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Chemical Biology 03 BLOOD. Biomolecular Structure Protein Structure 9/25/09. . Chemical Biology 03 BLOOD. Biomolecular Structure Protein Structure Lecture 8: 9/25/09. Hierarchy of Protein Structure. Primary structure is determined by sequence

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chemical biology 03 blood

Chemical Biology 03BLOOD

Biomolecular Structure

Protein Structure


chemical biology 03 blood1

Chemical Biology 03BLOOD

Biomolecular Structure

Protein Structure

Lecture 8: 9/25/09

hierarchy of protein structure
Hierarchy of Protein Structure
  • Primary structure is determined by sequence
  • Secondary structure is α-helix, β-sheets, and turns or loops
  • Tertiary structure is packing of secondary structural elements
  • Quaternary is assembly of monomers into dimers, tetramers, etc.

Fig 1.1 Branden & Tooz

protein structure helix
Protein Structure "-HELIX
  • Favorable H-Bondingexists between the O of one amino acid residue and the N-H of a different amino acid residue four amino acids away.
    • A H bond is created when the electronegative lone pair electrons on the carbonyl oxygen cause the amide H to shift slightly away from N and towards O and form a weak bond.
    • O- - - H-N bond distance is ~2.9 D
    • bond energy is ~ 10 kJ/mole or less (compared with 400kJ for a C-H bond).
  • Must remember that these atoms would be interacting with solvent if they weren’t involved in this internal H bonding.
protein structure helix1
Protein Structure "-HELIX

13 atoms between hydrogen bonds.


protein structure helix2
Protein Structure α-HELIX

* In this highly idealized image of myoglobin, all of the helices are represented as colored ribbons, which trace their backbone.

* For simplicity, all side groups are completely ignored.

* How many helices do you see?

See myoglobin file with Rasmol

protein structure helix4
Protein Structure α-HELIX
  • -R groups big and bulky side groups accommodated
    • charged amino acids project away from the cylinder
    • Ala, Glu, Leuand Metare good alpha helix formers
    • Pro, Gly, Tyrand Serare poor helix formers
  • Can see already from above observations that sequence determines structure.

R groups on the outside of helix:

protein structure strands and sheets
Protein Structure β strands and sheets
  • Anti parallel Beta Sheet:
    • Strands run in opposite N→C directions.
    • H bonds are made between different strands of the sheet, inter-strand H bonded network.
    • R groups:
      • alternate up and down the strand and across each strand,
      • are both up or both down across strands.
protein structure beta sheet
Protein Structure Beta Sheet
  • Beta sheet predicted (once again) by Pauling and found later experimentally in proteins.
    • Typical protein with extended beta sheet structure is fibroin, the major protein in silk which is a fabric produced by the silkworms of China and used for fabric since 2,000BC.

Here, silk worms munch on mulberry leaves.. YUM

protein structure silk
Protein Structure* Silk

Note the H bonding within sheet that is inter-strand, and spacing pattern.

Strand 1a

Sheet A

Strand 2a

Strand 1b

Sheet B

Strand 2b

protein structure silk1
Protein Structure* Silk

Note the R groups project above and below the sheets, and that the sheets aren’t perfectly flat, but are puckered in such a way that the knob of one R group fits into the upward pucker of sheet above.

Intersheet van der Waals interactions govern polymer properties

Sheet A

Sheet B

Identity of R groups determine tensile strengths, flexibilities, and price of different silks.

protein structure beta sheet1
Protein Structure Beta Sheet
  • Parallel β sheet
    • Adjacent strands run in the same direction (-N to -C terminal)
    • H bonding pattern is evenly spaced

Parallel Beta Sheet

protein structure molecular architecture
Protein Structure Molecular Architecture
  • Beta twists: a common structural motif in which a mostly parallel beta sheet naturally twists.

The twisted sheet is not purely parallel or antiparallel , but has a mixed orientation.

Note from the N-term.: strand/loop/helix/loop strand/loop/helix/loop repeat

Protein Thioredoxin

protein structure molecular architecture1
Protein StructureMolecular Architecture
  • Beta Barrels GFP, green fluorescent protein

This beta stranded barrel and tight loops create a shielded lantern to hold the fluorescing group (cyclized triad of amino acids shown below). If group were exposed to solvent, the light would be extinguished.

protein structure loops
Protein StructureLoops
  • Loops: Making the Connections
    • Loop regions connect regions of secondary structure, either helices or strands, or helices to strands. Previously thought to be unstructured random arrangements of amino acids, it is now realized that some regular features exist which can be described and in some cases rationalized.
    • They are usually at the surface of the molecule and therefore have polar charged or hydrophilic polar side groups. Glyis another favorite. Surprisingly, one can predict loop regions with more certainty than any other "structure". The carbonyl oxygen and amide proton hydrogen bond with the solvent.
protein structure loops1
Protein StructureLoops
  • Making Connections between α Helices
    • Alpha helices are often connected to each other by loop regions as seen in Mb.
    • The carbonyl and amide groups in the loops make hydrogen bonds with the solvent rather than internally.
    • Loop regions are often the sites of mutations, insertions, and deletions, since it seems you can perturb the loop and not affect the core structure.

Myoglobin again, see loops

protein structure loops2
Protein Structure Loops


Putting it together: What kinds of amino acid side groups would you expect to find on the surface of a protein like fibrinogen?