生物化学：Chapter 5-1 Protein function, modulation and evolution

1、Chapter 5-1 Protein function, modulation and evolution (Fibrous proteins),Biochemistry Lecture (Sept. 27, 2012),Fibrous proteins play protective, connective and supportive roles,Highly elongated and filamentous; forming rod or wire -like regular repeating structures. Water-insoluble and structurally

2、 inert. Keratin, collagen, silk fibroin and elastin are typical examples. Used to construct as hair, horns, wool, nails, feathers, scales, or connective tissues, tendons, bone matrix and muscle fibers, or silk cloth and spider webs. A preponderance of amino acids with small, nonreactive side groups

3、is characteristic for structural proteins, for which H-bonded close packing is more important than chemical specificity (unlike globular proteins). Amino acid sequences favor a particular kind of secondary structure which, in turn, confer particular mechanical properties on the proteins.,“Structure

4、dictates function” is nicely illustrated in fibrous proteins,Fibrous proteins give strength and/or flexibility to the structure containing them. Rope-like protein filaments are often cross-linked with covalent bonds (through Cys or Lys residues).,(b-keratins),Exact molecular structure of fibrous pro

5、teins are difficult to determine Because they do not form crystals and insoluble, thus can be examined neither by X-ray crystallography, nor by NMR., Abundant in amino acids Of non-bulky side chains (e.g., Gly, Ala); Pro or its hydroxylated form is rich in collagen and elastin (preventing the format

6、ion of typical a-helices) Certain amino acids are lacking in certain fibrous proteins (e.g., Cys, Tryp, His,Met for elastin; Cys and tryp for collagen, Cys and Met for fibroin). Collagen is not a good nutrient (low percentage of essential amino acids),Amino acid compositions of four typical fibrous

7、proteins,Each has a Unique Amino acid Composition!,Fibrous proteins often contain repetitive sequences forming extended structures of strength and flexibility,Collagen (triple helix),a-keratin (coiled coils),Silk fibroin (b sheets),Elastin,(a form),Every other residue is a Gly (others are Ala armor

8、or exoskeleton of arthropod (in combination with chitin); Durable, insoluble, chemically unreactive, pliable.,b-keratin,a-keratins,X-ray diffraction was used to study the structure of hair and wool (a-keratin) (1930s, Astbury;1950s, Pauling and Crick),a-Keratin (unstretched hair),-keratin was propos

9、ed to form coiled coils (Crick, 1952),This was soon after structure of a-helix suggested in 1951 by Linus Pauling. a-keratin gave reflexion at 5.15 A, but normal a-helix at 5.4 A. A coiled coil model explain the data better than a straight a-helix.,Crick, FHC (1952) Is -Keratin a coiled coil?. Natur

10、e 170: 882883,a-keratin forms coiled coils,The central segment of each polypeptide chain has a 7-residue pseudorepeat, a-b-c-d-e-f-g (a heptad), with nonpolar residues predominating at positions a and d. The two keratin helices are inclined about 18 relative to one another, resulting in the coiled c

11、oil arrangement, allowing the contacting side chains to interdigitate (i.e., the knobs into holes packing).,Hydrophobic strips,Coiled coils are also commonly found in globular proteins (e.g., in leucine zipper),a-keratin coiled coils assemble further to form larger supramolecular structures,Terminal

12、 heads and tails important; Level of S-S- bonds related to hardness and springness; Assemble mechanism still poorly understood.,Hairs are curled (“permanent waving”) by reestablishing the disulfide bonds between the a-keratin peptide chain,1,1,2,2,3,3,4,4,5,5,6,6,1,1,2,2,3,3,4,4,5,6,5,6,Collagen pro

13、teins found in connective tissues in mammals,Found in tendon, ligament, skin, cornea, cartilage, bone, blood vessels, the gut, etc. provide tensile strength. Make up 1/3 of all protein of an animal body.,Collagen proteins found to possess unique amino acid composition and repeating sequences,A main

14、extracellular protein of the connective tissues of mammals (often produced by fibroblasts). Having about 300 repeated Gly-Pro-X or Gly-X-HyPro sequences (X being any other residue) with intra-chain H-boding not favored. Half of the collagen sequence contain amino acid residues other than Gly and Pro

15、! Such high glycine and regular repetitions are found in other fibrous proteins (e.g, silk fibroin, elastin) but never found in globular proteins.,X-ray reflection studies hinted a regular molecular structure for collagen,The molecular and packing structures of collagen have eluded scientists over d

16、ecades of research. The reflection pattern of collagen (tendon) indicate the impossibility of having irregular structures (1935).,X-ray diffraction pattern of DNA fibers.,X-ray diffraction pattern of collagen fiber (tendon),Meridian arc Equatorial reflection,Triple helix structure models proposed fo

17、r collagen structure (Pauling),Pauling, L. Two of three NH groups in each turn of a chain form H-bonds with an O of each of the other two chains; The third NH from a Pro residue points outward from the cylinder. The collagen triple helix is thus called the “Madras helix” (the a-helix the “California

18、 helix”; the DNA double helix the “British helix”),1922-2001,The triple helices can either be homo- or hetero-trimers,Forming a long, rod-like structure, stiff but flexible, ,1.5 nm wide and over 300 nm long, topped at both ends by globular domains (procollagen). Genetic disease (e.g., Osteogenesis

19、Imperfecta) caused by a single substitution of the Gly residue!,Every third residue lies near the center of the triple helix and can only by Gly.,Biosynthesis and assembly of collagen.,Extensive posttranslational modifications before secretion. Head domains removed after secretion and before assembl

20、y into fibrils.,The C-propeptide seems to mediate chain assembly.,The sugars play key roles in tissue organization,Vitamin C (Ascorbate) is needed for the prolyl hydroxylase to function, deficiency causes scurvy,It seems to reduce ferric iron ( Fe3+ ) to ferrous (Fe2+), as well as to reduce O2.,Scur

21、vy:,Reduced,Oxidized,Vitamin C,The hierarchical supramolecular arrangement within collagen fibers is far more elusive,None of the models so far advanced is universally accepted. Major issue: Correlate the striated pattern with a molecular staggering pattern.,One model of molecular packing in collage

22、n fibrils,Triple-helices arranged in hexagonal or quasihexagonal array in cross-section.,Collagen triple helices are cross-linked via the Lys residues,Catalyzed by enzymes.,Fibroins,Produced by insects and spiders; tough but flexible. b-pleated sheets of antiparallel strands. The side chain R groups

23、 in silk are not very bulky The b sheets stack to form a microcrystalline array in which layers of contacting Gly side chains from neighboring sheets alternate with layers of contacting Ser and Ala side chains,Cocoon,Very flexible but relatively inextensible!,X-ray diffraction and Electron microscop

24、y were applied to study the structure of silk fibers,Suggesting a sheet-like structure.,X-ray photograph of powder silk (1954）,Electron micrograph of silk (1967),Spider web,Spiders silk fiber formation: spidroin protein adopts different secondary structure at different locations,The a-helice structu

25、re prevents them from assembly into b-sheet structures before they are secreted.,Elastin endow connective tissues with resilience,Permitting long-range deformability and passive recoil without energy input. Critical to the function of arteries, which undergo repeated cycles of extension and recoil,

26、and to lungs, skin and all other dynamic connective tissues.,aorta,Lys,Lys,Lys,Lys,Hydrophilic cross-linked domains (rich in Lys and Ala) Instrinsically disordered Hydrophobic domains (rich in Val, Pro, and Gly; dynamic conforamtion),An “oiled coil” model was proposed to explain elastins elastic behavior,Gray, W. R., Sandberg, L. B. form a network of 3D spring.,Highlights of the structure of elastin,(1) Occurrence of alternating regions rich in alanine and lysine (cross-link regions) and in glycine, valine and proline (hydrophobic regions). (2) Glycine, alanine, valine and proline com