Module 4: V11 - V16 Flashcards by Jack Wilkins

How is a disaccharide formed?

from two monosaccharides when the hemiacetal of one links with an -OH of another and condenses with the elimination of H2O to form a glycosidic bond

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What are some examples of common disaccharides?

sucrose - sugar cane
lactose - milk
maltose - derived from starch

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What are disaccharides defined by?

the monosaccharides and the nature of the linkage

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How are glycosidic linkages named?

identify C1 of sugar on the left -> determine if this sugar is the alpha or beta anomer -> determine which carbon on the second sugar is bound

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How many different ways can two D-glucose units be linked?

11 different ways

number of different types of oligosaccharides from a few different monosaccharides is enormous

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Where in nature are unbranched homopolysaccharides found?

structural elements of plant cells walls (cellulose) and animal exoskeleton (chitin)

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How are carbohydrates stored in plants and animals?

starch in plants and glycogen in animals

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What type of glycosidic linkages result in branching?

alpha-1,6 glycosidic linkages

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What are the two ends of an oligosaccharide called?

the nonreducing sugar (nonreducing end) and the reducing sugar (reducing end)

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How is sugar concentration measured?

by measuring formation of Cu2O in a redox reaction (because sugar is a reducing agent)

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What happens to the sugar when it is acting as a reducing agent?

the aldehyde of the sugar is oxidised to carboxylic acid

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Why is a reducing sugar able to take part in reactions?

because it is able to decyclise since it is only bound by one glycosidic linkage (end of the chain)

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How does cellulose provide structural support?

it is an unbranched homopolymer consisting of ~10,000 glucose units which are linked by beta 1-4 glycosidic bonds stabilised by hydrogen bonding

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What is the shape of a cellulose molecule?

long and flat

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How do cellulose molecules interact with other cellulose molecules?

many chains can pack together and form interchain hydrogen bonds (stable + great strength)

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What is chitin?

a linear homopolymer of N-acetylglucosamine residues which are in beta 1-4 linkages (acetylated amino groups = extra hydrogen bonding)

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How are glucose units linked together in glycogen and starch?

via alpha 1-4 linkages resulting in an open-helix structure rather than a linear structure

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How accessible is glucose in glycogen and starch compared to cellulose?

very accessible (↓ hydrogen bonding network)

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What are glycoproteins?

carbohydrate groups that are covalently attached to proteins

components of cell membranes

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What is the role of glycoproteins?

important functions in recognition e.g. cell adhesion

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What are the two ways in which carbohydrates are linked to proteins?

via Asn residues (N-glycans) in which sugars are linked to the amide side chain of Asn
via Ser or Thr residues (O-glycans) in which sugars are linked to the oxygen atom in the side chain of Ser or Thr

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Which enzymes are responsible for oligosaccharide assembly?

glycosyl (sugar) transferase which transfers a monosaccharide from a nucleotide-monosaccharide complex to an acceptor molecule

When a monosaccharide is transferred from the nucleotide sugar, which end of the carbohydrate sugar is this monosaccharide added?

the non-reducing end

What is a glycosyl (sugar) transferase always specific for?

the sugar added PLUS the linkage for the product

What are human ABO blood groups?

carbohydrate antigens on the surfaces of red blood cells

How do A and B blood group antigens differ from O antigen?

by one extra monosaccharide

How are A and B blood group antigens different?

same amount of monosaccharides except the last monosaccharides of the glycosphingolipids differ

What is the extra sugar in the A blood group antigens?

GalNAc = N-acetylglucosamine

What is the extra sugar in the B blood group antigens?

Gal = galactose

How many genes control the synthesis of ABO blood groups? What does it encode? How many alleles of this gene exist in the human population?

one a glycosyltransferase (enzyme) three an individual has one allele inherited from their mother and one allele inherited from their father

What does the A allele code for?

GalNAc transferase

What does the B allele code for?

Gal transferase

What does the O allele code for?

no active transferase

What does the AB allele code for?

both GalNAc and Gal transferase

Why does O allele result in no active transferase?

because this allele results in a premature stop codon and the final polypeptide chain is not able to fold properly

Why are blood transfusions a problem if you get the wrong blood type?

because antibodies are highly specific and very small chemical differences in oligosaccharide structures changes the affinity of antibodies

What happens when someone is infused with the wrong blood type?

antibodies will cross link the "foreign" red blood cells via agglutination + haemolysis which can be deadly

What are lectins?

broad family of proteins which recognise carbohydrates

How do C-type lectins get their name? What are their characteristics?

C type for calcium binding | 120 amino acid domain responsible for carbohydrate binding

How does calcium act in relation to C-type lectins?

acts as a bridge between the protein and the sugar through direct interaction with sugar hydroxyl groups

What happens as a result of changes in residues that interact with the sugar?

alter the carbohydrate binding specificity of the lectin

What are selectins and what are they involved in?

C-type lectins which are involved in cell-cell adhesion | bind white blood cells to site of injury and allow movement of cells from blood stream to site of infection

How do viruses attach to host cells? Give an example.

bind to receptors on the cell surface (sometimes carbohydrates) e.g. influenza virus recognises sialic acid residues present on cell surface glycoproteins

What is Relenza? How does it work as an antiviral?

an anti-flu drug which is a sialic acid analogue | inhibits the enzyme neuraminidase from flu virus which prevents cleavage of sialic acid (stuck to the cell)

What side of the cell is the T1 phase of the GLUT transporter open to?

the outside of the cell

What side of the cell is the T2 phase of the GLUT transporter open to?

the inside of the cell

How are glucose transporters able to allow the passage of glucose (large polar molecule) across the plasma membrane?

arrangement of 4 amphipathic helices (helix 1, 4, 7 and 10) which has a central polar region (glucose is shielded from the hydrophobic environment of the bilayer core)

How can cellulose and starch be so similar and yet so different?

they are both composed of glucose monomers, except bonding is different between these monomers cellulose is linked by beta 1 -> 4 glycosidic bonds which are stabilised by hydrogen bonding and starch is linked by alpha 1 -> 4 glycosidic bonds cellulose forms more of a linear structure, while starch forms an open helix structure which makes it much more accessible to the body

If there is a gene encoding every protein, is there or isn't there a gene for every oligosaccharide?

there isn’t a gene for encoding for every oligosaccharide

N-ter and C-ter; 5' and 3'; nonreducing and reducing. Where is this taking us?

similar to amino acid sequences, carbohydrate polymers have an equivalent N-terminus and C-terminus, the nonreducing and reducing ends

What exactly are the ABO blood group antigens? How and why are they different and why does this matter?

ABO blood group antigens are glycosphingolipids on surfaces of red blood cells

How can proteins recognise sugars and why is this important?

proteins are able to recognise sugars using C-type lectins in which calcium acts a bridge between the protein and the sugar through direct interaction with sugar hydroxyl groups this is important because protein recognition of sugars is essential for processes such as cell-cell adhesion

How is it that the same transporter can move glucose into and out of a cell? Is ATP required?

the transporter has two phases one phase allows the transporter to be open to the outside of the cell, and the other phase allows the transporter to be open to the inside of the cell ATP is required when moving glucose against its concentration gradient, and it is not required when glucose is being moved down its concentration gradient

A membrane transporter like GLUT1 can be considered an 'inside out' protein". What does this statement mean?

the GLUT1 transporter has a central polar region | therefore, it is considered an ‘inside out’ protein because most proteins have a central hydrophobic region