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Biochemistry 441B > Carbohydrate Structure & Function > Flashcards

Carbohydrate Structure & Function Flashcards

1
Q

What are three types of carbohydrates?

A

Simple sugars
Polysaccharides
Glycoconjugates

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2
Q

What is another name for carbohydrates?

A

Glycans

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3
Q

What are glycans?

A

Polyhydroxy aldehydes or ketones or substances that yield such compounds on hydrolysis

Very heterogeneous in size and composition

Named so because many have formula Cn(H2O)n, where n ≥ 3

Glycans = Carbohydrates

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4
Q

What is the most abundant biological molecules and how are they produced?

A

Glycans (Carbohydrates)

Produced from CO2 and H2O via photosynthesis in plants

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5
Q

What can carbohydrates covalently link to?

A

Proteins and lipids

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6
Q

What functions do carbohydrates fulfill?

A

Energy source and energy storage

Structural component of cell walls and exoskeletons

Informational molecules in cell-cell signaling

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6
Q

What are examples of simple sugars?

A

Monosaccharides
- glucose, galactose, mannose

Disaccharides
- sucrose, maltose, lactose

Oligosaccharides
- milk oligosaccharides, raffinose

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6
Q

What are examples of polysaccharides?

A

Glucose homopolymers
- cellulose, starch, glycogen

Disaccharide heteropolymers
- keratan, sulfate

Chitin is a homopolymer of N-acetyl glucosamine

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6
Q

What are examples of glycoconjugates?

A

Glycoproteins
- antibodies, viral coat proteins

Proteoglycans
- aggrecan, syndecan, glypican

Glycolipids
- blood antigens, membrane anchors

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7
Q

What are oligosaccharides?

A

Simple sugars that range from 3-20 unbranched and branched sugar residues

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8
Q

What are milk oligosaccharides?

A

They are derived from lactose

Lacto-N-tetrarose
Lacto-N-fucopenraose-1

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9
Q

What is the importance of human milk oligosaccharides?

A

Important functions in the infant intestinal tract

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10
Q

What does Lacto-N-tetrarose do in the infant intestinal tract?

A

Helps with digestion

Growth advantage to bifidobacteria

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11
Q

What does Lacto-N-fucopentaose 1 do in the infant intestinal tract?

A

Fools bacteria with itself so it binds to the carbohydrate instead of the receptor of the cell

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12
Q

What are examples of plant oligosaccharides?

A

Raffinose
Stachyose
Verbascose

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13
Q

What are plant oligosaccharides derived from?

A

Sucrose

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14
Q

Why can humans and non-ruminating animals (pigs, poultry) not digest plant oligosaccharides?

A

Humans and non-ruminating animals lack the a-galactosidase enzyme needed to hydrolyze the a-1,6 glycosidic bond.

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15
Q

What is beano?

A

A preparation of a-galactosidase that can aid in digestion of plant oligosaccharides, resulting in the release of free galactose and sucrose

Sucrose is further metabolized to glucose and fructose by the enzyme sucrase in the small intestine

These simple sugars are easily metabolized by our own enzymes

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16
Q

What are the types of polysaccharides?

A

Homopolysaccharides

Heteropolysaccharides

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17
Q

What are homopolysaccharides?

A

They can be unbranched (linear) and branched (every now and then)

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18
Q

What are heteropolysaccharides?

A

They can be unbranched with two monomer types and branched with multiple monomer types.

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19
Q

What do polysaccharides not have?

A

They do not have a defined molecular weight and composition

This is in contrast to proteins because unlike proteins, no template is used to make polysaccharides

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20
Q

What are the functions of polysaccharides?

A

Some serve as structural elements

Some serve as storage forms of monosaccharides that are used as fuels

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21
Q

What are examples of structural polysaccharides?

A

Cellulose (plant cell walls)

Chitin (animal exoskeletons)

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22
Q

What are examples of storage polysaccharides?

A

Starch in plants

Glycogen in animals

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23
Q

What is cellulose?

A

An unbranched homopolysaccharide consisting of ~1000 repeating units of a disaccharide cellobiose

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24
Q

What is cellobiose?

A

2 glucose (Glc) monomers linked by (B1–>4) glycosidic bonds

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25
Q

What bonds does cellulose have and where?

A

H-bonds between cellulose strands

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26
Q

What is cellulose a component of?

A

Component of the plant cell wall

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27
Q

What is the most abundant polysaccharide in nature?

A

Cellulose

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28
Q

What is nearly pure fibrous cellulose?

A

Cotton

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29
Q

Why are individual cellulose strands extensively hydrogen-bonded?

A

To build a polysaccharide fibril

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30
Q

What do most animals lack in terms of polysaccharides?

A

Most lack cellulase

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31
Q

What is cellulase?

A

The enzye required to hydrolyze the B1–>4 linkage between glucose molecules in cellulose

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32
Q

What is high in cellulose?

A

Plant material

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33
Q

What is plant material high in cellulose considered?

A

Considered “roughage in the diet”

- it passes through the digestive system without being degraded

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34
Q

What can fungi, bacteria, and protozoa do in terms of cellulose?

A

Secrete cellulase, that can use wood as source of glucose

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35
Q

What do ruminants and termites have in relation to cellulose?

A

They live symbiotically with microorganisms that produce cellulase

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36
Q

What is chitin?

A

Linear homopolysaccharide of N-acetyl Glucosamine (GlcNAc) units
- Contains B1–>4 glycosidic bonds

Structural exoskeletons in insects and crustaceans

Also found in cell walls of many types of fungi, including mushrooms

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37
Q

What does chitin provide?

A

Provides a strong body frame

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38
Q

What bonds does chitin have and where?

A

H-bonding within and between polysaccharide strands

39
Q

What is starch?

A

The main storage polysaccharide in plants

40
Q

What are the two forms of starch that plants synthesize?

A

Amylose

Amylopectin

41
Q

What is amylose?

A

an unbranched (linear) polymer of ~100 (a1–>4) linked glucose units

humans can metabolize this called amylase to break it down

42
Q

What is amylopectin?

A

a (a1–>4) linked glc polymer with (a1–>6) branching occuring every 15-30 residues

linear chain with branches
- homopolymers

Left handed

43
Q

How is starch different from cellulose and chitin?

A

Starch is an important dietary source of glucose for animals

44
Q

Why is starch an important dietary source of glucose for animals?

A

It can be hydrolyzed by the enzyme a-amylase which cleaves (a1–>4) glycosidic bonds

45
Q

What is glycogen?

A

Branched homopolysaccharide of glucose
- just like amylopectin but is more heavily branched

Molecular weight reaches several millions

Glycogen bonded to only 1 glucose

46
Q

In glycogen, what do glucose monomers form?

A

(a1–>4) linked linear chains

47
Q

In glyocogen, describe its branch points.

A

(a1–>6) linkers every 8-12 residues

48
Q

What is the main function of glycogen?

A

Main storage polysaccharide in animals

49
Q

Where is there an abundance of glycogen?

A

Abundant in liver (~7-10% wet weight of liver) and in skeletal muscles

50
Q

What does glycogen and starch often form?

A

Granules in cells

51
Q

What do granules contain?

A

Enzymes that synthesize and degrade these polymers

52
Q

What do glycogen and amylopectin have?

A

One reducing end but many non-reducing ends

Anomeric carbon has to attack at the non reducing end

53
Q

Where do enzymatic processing occur?

A

Occurs simultaneously at many non-reducing ends

Therefore, more branch points provide more ends for Glc retrieval and storage

54
Q

What is the advantage of storing glucose as starch or glycogen?

A

Greatly reduces osmotic pressure that would result from its storage in monomeric form

This is because osmotic pressure is proportional to the number of solute molecules (regardless of their given weight) in a given volume.

55
Q

What are important roles does glycans have?

A

In addition to the energy and structural roles, it has an important role as functional groups in glycoconjugates

56
Q

How much of all human proteins are glycoproteins?

A

~50%

57
Q

How much of the human genome encodes enzymes required for the synthesis and degradation of carbohydrates and glycojungates?

A

~1%

58
Q

What enzymes are required for the synthesis and degradation of carbohydrates and glycoconjugates?

A

Glycosyltransferases

Glycosidases

59
Q

What are glycosyltransferases?

A

Covalently linked glycans to proteins and lipids

60
Q

What are glycosidases?

A

Remove glycans through hydrolysis reactions

61
Q

What are the types of glycoconjugates?

A

Glycoproteins

Proteoglycans

Glycolipids

62
Q

What are glycoproteins?

A

Bulk of the macromolecule consists of protein

  • Some are transported by membrane vesicles to the plasma membrane for insertion or secretion
  • Others are targeted to cellular organelles such as mitochondria, peroxisomes
63
Q

What are proteoglycans?

A

Bulk of the macromolecule consists of carbohydrates

64
Q

In humans, where are proteoglycans found?

A

Extracellular matrix

65
Q

In bacteria, where are proteoglycans found?

A

Cell wall (peptidoglycans)

66
Q

Where are glycoproteins transported?

A

transported to the plasma membrane for insertion or secretion by membrane vesicles

67
Q

What targets glycoproteins?

A

Cellular organelles such as mitochondria, peroxisomes target some glycoproteins (the other half that i’snt transported)

68
Q

Glycan groups on glycoconjugates are recognize by glycan binding proteins called ______.

A

Lectins

69
Q

What are lectins?

A

Special class of proteins

Glycan group-lectin interactions play important role in cell signaling and immunity

70
Q

What are colored symbols for monosaccharides used as?

A

Used as building blocks of the glycan groups in most glycoconjugates

Simplifies representation of the glycan group of the glycoconjugate

Consortium for Functional Glycomics (CFG) notation

71
Q

Where on the protein do the glycans attach?

A

N- and O-Linked glycosylation

N-Linked = Asn always (GlcNac)

O-Linked = Ser or Thr (GalNAc)

75
Q

What is N-Linked Glycosylation?

A

Carbohydrate linkage occurs through the amide N atom in the side chain of Asn.

76
Q

What does N-glycosylating enzymes most often attach to?

A

Often attach the sugar to asparagine (Asn) residues contained within the consensus sequence Asn-X-Ser/Thr (X can be any amino acid except Pro)

77
Q

What is the consensus sequence?

A

Asn-X-Ser/Thr
(X can be any amino acid except Pro)

Matters how it is attached

78
Q

What is O-Linked Glycosylation?

A

Carbohydrate linkage occurs through the O atom in the side chain of Ser or Thr residues

79
Q

What is the consensus sequence for O-glycosylating enzymes?

A

Unlike N-glycosylating enzymes, the O-glycosylating enzymes do not have a preferred consensus sequence

TLDR: O-glycosylating enzymes does not have a consensus sequence

80
Q

What are mucins?

A

O-linked glycoproteins are often called mucins

Mucins function to protect epithelial cells in the intestinal, urinary and respiratory tracts from physical damage and pathogens

81
Q

Which enzymes determine the expression of human ABO blood groups?

A

a-1,3-N-acetylgalactosaminyltransferase (GTA)

a-1,3-galactosyltransferase (GTB)

These enzymes attach either a GalNAc (GTA) or Gal (GTB) sugar residue to a glycan subgroup called the O antigen

82
Q

If neither GTA or GTB enzymes are present, which blood type group is it?

A

O-type blood

83
Q

If only GTA enzyme is present, which blood type group is it?

A

A-type blood

Adds square (GalNAc)

84
Q

If only GTB enzyme is present, which blood type group is it?

A

B-type blood

Adds circle (Gal)

85
Q

If both GTA and GTB enzymes are present, which blood type group is it?

A

AB-type blood

Adds square and circle (GalNAc, Gal)

86
Q

What is the O antigen?

A

Glycan subgroup where a-1,3-N-acetylgalactosaminyltransferase (GTA) and a-1,3-galactosyltransferase (GTB) attach to either a GalNAc (GTA) or Gal (GTB) sugar residue

87
Q

Where is the O antigen present?

A

Present on glycoproteins and glycolipids on red blood cells

88
Q

Which blood group is a universal red blood cell acceptor?

A

Type AB blood

89
Q

Which blood group is a universal red blood cell donor?

A

Type O blood

90
Q

Which blood group is a universal plasma acceptor?

A

Type O blood

91
Q

Which blood group is a universal plasma donor?

A

Type AB blood

92
Q

What happens if anti-A and anti-B antibodies meet anti-O antibodies?

A

They will attack each other

93
Q

What are proteoglycans?

A

Proteins with multiple long glycosaminoglycan chains covalently attached to a relatively small core protein

Attached to a hyaluronic acid through a linker protein

Extracellular matrix proteoglycans and cell surface proteoglycans

94
Q

Where do proteoglycans function?

A

Membrane-bound through a Glyco-phosphatidyl inositol (GPI) anchor or a transmembrane a helix on the protein

Secreted into the extracellular matrix (most function here)

95
Q

What is a peptidoglycan?

A

Component of bacterial cell wall

Provides shape and protection against osmotic changes
- Linear chains of B(1–>4) linked MurNAc and GlcNAc tethered together by linkages between short peptides

MurNAc = N-Acetylmuramic acid
GlcNAc = N-acetylglucosamine
meso-DAP = Meso-diaminopimelic acid (derivative of lysozyme)
96
Q

What does lysozyme do in a peptidoglycan?

A

Breaks B(1–>4) bond which can neutralize bacteria

97
Q

What does penicillin do?

A

Blocks bacterial cell wall synthesis by inhibiting Transpeptidase

When transpeptidase is inactive, cell wall synthesis is inhibited and bacterial cells die.

Forms a suicide inhibitor complex between a Ser residue in transpeptidase and a carbonyl C in the B-lactam ring of penicillin

98
Q

What is transpeptidase required for?

A

This enzyme is required to form the oligopeptide linkages between polysaccharide chains (gives it strength)

99
Q

Why is penicillin considered a “suicide inhibitor”?

A

A suicide inhibitor is one that inhibits an enzyme and the mechanism of inhibition such that no conditions exist by which this inhibition can be overcome in the cell.

Penicillin forms an irreversible, covalent bond with transpeptidase.

The presence of this bond inactivates the enzyme and the irreversible nature of the bond makes penicillin a suicide inhibitor.

100
Q

How has bacteria developed mechanisms to resist B-lactam antibiotics?

A

Penicillin-resistant bacteria express the enzyme B-lactamase, which inactivates penicillin by cleaving the B-lactam ring

101
Q

What is the solution to bacteria with resistance to B-lactam antibiotics?

A

Synthetic compounds that block Transpeptidase activity without being a substrate for B-lactamase such as Methicillin

102
Q

What new problem arose after the use of methicillin?

A

Methicillin-resistant S. aureus (MRSA) bacterial strains arose that express a variant form of the transpeptidase enzyme that is not inhibited by the antibiotic

This variant transpeptidase has no reaction with methicillin which allows transpeptidase to be active and allowing bacterial cells to live

Biochemistry 441B (10 decks)

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