Three-Dimensional Structure of Proteins

Rotation around the -Carbon in a Polypeptide

A Sterically Nonallowed Conformation

The -Helix and -Pleated Sheet

-Helix:•3.6 residues/turn•Rise = 0.15 nm/residue•13-atom hydrogen-bonded loop

-Pleated Sheet:•anti-parallel or parallel•2.0 residues/”turn”•0.34 nm/residue (anti-parallel) or 0.32 nm/residue (parallel)

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Conformationally and allowable structures where backbone is optimally hydrogen-bonded (linear H-bonds)

Linus Pauling and Robert Corey, 1950

Linus Pauling and Robert Corey, 1951

Helices have electric dipoles.

Antiparallel and Parallel Pleated Sheets

Other Possible Secondary Structures

310 Helix:•3 residues/turn•0.20 nm/residue•10-atom hydrogen-bonded loop

Helix:•4.4 residues/turn•0.12 nm/residue•16-atom hydrogen-bonded loop

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Hydrogen Bonding Patterns for Different Helices

Ramachandran Plot

G.N. Ramachandran, 1963

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Fibrous Proteins

Proteins with an elongated or filamentous form, often dominated by a single type of

secondary structure over a large distance.

Most fibrous proteins are associated with connective tissue and help provide mechanical strength to the tissue.

Structure of Keratin and Keratin-Type Intermediate Filaments

Adjacent polypeptide chains also crosslinked by disulfide bonds.

Disulfide bond patterns between are what determine whether human hair is straight or curly.

Keratin is a principal component of hair, horn, nails and feathers.

Coiled-Coil -Helical Dimer of Keratin

Amphipathic helices: Residues a, d, a’ and d’ hydrophobic, other residues more hydrophilic

Structure of Silk Fibroin

Silk made by silkworms and spiders.

Composed of microcrystalline array of antiparallel pleated sheets where each strand has alternating Gly and Ala or Ser residues.

Structure of Collagen Fibers

•3 intertwined left-handed helices•3.3 residues/turn•Repeating Gly-X-Y (X often Pro, Y often Pro or hydroxyPro)

Collagen is the most abundant vertebrate protein and the major stress-bearing component of connective tissue (bone, teeth, cartilage, tendon) and fibrous matrix of skin and blood vessels.

The Collagen Triple Helix (Tropocollagen)

Tropocollagen with Gly Ala substitution (yellow)Interactions between strands

G.N. Ramachandran, 1955

Post-Translational Modifications in Collagen

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Collagen contains unusual oxidized and crosslinked lysine residues.

Lysyl oxidase is the enzyme that oxidizes lysine residues to the aldehyde allysine, which then forms the crosslinks.

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Hydroxyproline is also found in collagen. (Some lysine residues also hydroxylated.) The enzyme required for hydroxylation of proline residues is prolyl hydroxylase, a vitamin C-dependent enzyme.

Scurvy is caused by reduced hydroxyproline in collagen as a result of vitamin C deficiency.