Physics A. V. Finkelstein & O. B. Ptitsyn LECTURE 11

One classification of 1.Fibrous Proteins - mostly structural proteins - maintain a rigid/flexible - an elongated, unidimensional structure - most of them are insoluble - high helix or beta sheet content - Ex: α-, collagen, , , …

2.Membrane Proteins - reside within the cellular & intracellular membrane - transport molecules in and out of the - Ex: membrane associated enzymes,

3.Globular Proteins - short chains packed into a compact 25-40Å globule - closely folded structure - water soluble proteins - Ex: egg albumin, hemoglobin, many enzymes

Other classifications: SCOP, PANTHER Fibrous proteins: (33%) contained in the silk made by the and other Fibroin (67%)

-strong, chemically inert, insoluble - β-structural protein - repeating octamer (Gly, Ala) in the primary structure, interrupted by regions containing bulkier residues

Packed β-sheets Fibrous proteins: α-keratin Found in , , , tail, -strong, chemically inert, insoluble -α-structural protein - repeating heptamer in the primary structure - similar structure in myosin,


Held together by Hydrophobic interactions Tropomyosin mid-region A. : Actin • of G-actin bind ATP and reversibly polymerize into a double stranded filament. •ATP is hydrolized to ADP. •Microfilaments have polarity. Fig. 5-2. G-actin bound to ATP (green). •G-actin adds more quickly to the (+) end than to the (–) end.

Fig. 5-4. F-actin assembly. Cleft is ATP binding site.

5 Fig. 5-6. Crawling cells. The leading edge of a crawling cell has densely packed microfilaments that are polymerizing, while the trailing Removal of capping proteins causes edge is composed growth at that end. of depolymerizing actin filaments.

Fig. 5-5. (actin) treadmilling. Filaments can also branch. Severing proteins bind to polymerized regions and cause dissociation. 6 B. Myosin associates with actin • Form most of the thick (myosin) and thin (actin) filaments of muscle.

Structure of striated muscle. Note region of thick and thin filament overlap. Connections between regions are called cross bridges.

7 See Fig. 5-7. Myosin.

See Fig. 5-8. Myosin head interacting with actin.

Fig. 5-10. Contraction of muscle. 8 Cross bridge cycling in muscle (requires ATP

Actin thin filament

Myosin thick filament

9 See Fig. 5-11. Myosin function during contraction.

10 E. Keratin is an • Intermediate filaments are exclusively structural (no motor proteins), but can cross-link microfilaments and . • α-keratin: intracellular component of hair, , shell, fingernails, . • Made of α-helices rich in hydrophobic residues. • of α-helix pair around. • Coiled coils are held together with bridges.

The structure of hair alpha-keratin (see Fig. 5-20 to 5-22). 11 Fig. 5-22. Coiled coil of tropomyosin (similar structure to keratin). Hydrophobic residues aligned to make contact between helices.

Fig. 5-20. Scanning electron micrograph of skin. The upper layer consists of dead epidermal cells that are mostly made of keratin.

12 Tropomyosin – structure determination Helix Packing

• the surface of the helix “consists” of grooves and ridges, like a screw thread

• 2 helices can be packed when a ridge from each fits into the other’s groove

• the two interface areas should have complementary surfaces Fibrous proteins: Collagen Structural protein of , skin, tendons and ligaments - strong, insoluble and chemically inert - superhelix composed of three helices (1000 residues) - main motif: a triad of residues Gly – Pro – (3/4/5) HyPro/Pro - Gly faces the center – essential for H-bonding - boiled, the is destroyed -> - in helices we have found intra-chain H-bonds, in collagen H-bonds are between different chains - mutations – Gly replaced by another residue, Differences between Collagen and keratin F. Collagen is an extracellular fibrous protei • Collagen: , , tendons and fibrous network of skin and blood vessels. • repeating units of gly-X-pro or gly-X-hydroxyproline.


+ - + - H2NCCOO Requires ascorbic acid H2NCCOO H C CH H C CH 2 2 Hydroxyproline formed 2 2 C CH H2 after collagen synthesis HO hydroxyproline

Q: Low ascorbic acid causes what disease? Q: Where do you get ascorbic acid?

17 • Proline prevents α-helix formation. • Forms a left-handed helix (3 residues/). • 3 left handed helices form a right handed triple helix

18 Collagen structure. See Figs. 5-24 to 5-27. Collagen – the next higher structural level

Collagen superhelices associate into collagen fibrils Fibrils are strengthened by intrachain Lys-Lys and interchain Hydroxy-pyridinium crosslinks “Fibrous proteins”: – matrix protein

Abundant in ligaments, lungs, skin, artery walls

- flexible, insoluble - 1/3 of Gly, 1/3 of Ala+Val, many Pro, but no HyPro - lacks regular secondary structure - arranged in relaxed coils - elastin chains are higly cross-linked into 3D network of fibers, in which other more structural proteins are immersed