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CMCB > Lecture 5 > Flashcards

Lecture 5 Flashcards

1
Q

what is the Bombay blood type? Why does it exist?

A

Individuals whose blood type is incompatible with all of the major types, A, B, AB and O. These patients have antibodies in their blood that react against all the major blood types. this is a disease due to glycosylation.

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2
Q

what are glycoproteins?

A

proteins that contain covalently attached carbohydrate.

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3
Q

where are glycoproteins found?

A

in extracellular spaces or on the noncytosolic side of membrane systems, but a small number have been found in the cytosol, the nuclear envelope, and the nucleoplasm.

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4
Q

what is the range of function of glycoproteins?

A

structural, immune, coagulation, hormone, and protection.

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5
Q

Which glycoproteins have the highest percentage of carbohydrate?

A

mucin, for mucus. Its sliminess is due to carbohydrate. Proteins just hold the carbs together.

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6
Q

What glycoproteins have the highest percentage of carbohydrates? the lowest?

A

the highest three, from top to bottom, are mucin, cartilage proteoglycan, and interleukin-2. The lowest would be collagen.

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7
Q

Name seven functions of carbohydrates

A
  1. recognition 2. trafficking 3. physical properties 4. stabilize proteins 5. modulate activity 6. protein quality control and 7 store energy
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8
Q

what is the role of carbohydrates in recognition?

A

carbohydrates act as trafficking signals for transport to lysosomes, for removal from serum, for receptor-mediated endocytosis, which is all trafficking. Carbohydrates help with cell to cell recognition, as in during tissue formation or during lymphocyte invasion at an inflammation site.

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9
Q

What role do carbs play in physical properties?

A

mucins line and protect the stomach; antifreeze GPs in cold water fish depress freezing point of water; hyaluronic acid increases viscosity of water, makes viscous lubricating fluids for joints.

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10
Q

How do carbs stabilize proteins?

A

erythropoietin (40% carb) more susceptible to removal and degradation if carbs are removed.

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11
Q

how do carbs modulate protein activity?

A

some are transcription factors

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12
Q

how do carbs control protein quality?

A

They flag incompletely folded proteins for chaperones in ER.

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13
Q

what does structural complexity of sugars depend on?

A

attachment points, anomers (alpha or beta linkage), and multiple attachment points that permit branching

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14
Q

what linkages are common for sugars?

A

1,2- or 1,3- or 1,4- or 1,6- linkages are common

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15
Q

where do sugars attach to other sugars?

A

-OH groups

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16
Q

what is carbohydrate most often attached to?

A

Asn or Ser or Thr

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17
Q

How is carb attached to Asn?

A

N-linked

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18
Q

How is carb attached to Ser or Thr?

A

O-linked

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19
Q

how many carbohydrate chains may an individual glycoprotein contain? what kinds?

A

> 1 carb chains. It may have different types of carb chains.

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20
Q

how are sugars attached? what does attachment depend on?

A

one at a time. This depends on presence of specific glycosyltransferase enzymes, and on the presence of the proper substrate, and on the presence of the necessary activated sugars.

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21
Q

what is an activated sugar?

A

a sugar with a nucleoside diphosphate or a nucleoside phosphate.

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22
Q

What is unique about glycoprotein synthesis?

A

it is sequential but not templated. there is no genetic information that tells us how a carb tree needs to be built.

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23
Q

How is the synthesis of blood group A and B antigens sequential?

A

the GaINAc or GaI cannot be added until the fucose is in place.

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24
Q

what does glyprotein synthesis require?

A

that the appropriate precursors and enzymes are present

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25
Q

what is microheterogeneity? why is it observed?

A

small variations may be observed as a result of the nontemplated synthesis of these chains.

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26
Q

what are N-linked glycoproteins?

A

these carbs have from about 8 to 25 residues and are linked via an N-glycosidic bond to Asn residues.

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27
Q

What sequence is Asn in usuallay in N-=linked glycoprotein?

A

in the sequence Asn-X-Ser/Thr.

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28
Q

what is a conserved core structure?

A

the carbohydrate chains contain a conserved core structure and variable peripheral structures (R’s)

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29
Q

what is a high mannose N-linked glycoprotein?

A

has mannosyl-mannose substitutents on the core structure

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30
Q

what is a complex N-linked glycoprotein?

A

substituents on the core consist of a short chain composed of multiple sugar types

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31
Q

what are hybrid N-linked glycoproteins?

A

contain both high mannose and complex N-linked carbohydrates.

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32
Q

How is the N-linked carbohydrate chain, without the protein, first formed?

A

it is formed on a lipid carrier, a dolichol, located in the membrane of the ER. The first stage takes place in cytoplasm on a phosphate of a membrane-embedded dolichol. the oligosaccharide is assembled sugar by sugar on the carrier lipid dolichol.

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33
Q

what is a dolichol? what are its characteristics?

A

it is a lipid carrier. long and very hydrophobic. its 22 five-carbon units can span the thickness of a lipid bilayer more than three times.

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34
Q

how is the first sugar linked to a dolichol?

A

by a phrophosphate bridge, which is a high energy bond that activates the oligosaccharide for its eventual transfer from the lipid to the protein.

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35
Q

where does the synthesis of the oligosaccharide begin? to where does it continue?

A

on the cytosolic side of the ER membrane and continues on the lumenal face after the (Man)5(GlcNAc)2 lipid intermediate is flipped across the bilayer by a tranporter protein.

36
Q

How are subsequent glycosyl transfer reeactions on lumenal side of ER done?

A

they involve transfers from dolichol-P-glucose and dolichol-P-mannose; thes activated, lipid-linked monosaccharides are synthesized from dolichol phosphate and UDP-glucose or GDP-mannose on the cytosolic side of the ER and are then thought to be flipped across the ER membrane.

37
Q

what happens after oligosaccharide is assembled?

A

it can be transferred en bloc to an asparagine side chain of a nascent polypeptide as it enters the lumenal side of the rough ER.

38
Q

how does tunicamycin work? What is it used for?

A

it is an analog of UDP-GlcNAc, and inhibits the first reaction in the pathway of building the N-linked carbohydrate. It is used to understand glycoproteins.

39
Q

what happens to carbohydrate after it is transferred to the protein?

A

the chain is processed by enzymes that remove some sugar residues and add others.

40
Q

Where does removal of sugars generally occur?n what about subsequent modifications?

A

in the ER and/or early Golgi. subsequent modifications of basic structrue happen in specific “parts” of the golgi.

41
Q

where does processing the carbohydrate chain start?

A

It starts in the ER with the removal of two glucoses from the oligosaccharide initially transferred to the protein.

42
Q

how many mannoses in a complex oligosaccharides

A

three mannoses

43
Q

to what do the two N-acetylglucosamines in the core become resistant to?

A

attacck by a highly specific endoglycosidase (Endo H).

44
Q

What is Endo H. what is it used for?

A

endoglycosidase. treatment with this enzyme is widely used to distinguish complex from high-mannose oligosaccharides.

45
Q

where do the final steps in the synthesis of a complex oligosaccharide occur?

A

in the cisternal compartments of the golgi apparatus.

46
Q

how many types of glycosyl transferase enzymes act sequentially in the golgi?

A

three kinds. the membranes of the golgi cisternae contain specific carrier proteins that aloow the sugar nucleotides to enter, in exchange for the nucoleotides that are released after the sugar is attached to the protein on the lumenal face.

47
Q

What does an oligosaccharide need in order to bind to calnexin? what is calnexin?

A

the oligosaccharide needs one glucose. calnexin is an ER membrane bound folding chaperone.

48
Q

what releases oligosaccharide frm calnexin

A

glocusidase

49
Q

what determines if oligosaccharide is folded or not?

A

glucosyl transferase. if not folded properly, will add glucose from UDP-glucose and make it reenter chaperone cycle until it is folded.

50
Q

two protein folding chaperones besides calnexin?

A

calreticulin and ERp57

51
Q

what is a terminal mannosyl residue?

A

i think it is a end of chain mannose, in this case on lysosome enzyme.

52
Q

What transfer phosphjate to lysosomal enzyme? where?

A

GlcNAc phosphatase to mannose reside on lysosomal enzyme in cis golgi.

53
Q

how is GlcNAc removed from enzyme?

A

with phosphodieesterase. then sent to lysosome.

54
Q

how are carbs attached to O-linked glycoproteins?

A

to side chain -OH of Ser, Thr, or hydroxy-Lys

55
Q

where does glycosylation of o-linked glycoproteins take place?

A

in golgi

56
Q

how large are O-linked sugar chains? how are they synthesized?

A

usually

57
Q

what is first enzyme involved for secreted and extracellular glycoproteins?

A

N-acetylgalactosaminyl transferace

58
Q

what sugar is attached to Ser or Thr for intracellular glycoproteins?

A

N-acetyl-glucosamine (GlcNac)

59
Q

what major group of glycoproteins carries O linked carb chains

A

mucins

60
Q

which glycoproteins are known as bottle brush glycoproteins?

A

mucins

61
Q

what are proteoglycans?

A

linear polysaccharides of form (X-Y)n, where n > 20. Y is uronic acid and X is hexoamine.

62
Q

uronic acid examples

A

glucuronic or iduronic acid

63
Q

hexoamine example

A

N-acetylglucosamine or N-acetylgalactosamine

64
Q

proteoglycans are also called

A

glycosaminoglycans (GAGs)

65
Q

what are GAGs usually attached to

A

Ser residues in the sequence of Ser-Gly of core proteins via a unique sugar sequence

66
Q

what knd of charge is found in GAGs? why?

A

negative charge. uronic acid and sulfate presence

67
Q

what forms GAGs disaccharide repeating structure?

A

an enzyme complex that contains 2 transferases, which act together to form the disaccharide repeating structure.

68
Q

when does sulfation happen in GAGs? where does it happen?

A

sulfation and coversion of D-glucoronic to L-iduronic acid occurs after chain polymerikzation

69
Q

does hyaluronate exist as a proteoglycan

A

no. just as a GAG, a polysaccharide.

70
Q

where are proteoglycans found

A

in cartillage. help make bone to bone contact smooth.

71
Q

are proteoglycans hydrophobic?

A

no. they are hydrophilic

72
Q

where do carbohydrates attached to GPI anchor attach to c-terminal of protein?

A

ER lumen

73
Q

what does GPI anchor stand for?

A

glycosylphsphatydil-inositol anchor

74
Q

what is the main carbohydrate core that is linked to the C terminal?

A

phosphatydil inositol, GlcNac, mannose, mannose, mannose

75
Q

what is core protein of glycogen? how does it add glucose to itself? what is the limit of how much glucose it can add? how does it keep adding glucose after it hits that limit?

A

glycogenin. adds glucose from UDP-glucose to OH group on Tyr side chain. up to eight molecules. then glycogen synthase takes over.

76
Q

what is an exoglycosidase? endoglycosidase?

A

exoglycosidase cleaves glycosidic bonds at terminal residue. endoglyosidase cleaves polysaccharide chains between residues that are not the terminal residue.

77
Q

how is mass spectometry useful in determining carbohydrate content and structure of a glycoprotein?

A

it can identify molecules or hydrolysis products on the basis of molecular weight. we can use it and glycosidases to determine the carbohydrate content and structure of a glycoprotein.

78
Q

what are lectins?

A

carbohydrate binding proteins, distinct from antibodies or enzymes, that have specificity for specific carbohydrate structures.

79
Q

What is ELAM-1? how does it work?

A

endothelial-leukocyte adhesion molecule, a lectin. in vascular endothelium after stimulation by specific polypeptide growth factors. It allows circulating neutrophils to bind to vascular linking and subsequently extravasate surrounding tissue to aid in the defense against infection.

80
Q

at neutral pH, what does aldehyde function of sugars react and combine with?

A

e-amino groups of Lys and free N-terminals of proteins. Results in ketoamine or advanced glycation endproducts (AGEs)

81
Q

what is HbA1C? What can it measure?

A

glycated hemoglobin. Reflects average blood clucose concentration over preceding 1-3 months, as opposed to blood glucose, which tells current blood glucose concentration.

82
Q

what gene are bombay blood patients missing

A

the gene for fucosyl transferase, which completes the O antigen

83
Q

carbohydrate from cytosol/dolichol to inside ER lumen, complete process

A

start outside of ER membrane in cytosol. Dolichol embedded in membrane. One CTP to CDP, dolichol -P. Add 2 UDP GlcNAc to UDP UMP, and you get dolichol-P-P-(GlcNAc)2. Add 5 GDP Man to 5 GDP. Get dolichol, P, P, GlcNAC2, Mann 5, and flip to other side. Then Mann-P-dolichol x 5, end up with 9 Mannose. Then Glc-P-dolichol x 3, and and up with 3 glucose, 9 mannose, 2 GlcNAc, and 2 phosphates on side of ER lumen.

84
Q

How does oligosaccharide that is attached to dolichol get moved to growing polypeptide on ER membrane?

A

oligosaccharide protein transferase transfers oligosaccharide, severed between GlcNAc and phosphate, and adds it to ASN residue on polypeptide chain.

85
Q

process n linked carb from ER lumen to golgi lumen

A

start with full oligosaccharide at ER lumen. glucosidase I and II remove 2 and 1 gluocose. ER mannosidase removes one mannose. Move to golgi. Golgi mannosidase I removes three mannoses. GlcNAc transferase i adds one GlcNAc. Golgi mannosidase II removes 2 mannose. Then add one GlcNac, one galactose, and one sialic acid, and end up with complex oligosaccharide.

CMCB (17 decks)

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