biochemistry生物化学西文chapter5-1protein function,modulation and evolution fibrous proteins

1、Chapter 5 Protein function, modulation and evolution (Fibrous proteins)By Prof. Zengyi Chang （昌增益教授）Biochemistry Lecture (Sept. 25, 2012)Fibrous proteins play protective, connective and supportive roles Highly elongated, single type of secondary structure, forming rod or wire -like shapes. Water-ins

2、oluble and structurally inert. Keratin, silk fibroin and collagen are the three best characterized types.Used to construct connective tissues, tendons, bone matrix and muscle fiberA preponderance of amino acids with small, nonreactive side groups is characteristic for structural proteins, for which

3、H-bonded close packing is more important than chemical specificity. “Structure dictates function” is nicely illustrated in fibrous proteinsFibrous proteins give strength and/or flexibility to the structure containing them.Rope-like filaments often cross-linked with covalent bonds.Amino acid composit

4、ions of four different fibrous proteins Abundant in amino acids Of non-bulky side chains;Pro is rich incollagen and elastinFibrous proteins contain repetitive sequences forming extended structures of strength and flexibilityCollagena-keratinSilk fibroinElastinKeratins present as coiled coilsMain con

5、stituent of tough and insoluble non-mineral structures that grow from the skin like hair (wool), horns, claws, nails and hooves in mammals (a-keratins), shells, scales and claws in reptiles, beaks and feathers in birds(b-keratins); armor or exoskeleton of Arthropod (in combination with chitin); Dura

6、ble, insoluble, chemically unreactive, pliable.X-ray photograph of hair (keratin) and silk (fibroin) (1934, Astbury)a-Keratin(unstretched hair)b-Keratin(stretched hair)Stretched human hairFibroin(silk)-keratin was proposed to form coiled coils (Crick, 1952) This was soon after structure of a-helix s

7、uggested 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?. Nature 170: 882883 a-keratin forms coiled coilsThe central segment of each polypeptid

8、e chain has a 7-residue pseudorepeat, a-b-c-d-e-f-g, 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 coil arrangement, allowing the contacting side chains to interdigitate.Hydrophobicstripsa-kera

9、tin coiled coils assemble further to form larger supramolecular structuresTerminal heads and tails important; Level of S-S- bonds related to hardness and springness; Assembling mechanism poorly understood.Hairs are curled (“permanent waving”) by reestablishing the disulfide bonds between the a-kerat

10、in peptide chain Collagen proteins found in connective tissues in mammalsFound in tendon, ligament, skin, cornea, cartilage, bone, blood vessels, the gut, etc. provide tensile strength.Make up 1/3 of all protein of the animal body. Collagen proteins found to possess unique amino acid composition and

11、 repeating sequencesA main extracellular protein of the connective tissues of mammals.Having about 300 repeated (Gly-X-Y) sequences, with X and Y frequently being Pro or hyPro (intra-chain H-boding is not favored).(Half of the collagen sequence is not Gly, Pro, or hyPro!)Such high glycine and regula

12、r repetitions are found in a few other fibrous proteins (e.g, silk fibroin, elastin) but never found in globular proteins. X-ray reflection studies hinted a regular molecular structure for collagenThe molecular and packing structures of collagen have eluded scientists over decades of research. The r

13、eflection pattern of collagen (tendon) indicate the impossibility of having irregular structures (1935).Triple helix structure models proposed for collagen structure (Pauling)Pauling, L. & Corey, R. B. (1951) The Structure of Fibrous Proteins of the Collagen-Gelatin Group. PNAS, 37:272-281.BUT alter

14、nate of cis- and trans- peptide bonds incorrectly proposed!1901-1994Triple helix structure models proposed for collagen structure (Ramachandran)Ramachandran, G.N. and Kartha, G. 1954. Structure of collagen. Nature 174:269270.All in trans-configuration.Two of the NH groups in each turn of a chain for

15、m H-bonds with the 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 helix”; the DNA double helix the “British helix”)1922-2001 The triple helices can either b

16、e homo- or hetero-trimersForming 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 Imperfecta) caused by a single substitution of the Gly residue!Only Gly allowed in the center!

17、Pro, hyPro and other residues on the surfaceBiosynthesis and assembly of collagen. Extensive posttranslational modifications before secretion.Head domains removed after secretion and before assembly into fibrils.The C-propeptideseems to mediatechain assembly.The intracellular and extracellular event

18、s in the formation of a collagen fibrilVitamin C (Ascorbate) is needed for the prolyl hydroxylase to function, deficiency causes scurvyIt seems to reduce ferric iron ( Fe3+ ) to ferrous (Fe2+), as well as to reduce O2. Scurvy:ReducedOxidizedThe hierarchical supramolecular arrangement within collagen

19、 fibres 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 collagen fibrilsTriple-helices arranged in hexagonal or quasihexagonal array in cross-section. Coll

20、agen triple helices are cross-linked via the Lys residuesCatalyzed by enzymes.FibroinsProduced by insects and spiders; tough but flexible.b-pleated sheets of antiparallel strands.The side chain R groups in silk are not very bulky The b sheets stack to form a microcrystalline array in which layers of

21、 contacting Gly side chains from neighboring sheets alternate with layers of contacting Ser and Ala side chainsCocoonX-ray diffraction and Electron microscopy were applied to study the structure of silk fibers Suggesting a sheet-like structure.X-ray photograph of powder silk (1954）Electron micrograp

22、h of silk (1967)Spider webSpiders silk fiber formation: spidroin protein adopts different secondary structure at different locationsThe a-helice structure prevents them from assembly into b-sheet structures before they are secreted.Elastin endow connective tissues with resiliencePermitting long-rang

23、e deformability and passive recoil without energy input.Critical to the function of arteries, which undergo repeated cycles of extension and recoil, and to lungs, skin and all other dynamic connective tissues.aorta An “oiled coil” model was proposed to explain elastins elastic behaviorGray, W. R., Sandberg, L. B.& Foster, J A.(1973) Molecular Model for Elastin Structure and Function Nature 246, 461-466.Mainly based on striking sequence features.Each monomer is fibrillar, of alternating segments of cross-linked regions and “oiled