fibrous proteins

Question 1

what are examples of fibrous proteins

Answer 1

collagen, elastin and myosin

Question 2

what are differences between globular and fibrous proteins

Answer 2

globular are compact, fibrous are extended

globular is water soluble

fibrous is insoluble in water and lipid bilayers

secondary structure of globular is more complex with a mixture of alpha helices, beta sheets and loops

fibrous secondary structure is based on one type only

globular quaternary structure is held together by noncovalent forces

fibrous quaternary structure is usually held together by covalent bonds

Question 3

what are functions of fibrous proteins

Answer 3

bone, muscle, skin, hair etc

collagen in connective tissues relates to tensile strength

elastin in connective tissues relates to elasticity and stretchability

alpha keratin provides external protection and gives toughness to hair, nails and outer skin

Question 4

what proportion of body protein is fibrous

Answer 4

half fibrous half globular

Question 5

what are examples of mixed fibrous-globular protein

Answer 5

fibrinogen and myosin

Question 6

what is the extracellular matrix

Answer 6

compex structural entity surrounding and supporting cells that are found within mammalian tissues, often referred to as connective tissue

composed of 3 major classes of biomolecules; structural proteins (collagen and elastin), specialised proteins (fibrillin and fibronectin) and proteoglycans

the extracellular matrix consists of fibrous proteins in a hydrated polysaccharide gel, connective tissue is this plus cells

Question 7

what is structure of proteoglycans

Answer 7

protein core attached to long chains of repeating disaccharide units termed glycosaminoglycans (GAGs), forming extememly complex hgigh molecular weight components of the ECM

long carbohydrate chains which are highly negative charged (2 or 3 negative chargers per dimer)

Question 8

what does the extracellular matrix effect

Answer 8

effects dynamic behaviour of cells by effecting; growth, migration, proliferation, shape and metabolism

Question 9

what are cells found in the ECM

Answer 9

fibroblasts that secrete the matrix

chondroblasts which give rise to cartilage

oesteoblasts and oesteoclasts which are responsible for bone growth

these are all underlying an epithelial cell sheet

Question 10

how many types of collages are there

Answer 10

over 20 types, type 1-3 are most abundant, they form fibrils of similar structure

Question 11

describe the structure of a single collagen chain

Answer 11

2/3 of residues are proline (no alpha helices are formed, forms collagen helix instead)

every 3rd residue is glycine, one surface is relatively flat without bulk, this twists gently about the axis

produces a stable intertwined triple helix

substantial posttranslational processing of sidechains leads to strong cross links

Question 12

describe the process of collagen assembly/ synthesis

Answer 12

synthesised as procollagen which is secreted from the cell

cleaved to tropocollagen by procollagen peptidase

assembly of tropocollagen leads to collagen fibre

chemical crosslinks of tropocollagen strengthens the fibre

Question 13

describe the synthesis of procollagen

Answer 13

three seperate pro- a chains are synthesised in the cells

hydroxylation of selected proline and lysine residues occurs, uses proline and lysine hydroxylases which are dioxygenase enzymes

scurvey due to lack of vitamin C results in weakening of fibres at this stage

selected hydroxylated lysine residues are glycosylated

the pro-a chains assemble at the C terminus end starting with disulphide bridge formations between 3 chains, 3 chains then zip up to form procollagen

collagen is only protein to fold up prom C terminus onwards

Question 14

describe basic structure of type 1 collagen

Answer 14

900 residues long, standard sequence is proline, hydroxy-proline, glycine which is repeated

Question 15

how is tropocollagen formed

Answer 15

tropocollagen is formed by cleaving top and bottom loose strands to leave just the triple helix

N and C terminal peptides of procallagen are cleaved by procollagen peptidase

Question 16

describe general structure of tropocollagen

Answer 16

tropocollagen is one of the most elongated proteins, length of 300nm

composition of tropocollagen; 35% glycine, 21% pro/hydroxy-pro, 12%ala and 32% other

secondary structure of tropocollagen is triple helix (not alpha helices or beta sheets)

Question 17

describe features of tropocollagen secondary structure

Answer 17

features of triple helix;

3 seperate polypeptide chains arranged as a left handed helix, 3.3 residues per turn, each chains forms H bonds with other 2

there is a glycine every 3rd place, sidechains (H) point towards the centre of triple helix, leads to lots of H bonds, glycine is only source of H bonds between strands

glycine mainchain N-H protons form H bonds with C=O oxygen of adjacent polypeptide

pro and hy-pro are ‘imino’ acids and the side chain forms a covalent link with N atom so it cannot form H bonds, so glycine is buried in the middle while other 2 amino acids are on outside of triple helix

electron micrographs of a collagen fibre show cross striations with a repeat of about every 67nm, this is accounted for by the formation of one quater staggered array of tropocollagen molecules, since tropocollagen is 300nm long and 67nm is almost a quarter of that

Question 18

how do tropocollagen molecules join with eachother

Answer 18

there are 40nm gaps between the tropocollagen molecules which is where calcium phosphate is deposited in bone formation

this assembly forms spontaneously by means of noncovalent H bond interactions involving the OH group of hydroxyproline

Question 19

what causes scurvy, what are symptoms

Answer 19

a synthetic polymer made up of repeats of pro-pro-gly has a melting/ unfolding temp of 24 degrees (less than body temp)

if the second pro is replaced with hydroxy-pro this changes to 58 degrees, absence of this OH group leads to scurvy

symptoms are haemorrhages and blotchiness of skin, teeth fall out and people become lethargic

due to vitamin C (ascorbate) deficiency, so proline cannot be hydroxylated

Question 20

how is tropocollagen converted to collagen

Answer 20

tropocollagen subunit assembly is strengthened by covalent crosslinks that are formed between lysine residues

lysyl oxidase generates a reactive aldehyde form of lysine which makes cross links

joining the CHO aldehyde group and the amino group of the other lysine residues leads to lysinonorleucine

if lysyl oxidase is hyperactive then both residues will be oxidised to aldehydes forming an adol link instead

after these covalent links are formed what is left is collagen

Question 21

describe the alpha chains of collagen types 1-5

Answer 21

type 1: 2 alpha1 chains and 1 alpha2 chain

type 2 has 3 alpha1 chains

type 3 has 3 alpha 1 chains

type 4 has 2 alpha 1 and 1 alpha 2 chains

type 5 has 2 alpha 1 and 1 alpha 2

Question 22

compare basic structure/function of collages types 1-6

Answer 22

type 4 collagen does not make a fibre

types 1-3 and 5-6 use fibronectin as an anchor, type 4 uses laminin

types that use fibronectin use cell surface receptor integrin, type 4 uses laminin receptors

Question 23

describe structure of type 4 collagen, where is type 4 found

Answer 23

is a segmented molecule with kinks in it, monomers associate rapidly with head to head associations via C-terminal globular domains

dimers then form lateral associations via triple helical domains to form a sheet like meshwork

type 4 is present is epithelial cells, endothelial cells and regenerating hepatocytes, useful since epithelial and endothelial cells are flat

Question 24

what is osteogenesis imperfecta

Answer 24

causes by a mutation in one or the other of 2 genes whose products are used to make type 1 collagen

all inherited collagen diseases, including this are dominant, since assembly of normal gene product with mutant gene product produces defective collagen fibres

common mutation is buried glycine residue to mutated cystine

clinical symptoms are brittle bones and skeletal deformaties

Question 25

what is rubber man syndrome

Answer 25

caused by mutations in a type 1 collagen gene

symptoms are hyperextensible joints, tendons and skin

this is a type of ehlers danlos syndrome (type 7)

results from reduced levels of procollagen peptidase so not fully converted to tropocollagen

the 40nm gaps between tropocollagen molecules become blocked by uncleaved peptides which prevent lysyl oxidase from acting on tropocollagen to create the cross links

Question 26

describe equation of hydroxylation of proline in procollagen

Answer 26

proline + oxygen + alphaketoglutarate - hydroxyproline + CO2 + succinate

Question 27

what is lathyrism

Answer 27

an animal disease caused by ingestion of sweet pea seeds with beta aminopropionitrile or from copper deficiency

also occurs from low levels of lysyl oxidase (another form of ehlers danlos syndrome)

lysyl oxidase is a copper binding enzyme, beta aminopropionitrile prevents conversion of lys to aldehyde by irreversibly inhibiting lysyl oxidase

Question 28

descibe structure of glycosaminoglycans

Answer 28

disaccharide repeating units, derivates of amino sugar, negatively charged (like proteoglycans)

GAGs are incorporated into proteoglycan aggregates

Question 29

describe proteoglycan aggregates

Answer 29

form bottle brush structure:

a long chain of hyaluronic acid, link proteins are attached to long chain of hyaluronic acid, core protein of proteoglycans are attached to this, to the core proteins side chains of keratin sulphate and chondrotin sulphate split off

hyaluronic acid, keratin sulphate and chondrotin sulphate are glycosaminoglycans

when aqueous this has viscoelastic properties which helps support joints such as the knee

Question 30

what forms cartilage

Answer 30

proteoglycans along with collagen

Question 31

what is elastin, descibe primary structure

Answer 31

major component of elastic fibres in vasculature, found in major blood vessels and ligaments

the dominant sequence is pro-gly-val-gly-val-pro

a third of residues are gly

rich in pro and in small non polar residues

10% pro and hydroxy-pro, 23% ala, 13% val

Question 32

descibe secondary structure of elastin

Answer 32

helical structure, not alpha helical or collagen triple helix though

it is able to stretch and relax like a coiled spring, it is called a beta spiral and is a helix constructed of beta turns

Question 33

describe formation of elastin

Answer 33

synthesised as a proelastin, converted to tropoelastin (mol weight 72,000)

crosslinking of tropoelastin via lysine residues gives elastin crosslinks:

either desmosine (4 lysines) linking 2,3 or 4 molecules of tropoelastin

or lysinonorleucine (2 lysines)or adol link (2 lysines) linking 2 tropoelastin molecules

Question 34

how is desmosine formed

Answer 34

4 lysines come together and are oxidised to an aldehyde and join together to form an aromatic ring, 3 are oxidised to an aldehyde, one remains as lysine

when lysine is converted to aldehyde the end CH2-NH2 group is converted into a CHO group

Question 35

how is lysinonorleucine formed

Answer 35

one aldehyde reacts with lysine to form a “schiff base” where the end N of the lysine forms a double bond with the aldehyde forming a -N=CH- group

this is reduced to lysinonorleucine which has -N-CH2-