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Flashcards in protein structure and function Deck (43):
1

Examples of Silk Proteins

1. Collagen
2. Elastin
3. Keratin
4. Silk Fibroin

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▪ insoluble, extracellular (found outside the cell)


Collagen

3

▪ very strong due to aldol crosslinks (covalent bond)
▪ found in all animals

Collagen

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▪ most abundant in the human body

Collagen

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▪ essential in all connective tissues e.g. cartilage, bone, tendons, ligaments, and skin

Collagen

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▪ triple helical structure (tropocollagen) stabilized by

aldol crosslinks

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▪primary structure of collagen

repeating tripeptide (Gly-X-Y) where X is usually Pro and Y is usually hydroxyproline

8

they also exhibit H-bonding because of hydroxyproline

Collagen

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they are not α-helical; just helical

Collagen

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– excessive collagen synthesis

Fibrosis

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▪ overproduction of type 1 collagen in lungs reduces the lung’s ability to expand for air intake and interferes with normal breathing; scarring of lungs

Pulmonary fibrosis

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▪ excessive deposition in liver leads to scar tissue formation due to overconsumption of alcohol; may lead to cirrhosis

Liver fibrosis

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caused by insufficient collagen production where skin and tendons are weak and easily stretched producing loose joints and hyperextensive limbs

Ehler’s-Danlos (Rubberman syndrome)

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▪ type 1 procollagen molecules fail partially or completely to assemble into triple helices and are therefore degraded.
▪ reduced amount of collagen leads to fragile bones; easily fractured

Osteogenesis Imperfecta (Brittle bone syndrome)

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▪ found predominantly in walls of arteries, lungs, intestines, skins, etc.

Elastin

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▪ structural proteins that gives elasticity to the body’s tissues and organs

Elastin

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MW of elastin

72 kDA

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AAs in elastin

Small nonpolar
A,V, L,G

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▪WHY hydrophobic elastin molecules “slide and stretch” over one another

to maintain structural integrity and provide recoil

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▪ elastin protein molecules are crosslinked by WHAT TYPE OF BONDS

covalent bonds

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tough, fibrous, insoluble protein that makes up the skin, hair and nails

KERATIN

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also found in claws, hooves, feathers and horns

KERATIN

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human hair is __% cysteine;

14

24

the more disulfide bridges present, the _ curly

more

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Soft Keratin –____disulfide bonds
Hard Keratin – ____disulfide bonds

soft - less
hard-more

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-most abundant protein in vertebrates
-has 3 polypeptide chains wrapped around each other in a ropelike twist or triple helix

collagen

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collagen fibers are both intra and intermolecularly linked by covalent bonds formed by reactions of ______ residues

lysine and histidine

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side chains of collagen are on the __ of the triple-helical molecule

surface

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in collagen, the AA which facilitates formation of the helical conformation of each alpha chain because it causes kinks

Proline

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in collagen, the AA which fits into the restricted spaces where the 3 chains come together

Glycine

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in collagen, the AA important in stabilizing the triple-helical structure because it maximizes interchain H-bond formation

Hydroxyproline

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*In collagen, Every 3rd position must be occupied by _

glycine.

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300 nm, 1.5 nm in diameter
-held together by H-bonds (involving Hyp/P)
-MW: 300, 000
-each strand has 800 AA residues

tropocollagen

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-precursor: tropoelastin
-linear polypeptide with about 700 AA, primarily small and nonpolar (A, V, L, G)
-also rich in protein and lysine, but contains only little hydroxylysine and hydroxyproline

elastin

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formed by 3 allysyl side chains + 1 unaltered lysyl side chain from the same or neighboring peptide
-produces elastin

desmosine cross-link

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– group of specialized proteins that contain heme as a tightly bound prosthetic group


Hemeproteins

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has 8 alpha-helical regions and no beta-pleated sheet regions
-2 polar histidine residues are found in the interior (F8 and E7)
-proximal histidine (F8) binds directly to the iron of heme
-distal histidine (E7) does not directly interact with heme group, but helps stabilize binding of oxygen to the ferrous iron

myoglobin

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prosthetic group of myoglobin

heme group

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= complete destruction of tertiary structure
Primary structure determines tertiary structure.

Denaturation + reduction of disulfide bonds

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-The 2 alpha chains and the 2 beta chains are identical.
-Many of the AA of alpha chain, beta chain and myoglobin are homologous (same AA in same position).
-must bind strongly to oxygen and release oxygen easily
-gives up O2 easily in capillaries, where need for it is great
-different quaternary structures in the bound and unbound forms

hemoglobin

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-has 4 heme groups, so it can bind 4 oxygen molecules

hemoglobin

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when one oxygen molecule is bound, it becomes easier for the next to bind

Positive cooperativity –

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IS the oxygen binding ability of myoglobin affected by the presence of H+ and CO2?

No!

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