAs. Assume each
 AA’s backbone can take up 10 different conformations,
 defined by ф and ψ values. Altogether we get:
 10100 conformations.
 If each conformation were sampled in the shortest
 possible time (time of a molecular vibration ~ 10-13 s) it
 would take an astronomical amount of time
 (~1077 years) to sample all possible conformations, in
 order to find the Native State.
              NPC even in the 2D case
Luckily, nature works out with these sorts of numbers and the
correct conformation of a protein is reached within seconds.
   From Sequence to Structure
Folding Models:
 The Backbone-Centric view:

                          •Sequence order dependent
                          interactions (фψ - propensities and H-
                          bonds), produce local secondary
                          structure elements (SSEs).
                          •Local SSEs later overgo longer-
                          range interactions to form
                          supersecondary structures.
                          •Supersecondary structures of
                          ever-increasing complexity thus
                          grow, ultimately into the native
                          conformation.
   From Sequence to Structure
Folding Models:
 The Sidechain-Centric view:

                             •Hydrophobic sidechain interactions are
                             the strongest for AAs in a water solution.
                             •A few key hydrophobic residues are
                             responsible for a “hydrophobic collapse”
                             to the “molten globule” state.
                             •The “molten globule” might not include
            Molten globule
                             SSEs, yet about this structure the
                states
                             remainder of the polypeptide chain
                             condenses.
                             •The conformation space is viewed as
                             “funnel shaped”.
   From Sequence to Structure
Folding Models:
 The Sidechain-Centric view - Larger proteins:
                         •Intermediate states exist, which are highly
                         populated.
                         •These states may assist in finding the
                         Native Structure or may serve as traps that
                         inhibit the folding process.
                         •Structurally aligning intermediate states
                         against the SCOP found the corresponding
                         Native Structures to have the highest
                         scores.
                         •But, many features were missing:
                                • Well defined SSEs.
                                • A well formed hydrophobic core.
                                • High RMSDs (7-10Å).
                         [Dobson C. M.; TRENDS in Biochemical Sciences; Jan 2005]
              From Sequence to Structure
    Folding Models:
Post-translational                                       Vs.                Co-translational
                                    Anfinsen’s experiments:
                                    •Exposure of a purified
                                    RNase-A enzyme to a
                                    concentrated urea solution
                                    in the presence of a
                                    reducing agent denaturizes
                                    the folded conformation
                                    resulting in a complete loss
                                    of catalytic activity.            •Denaturation-Renaturation
                                    •Removal of the urea and          experiments are biased.
                                    reducing agent causes the         •An AA is added to the
                                    enzyme to accurately refold       polypeptide chain in: 10-2 s.
                                    to its native structure and
                                    restore its catalytic activity.   •The rate at which an SSE is
                                                                      formed is: 10-7 – 10-4 s.
[Anfinsen C. et al.; PNAS (1961)]
        Determining the Structure
Crystallization:
•   Assembling a solution of protein molecules
    into a periodic lattice.

X-Ray Diffraction:
•   The crystal is bombarded with X-ray beams.
•   The collision of the beams with the electrons
    creates a diffraction pattern.
•   The diffraction pattern is transformed into an
    electron density map of the protein from which
    the 3D locations of the atoms can be deduced.


             F                                       F
        Determining the Structure
Nucleotide Magnetic
 Resonance:
•   A solution of the protein is placed in a
    magnetic field.
•   spins align parallel or anti-parallel to the
    field.
•   RF pulses of electromagnetic energy
    shifts spins from their alignment.
•   Upon radiation termination spins
    re-align while emitting the energy they
    absorbed.
•   The emission spectrum contains
    information about the identity of the
    nuclei and their immediate
    environment.
•   The result is an ensemble of models
    rather than a single structure.
                      Structure Similarity
     Protein Families:
     •   Structures seem to be preserved much more than sequences,
         which is easily explainable due to neutral mutations.
1BRU:
Pancreatic Elastase
(Sus scorfa).                                                     Global Alignment:
                                                                  39% identity
1CHG:
Chymotrysinogen
(Bos taurus).


                                                             Rigid Cα Alignment:
                                                             RMSD 1.26Å
                                                           1CHG
   1CHG                       1BRU                         1BRU
                Structure Similarity
Protein Families:
•   Structures seems to be preserved much more than sequences,
    which is easily explainable due to neutral mutations.
•   Structural Biologists claim that there are a limited number of
    ways in which protein domains fold. There may be as few as
    ~2000 different folds (differing by their backbone topology).
•   Nearly a 1000 different folds have already been resolved.




                           http://scop.mrc-lmb.cam.ac.uk/scop/
                    Structure Prediction
Homology (Comparative) Modeling:

Guideline: At least 30% sequence identity is needed between
       probe and template.

     1.    Template Assignment: creating a robust probe-
           template alignment (PWA/MSA).
     2.    Model Construction:
          a.    Generation of coordinates for conserved segments:
                superimposing/averaging/restrain based.
          b.    Generation of coordinates for variable segments:
                DB scanning/Ab Initio/restrain based.
          c.    Generation of coordinates for sidechain atoms:
                superimposing/rotamer libraries/restrain based.
     3.    Model Evaluation:
          a.    Assessment of to the ability to functionally identify
                the active site of the model.
          b.    Assessment of physico-chemical or structural
                environment based on statistical analyses of DBs
                for characteristics such as:
                    Intramolecular packing.
                    Bond geometry.
                    Solvent accessibility.
                                                                                         bFGF
                                                                        [Peitsch et al. (1999)]
                   Structure Prediction
Threading (Sequence-Structure Alignment):
  Identifying evolutionary unrelated proteins that have converged to similar
  folds.
         • Scoring Scheme: describes the propensity of each AA for its structural/physico-
             chemical environment: SS type, solvent accessibility, coordination number, etc…
          • Profile construction: encoding the template’s AAs structural features to a 1D profile
             and predicting such a profile for the probe.
          • Threading Algorithm: Aligning the 1D profiles of the template and the probe using
             DP and the defined scoring scheme.
                                      template                  probe




                                                                               [Bryant, Lawrence; Proteins (1993)]


 But:   No adjustments to the template profile can be made thus substantial rearrangements are ignored
                   Structure Prediction
Ab Initio Techniques:
Simulating the folding process

Simplifying the energy landscape:
•  Reducing the number of degrees of freedom:
     •   Representing a group of atoms by a single atom.
     •   Reducing the number of atom interactions.
•   Sampling the conformation space:
     •   Monte Carlo sampling.
     •   Genetic Algorithm.
     •   Simulated Annealing.
•   Hierarchical folding simulation.
                       Blind Prediction
Critical Assessment of Protein Structure Prediction – CASP




   Goal: “ to obtain an in-depth and objective assessment of our current abilities and
    inabilities in the area of protein structure prediction”.
   Groups use their tools to model proteins with pre-published structures.
   The predictions are thus evaluated against the subsequently determined structures.
   CASP6 (2004) shows limited improvements compared to CASP5 (2003).

								
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