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Biochemistry > Sugars (lectures 11&12) > Flashcards

Sugars (lectures 11&12) Flashcards

1
Q

What biological processes are carbohydrates involved in?

A 
Storage & transport of energy 
Cell-cell communication 
Host-pathogen interactions 
Structural 
Components of DNA & RNA
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2
Q

Composition of carbohydrates

A

All contain C, H, O & sometimes S & N
Molecule contains
• 2 or more hydroxyl groups
• An aldehyde or ketone group
• Empirical formula is (CH2O)n
• Also known as sugars, saccharrides & glycans

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3
Q

What happens if a sugar is asymmetric?

A

it is chirally active
attached to 4 different groups
exhibits stereoisomerism
aldoses have stereoisomerism but ketoses don’t

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4
Q

How do carbohydrates form rings?

A

sugars with 5 or more C generally form rings
they can then react with themselves to form a hemiketal or a hemiacetal
• the aldehyde/ketone & the alcohol group on the same molecule can react to form a ring
• a reversible reaction
• more energetically favourable when the ring is closed

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5
Q

Why are sugar rings not flat?

A

Because of the tetrahedral geometry of the carbon atoms, the cyclic form of pyranose (6 membered ring) sugars usually assume a non-planar ‘chair’ configuration

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6
Q

What are the 2 orientations of substituents of a ring of C atoms?

A

Axial - pointing up or down
• Axial atoms point in opposite directions on adjacent carbons
• If they pointed in the same direction the axial atoms would physically clash
• All hydrogens are axial

Equatorial - same plane as the ring
• Equatorial is less crowded
• OH groups are mainly equatorial as they are bigger constituents so are less likely to be in the way if they are equatorial
• Lots more space so more energetically favourable

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7
Q

What are anomers?

A

Epimers at C1
Forms alpha or beta forms of the sugar
The 2 sugar isomers are called anomers

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8
Q

How do you define alpha & beta anomers?

A

Configuration of the anomeric carbon compared to the anomeric reference atom which is the stereocentre that is farthest from the anomeric carbon in the ring

Alpha = opposite configuration 
Beta = same configuration

Beta form is most energetically favourable in most sugars

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9
Q

How do you distinguish between the 2 stereoisomers of glucose (D & L)?

A
  • Determined by the configuration of the asymmetric carbon (chirally active carbon) furthest from the aldehyde or ketone group
  • L & D glucose are mirror images of each other (enantiomers) as all 4 chiral centres are different isomers
  • In most cases only one enantiomer is found in nature – most sugars only in the D form
  • Designations are based on the configuration about the single asymmetric carbon in glyceraldehyde (simplest sugar with asymmetric carbon)
  • Called L & D from the direction in which sugars rotate the plane of polarised light
  • To the right are called dextrorotary (D)
  • To the left are called laevorotary (L)
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10
Q

What about when only asymmetric carbon 2, 3 or 4 are in a different configuration to glucose?

A
  • Then the molecule is a different sugar
  • The 2 sugars are epimers of each other
  • Glucose & Mannose are epimers at C2
  • Glucose & Galactose are epimers at C4
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11
Q

How many D-configured aldose epimers are there with 6 carbons?

A

2^3 = 8

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12
Q

Carbohydrate polymers

A

• Dissacharides = 2 monosaccharides
• Oligosaccharides = 2-10 monosaccharides
• Polysaccharides = 10+ monosaccharides
The covalent linkage between the sugars in these polymers is called a ‘glycosidic bond’

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13
Q

Glycosidic bond formation

A

A glycosidic bond is formed between the hemiacetal (or hemiketal) group of a sugar and the oxygen of the hydroxyl group of another sugar with loss of a water molecule - condensation reaction
Glycosidic bonds have different configurations
• Can be either alpha or beta according to the configurational relationship between the anomeric centre and the anomeric reference atom
• Alpha = opposite configuration
• Beta = same configuration

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14
Q

Structure of starch & glycogen

A

Backbone of D-glucose molecules linked by alpha-1,4-glycosidic bonds
Branches linked to backbone via an alpha-1,6-glycosidic bond
Reducing sugar is the terminal sugar in which the C1 is unattached & thus the ring can open & an aldehyde (which can reduce other molecules) can form
Non-reducing sugar is the terminal sugar in which C1 is involved in a glycosidic bond & this will prevent the sugar ring from opening so it csnt reduce other molecules as an aldehyde isn’t generated
1,4 bonds are linear

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15
Q

Reducing sugar

A

Reducing end – open form of sugar with free aldehyde group
A reducing sugar is any sugar that is capable of acting as a reducing agent because it has a free aldehyde or ketone group
All monosaccharides & most disaccharides/polysaccharides are reducing sugars

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16
Q

Why are alpha-glucans good storage molecules?

A

The alpha-1,4-linkage is kinked
• Linkage causes polysaccharide to twist into a helical structure
• More compact – takes up less room in the cell & easily accessible to enzymes so can be degraded quickly when needed

17
Q

Glycogen & starch have lots of branches. What does this mean?

A
  • Branching also leads to a very compact structure
  • It makes lots of non-reducing (NR) terminal sugars so easy to digest as multiple attack sites for enzymes (alpha-glucosidases)
  • If it was only a linear molecule then there is only one end for enzymes to attack
18
Q

Cellulose

A
  • Structural plant polysaccharide
  • Long beta-1,4 linked glucose chains pack together to form highly crystalline microfibrils
  • Very strong
  • Most abundant organic molecule on the planet
19
Q

Cellulose structure

A

Beta-1,4- bond means chains are planar
Allows multiple chains to pack tightly together via multiple hydrogen bonds unlike the alpha bonds in starch
Highly crystalline & ordered
Very strong & hard for enzymes to degrade

20
Q

What are cellulosic biofuels?

A

Glucose locked up in plant cell walls provides a possible renewable energy source
Hydrolyse cellulose to glucose with enzymes then convert glucose to ethanol by microbial fermentation

21
Q

Advantages of cellulosic biofuels?

A
  • It is an alternative to fossil fuels – more environmentally friendly & no net release of CO2
  • Can use agricultural wastes & non-food crops (cellulosic rather than starch based)
  • USA could alone potentially produce cellulosic biofuel equivalent to 4 billion barrels of oil
22
Q

Disadvantages of cellulosic biofuels?

A
  • Cellulose very recalcitrant to enzyme digestion due to structure
  • Lots of money being spent researching better more efficient enzyme systems to use
23
Q

What is peptidoglycan?

A

Main component of the bacterial cell wall
Usually 1 or 2 layers in gram-negative bacteria
40+ layers in gram-positive bacteria

24
Q

What are glycoconjugates?

A

carbohydrates covalently linked to other molecules such as proteins & lipids

25
Q

What is glycosylation?

A

The modification of a protein but the addition of a sugar (carbohydrate) group

26
Q

What are the 2 types of glycosylation?

A 
Nitrogen (N-linked)
• Joined to the amide of asparagine’s 
• Usually on secreted proteins only 
• Complex oligosaccharide structures 
• N-glycoslation plays roles in protein folding & stability & cell recognition

Oxygen (O-linked)
• Joined to hydroxyl of serine or threonine
• Common addition is GlcNAc
• Cytoplasmic
• Thought to be reciprocal to phosphorylation

27
Q

What are mucins?

A

O-linked glycoproteins that are key components of mucus

Different mucins cover all the epithelial surfaces & provide protection & lubrication

28
Q

What are glycosaminoglycans (GAGs)?

A

Produced by virtually all mammalian cells
GAG component determines function
Often heavily sulphated

function as:

1) joint lubricants
2) structural components of ECM
3) mediate adhesion of cells to ECM
4) bind factors that stimulate proliferation

29
Q

How are complex carbohydrates synthesised?

A

Glycotransferases (GTs)
Transfers sugars from nucleotides onto other molecules
GTs make the molecules (antigens) that comprise our blood groups

30
Q

How is blood group determined?

A

We all contain the O antigen core on the surface of our blood cells
But we contain different GTs that add an extra sugar onto the O antigen
2 GTs (GTA & GTB) are in the population that do this
• Encoded by different forms of the same gene (different alleles)

31
Q

What are the different blood groups?

A
  • Blood group A produce GTA that puts GalNAc on
  • Blood group B produce GTB that puts a Gal on
  • In O blood groups the GT is non-functional
  • AB blood groups produce both GTA & GTB
32
Q

Host pathogen recognition in the Influenza flu virus?

A

Contains 2 key surface proteins

1) Hemagglutinin
2) Neuraminidase

Hemagglutinin recognises the sugar sialic acid on the surface of target cells
Mediates viral invasion of cells
Neuraminidase is required to remove sialic acid from the cell surface
1) Influenza hemagglutinin (Ha) binds to sialic acid (Sa) on cell surface leading to invasion
2) Neuraminidase then removes cell surface Sa
3) Virus releases as cell can no longer bind to Ha

Biochemistry (16 decks)

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