Lectins and Working with carbohydrates

Question 1

How are sugars able to contain information?

Answer 1

They have huge amounts of structural and functional diversity, way more than DNA or proteins

Question 2

Even though oligosaccharides can be assembled in hundreds of billions of different structures, why are we limited to which ones we have?

Answer 2

We need the right enzymes to assemble them, so only the ones that have the enzymes around will get made

Question 3

What are lectins?

Answer 3

Proteins that specifically recognize and bind to sugars

Question 4

What do lectins do?

Answer 4

Cell to cell recognition and adhesion
Protein targeting
Signalling

Question 5

What are polyvalent interactions?

Answer 5

Interactions are weak on their own, but strong in multiples

Question 6

How do pathogens use lectins?

Answer 6

They have their own lectins that specifically recognize and bind to oligosaccharides on the host cell surface to infect the cell

Question 7

What type of protein usually gets a mannose-6-phosphate attached to it?

Answer 7

Hydrolases

Question 8

How do proteins with a mannose-6-phosphate dissociate from the receptor once in the endosome?

Answer 8

Endosomes have lower pH, so a His residue in the receptor gets protonated and the oligosaccharide dissociates

Question 9

What are 3 endoplasmic reticulum lectins?

Answer 9

Chaperones, calnexin, and calreticulin

Question 10

What is the difference between calnexin and calreticulin?

Answer 10

Calnexin is an integral membrane protein and calreticulin is soluble

Question 11

How do liver lectins differentiate between young and old red blood cells?

Answer 11

Newly made red blood cells have sialic acid on their surfaces, which aren’t recognized by the liver lectins. Older red blood cells start to lose their sialic acid and get degraded

Question 12

What type of lectin gets white blood cells to the site of inflammation?

Answer 12

Selectins

Question 13

How do selectins get white blood cells to stop moving?

Answer 13

Selectins are transmembrane proteins expressed on both the white blood cell surface and the endothelial cell surface of blood vessels. P-selectin on the endothelial cells attach to the ones on the white blood cell and cause it to roll and slow down, then it slips through the blood vessel to the site of inflammation

Question 14

What 2 parasitic infections use lectins to get into cells?

Answer 14

Malaria and African sleeping sickness

Question 15

What 4 toxins use lectins to get into cells?

Answer 15

Cholera, pertussis, ricin, diphtheria

Question 16

How do the cholera, pertussis, ricin, and diphtheria toxins work?

Answer 16

They have a lectin targeting protein and a toxic peptide. The lectin portion binds to the target cell, then undergoes a conformation change to inject the toxic peptide into the cell

Question 17

Why are carbohydrates often difficult to work with?

Answer 17

They have huge structural variation, and it’s difficult to study all the different combinations. Many carbohydrates are also sulfated, and sulfate esters are unstable

Question 18

What enzyme would you use to cleave an oligosaccharide from a glycoprotein?

Answer 18

Endoglycosidase

Question 19

What enzyme would you use to cleave an oligosaccharide from a glycolipid?

Answer 19

Lipase

Question 20

What are 3 types of chromatography that can be used to separate out sugars?

Answer 20

Ion exchange, size exclusion, and affinity using lectins

Question 21

What are 3 ways to identify an oligosaccharide?

Answer 21

Mass spec, NMR, chemical synthesis

Question 22

What is exhaustive methylation?

Answer 22

Uses methyl iodide to replace all OH with a OCH3 in the presence of a base. Then treating with an acid breaks up the whole thing into monosaccharides, but any OH that were in a glycosidic bond stay as an OH and we can piece the oligosaccharide back together

Question 23

What is sequence elucidation?

Answer 23

Cleavage with exoglycosidases. Use a glycosidase that is specific to one monosaccharide and look for cleavage