Carbohydrates

Question 1

What are pyranose structures and how are they formed?

Answer 1

A pyranose structure is the collective term given to a saccharide which forms a 6 membered ring of 5 carbons and 1 oxygen.

They are formed by the reaction of the OH group of C5 in the sugar’s linear conformation with the aldehyde C1, to form a hemiacetal. D-Glucose has a pyranose structure.

Question 2

What are furanose structures and how are they formed?

Answer 2

A furanose structure is the collective term given to a saccharide that forms a 5 membered ring of 4 carbons and 1 oxygen.

They are formed by the reaction of the OH group of C4 in the sugar’s linear conformation with the aldehyde C1, forming a hemiacetal. D-Fructose has a furanose structure.

Question 3

What is the name given to proteins that specifically bind to sugar molecules? What important roles might they play in the body?

Answer 3

Lectins

Cell-cell adhesion, glycoprotein synthesis regulation and cell signalling, for example

Question 4

Outline the science behind a historical blood sugar test

Answer 4

Use of Fehling’s solution
Cu(II) complex is blue- it oxidises the aldehyde group of the glucose to a carboxylate anion
This process reduces Cu(II) to Cu(I) which is red.
Red is positive for high glucose levels

Question 5

Outline the science behind modern day blood sugar tests

Answer 5

Glucose can be oxidised (reacted with oxygen) to gluconolactone and hydrogen peroxide by glucose oxidase.

Glucometer: Electrons released in the reaction induce a current which can be measured by an electrode. The current is proportional to glucose concentration

Colourimetric strips: Peroxidase immobilised in strip can give a colour change when reacted with the hydrogen peroxide

Question 6

How can carbohydrate molecules reinforce a structure’s resistance to compression

Answer 6

E.g. in cartilage, carbohydrates can form long chains called glycosaminoglycans, which attach to core protein molecules such as aggrecan to form proteoglycan super-structure.

This super-structure can become totally saturated in water due to the presence of hydrophilic sugar molecules as well as sulphate residues frequently found within the GAG chains.

The water enables these superstructures to act as shock absorbers, releasing water molecules slowly with impact and taking it back to reform a structure when the pressure of impact is released.

Question 7

What are the two ways that carbohydrates can bind to proteins?

Answer 7

N-linked glycosylation - where the hydroxyl/OH group of C1 of the sugar reacts with the amine/NH2 in an asparagine R-group to release water

O-linked glycosylation - where the hydroxyl/OH group of C1 of the sugar reacts with the hydroxyl/OH in a serine or threonine R-group to release water

Question 8

Describe the process of N-linked glycosylation

Answer 8

1. Form lipid-linked precursor oligosaccharide
   - ->synthesised first in cytoplasm, then in the lumen of the ER (precursor bound to the lipid dolichol phosphate in ER membrane but pointing out to cytoplasm, then transferred in to lumen for further addition of sugars)
   - ->almost always the sugar linking to the lipid is N-acetylglucosamine
2. The whole oligosaccharide unit (without the phospholipid) is transferred to the asparagine residue of a core protein that has been synthesised by a fixed ribosome and is in the lumen
3. Trimming and addition of saccharides (by glycosidases and transferases respectively) occurs in ER and Golgi, and then the proteoglycan is secreted in a secretory vesicle

Question 9

Describe the process of O-linked glycosylation (using mucin as an example)

Answer 9

Less methodical, but a mucin glycosylation is based on two core structures, core 1 and 2

N-acetylgalactosamine is linked to serine or threonine from a precursor by a transferase, and then:

• ->to form a core 1 structure galactose is added
• ->to form a core 2 structure both galactose and N-acetylgalactosamine are added

These cores then undergo certain modifications, such as sialylation, giving them a strong negative charge so that they can bind large amounts of water

This whole process occurs in the ER/Golgi

Question 10

Where are glycoconjugates made?
Where can they be secreted?
What are their functions?

Answer 10

Lumen of ER and Golgi

Secreted into extracellular fluids and matrices

Intrinsic functions such as providing structural components (ECM) and modifying protein properties

Extrinsic functions such as trafficking of glycoconjugates, cell-cell and cell-matrix adhesion, and signalling,