Vwf cleaving protease - ADAMTS13 Flashcards
What is the function of ADAMTS13? Where does it perform its function?
It regulates the activity of vwf by proteolytic cleaving to reduce multimer size
It has high specificity for cleaving at the A2 domain
How do vwf multimers circulate? What are the binding sites observed?
They circulate in globular form so binding sites are hidden (A1 domain)
but the A3 domain (collagen binding) is exposed so allows binding in the quiescent form to damaged collagen quickly
What happens to vwf following binding to the collagen?
The shear forces of blood flow are exerted upon vwf and this causes it to lose its globular conformation and exposure of platelet binding sites
What is the relationship between vwf multimer size and its function?
Smaller vwf multimers have lower binding sites so lower haemostatic ability in comparison to the large ones
Large multimers are prone to spontaneous unravelling hence cause problems
How are large multimers different to smaller multimers of vwf?
Large multimers bind platelets and collagen with higher affinity than dimers
more binding sites
more reactive to shear forces
At what site does ADAMTS13 bind and cleave vwf?
A2 domain, Tyr1605-Met1606
Why can’t ADAMTS13 cleave vwf all the time?
Vwf needs to be unravelled in order to allow for ADAMTS13 binding and cleaving, unravelling exposes the A2 domain
What is the importance of the c terminus in ADAMTS13?
The cub1 domains are implicated in binding to globular vwf - some ADAMTS13 circulates in complex with globular vwf
What is the function of the TSP-1 repeats?
They bind to vwf d4 and ck
What is the function of the spacer domain?
also binds vwf and is essential for function, it binds with high affinity to the a2 domain
involved in binding and recognition
How does ADAMTS13 recognise vwf?
the cystein rich domain recognises the a2 domain
What is the function of the disintegrin domain?
It is important for proteolysis
What does the metalloprotease domain do?
I has 3 his residues which hold a Zn2+ ion
ADAMTS13 is a zinc dependent metalloprotease
How does ADAMTS13 recognise shear-unfolded vwf?
there are complementary exosites and when the vwf unravels there is interaction between these and the domains
the scissile bond is exposed when there is unwinding
How does ADAMTS13 have high specificity for the a2 domain of vwf?
There is sequential recognition of the binding sites which must occur before there can be proteolytic cleavage.
4 sites identified so far
When does ADAMTS13 cleave vwf?
The proteolysis occurs when the vwf is secreted from the endothelium, it is secreated in strings and these are proteolysed into smaller multimeric forms by ADAMTS13
Where does ADAMTS13 act?
- Release from endothelium
- In free circulation - only UL-VWF that spontaneously unravels
- At sites of vessel damage - to limit platelet plug growth
What happens when there is aberrant ADAMTS13 activity?
- excessive vwf proteolysis - loss of HMW multimers leading to bleeding - vwd
- impaired proteolysis leading to accumulation of UL vwf in plasma - thrombotic thrombocytopenic purpura
What is vwd 2a?
mutations in vwf in the A2 region which destabilise the A2 fold
leads to promotion of unfolding under lower shear forces - proteolytic susceptibility
What is VWD 2B?
Mutations lie in proximity to vwf A1 domain and promote exposure of the platelet binding site in the A1 domain- increased platelet binding leading to promotion of A2 domain unfolding - proteolysis
Platelets act as pull on the vwf which can increase its unravelling from shear force
What is Heyde’s syndrome?
Acquired vwd
Aortic stenosis leads to increase of shear stress on the blood- increased vwf unravelling - increased proteolysis
Leads to angiodysplasia and epistaxis
How does TTP occur?
What is congenital TTP called?
High concentrations of plasma UL-VWF
this spontaneously unravels in the circulation, which is not cleaved by ADAMTS13
this then binds platelets leading to platelet rich thrombi accumulating in the microvasculature
Upshaw Schulman syndrome - both alleles for ADAMTS13 affected
What is aquired TTP?
Autoimmune TTP
caused by production of IgG against ADAMTS13 leading to inhibition and clearance
