Lecture 2 Flashcards
MUC =
Mucins in humans
Muc =
Mucins in mice
What is the major mucin in the small intestine?
MUC2
What happens to aspirin when it goes into the stomach?
When combined with the acid it becomes uncharged and so it can cross the cell membrane
What happens do ethanol when it is in the stomach?
It has its own charge and so it can be absorbed across the stomach
What type of organ is the stomach?
It is a non-absorptive organ
What types of mucin are absorbed in the stomach?
MUC5AC and 6
What is the MW of a mucin polymer?
2million MW and when it forms complexes the MW is 10 million
What is the structure of a mucin polymer?
Has a variable tangent repeat, a density glycosylated region, Globular domains and disulphide bridges linking subunits into polymers
What are VNTR also called?
An STP region as they contain serine, theronine and proline
What can attach to the STP region?
Carbohydrates can attach
What are the regions between the VNTR?
They are exposed to proteolytic enxymes
How do the mucin polymers polymerise?
By disulphide brides - can polymerise at the C and N terminal
What needs to happen in order for mucin to form a gel?
Mucin needs to be polymerised
What can the gaps in between the VNTR be degraded by?
Can be degraded by pepsin
What ends up in the colon?
A soluble mucin glycoprotein that can’t be further digested by proteinase, so it wont be able to form a gel.
What type of balance is needed in the stomach?
A dynamic balance - between secretion and the pepsin that is degrading the mucus from the surface - a dynamic balance is needed to maintain the thickness of the mucus
What is pepsin?
It is a damaging agent and is found in the stomach in gastric juice
What pepsin subgroups are found in gastric juice in humans?
A and C
Can pepsin diffuse through mucus?
No because it is too large
How does pepsin get to the cell surface?
Mucus would need to be digested
What pepsin is the most negatively charged?
Pepsin 1
What type of pepsin is the least negatively charged?
Pepsin 6
What are two ways to separate pepsins?
Gel electrophoresis and HPLC
What are the main pepsins in gastric juice?
5,3,1- and they are separated by anion exchange
What happens to pepsin during reflux?
Some of the gastric juice will end up on your larynx, this will be neutralised but the pepsin will be stuck to the larynx. Th next reflux there is a drop in pH, as the pepsin that got stuck is already sitting there
When is pepsin activated?
When there is a change in pH
What covers the active site of pepsin?
A lid which is formed by the n terminal
What are the two points of cleavage on the pepsin molecule?
16 and 17 and 44 and 45
Where does pepsin cleave?
Between 2 hydrophobic amino acids
What is the difference between pepsin and pepsinogen?
Pepsin has a bit missing, and end chopped off
What happens to the lid at a pH of 5?
To is held away from the active site, and it is held up by salt bridges or charge-charge interactions
What happens above pH5?
Histamine, lysine and arginine and +ve charged and aspartate and glutamate are -ve charged
What happens when the pH decreases below pH5?
Aspartate and glutamate will get protonated - they wont be charged anymore, they have lost the negative charge
What happens when the bridge is lost? Below pH5
The lid will drop into the active site, the active site becomes activated and cleaves the end terminal between 1-16 amino acids
What can propepsin interact with?
Other propepsin molecules or full active pepsin molecules
What is the pepsin structure?
N terminal is slightly different to the c terminal, a central valley is where the active site is held
What is pepsinogen structure?
The N and C terminals are almost identical
What does the active site in the pepsin structure rely on?
Relies on two aspartate residues (32 and 215)
What is the active site residue reliant on?
Relies on one aspartate being ionised and the other being protonated
Can you reform pepsinogen from Pepsin?
No - because pepsin has the N terminal chopped off
When does pepsin denature?
At pH8
What is the structure/ stability of pepsin at pH 8?
Long and denatured, the molecule has been unfolded
What happens if pepsin was to be refolded after it had been denatured?
It won’t fold properly because the n terminal is not in the right shape, instead of a valley for the active site there is a an open plane - you can reactivate pepsin once it has been denatured
How long is pepsin in humans?
47 amino acids long
Why is pepsin a good antimicrobial?
Because it will digest the bacteria. The bits cleaved off the pepsin will also have good antimicrobial activity
