Vwf cleaving protease - ADAMTS13

Question 1

What is the function of ADAMTS13? Where does it perform its function?

Answer 1

It regulates the activity of vwf by proteolytic cleaving to reduce multimer size
It has high specificity for cleaving at the A2 domain

Question 2

How do vwf multimers circulate? What are the binding sites observed?

Answer 2

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

Question 3

What happens to vwf following binding to the collagen?

Answer 3

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

Question 4

What is the relationship between vwf multimer size and its function?

Answer 4

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

Question 5

How are large multimers different to smaller multimers of vwf?

Answer 5

Large multimers bind platelets and collagen with higher affinity than dimers
more binding sites
more reactive to shear forces

Question 6

At what site does ADAMTS13 bind and cleave vwf?

Answer 6

A2 domain, Tyr1605-Met1606

Question 7

Why can’t ADAMTS13 cleave vwf all the time?

Answer 7

Vwf needs to be unravelled in order to allow for ADAMTS13 binding and cleaving, unravelling exposes the A2 domain

Question 8

What is the importance of the c terminus in ADAMTS13?

Answer 8

The cub1 domains are implicated in binding to globular vwf - some ADAMTS13 circulates in complex with globular vwf

Question 9

What is the function of the TSP-1 repeats?

Answer 9

They bind to vwf d4 and ck

Question 10

What is the function of the spacer domain?

Answer 10

also binds vwf and is essential for function, it binds with high affinity to the a2 domain
involved in binding and recognition

Question 11

How does ADAMTS13 recognise vwf?

Answer 11

the cystein rich domain recognises the a2 domain

Question 12

What is the function of the disintegrin domain?

Answer 12

It is important for proteolysis

Question 13

What does the metalloprotease domain do?

Answer 13

I has 3 his residues which hold a Zn2+ ion

ADAMTS13 is a zinc dependent metalloprotease

Question 14

How does ADAMTS13 recognise shear-unfolded vwf?

Answer 14

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

Question 15

How does ADAMTS13 have high specificity for the a2 domain of vwf?

Answer 15

There is sequential recognition of the binding sites which must occur before there can be proteolytic cleavage.
4 sites identified so far

Question 16

When does ADAMTS13 cleave vwf?

Answer 16

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

Question 17

Where does ADAMTS13 act?

Answer 17

1. Release from endothelium
2. In free circulation - only UL-VWF that spontaneously unravels
3. At sites of vessel damage - to limit platelet plug growth

Question 18

What happens when there is aberrant ADAMTS13 activity?

Answer 18

1. excessive vwf proteolysis - loss of HMW multimers leading to bleeding - vwd
2. impaired proteolysis leading to accumulation of UL vwf in plasma - thrombotic thrombocytopenic purpura

Question 19

What is vwd 2a?

Answer 19

mutations in vwf in the A2 region which destabilise the A2 fold
leads to promotion of unfolding under lower shear forces - proteolytic susceptibility

Question 20

What is VWD 2B?

Answer 20

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

Question 21

What is Heyde’s syndrome?

Answer 21

Acquired vwd
Aortic stenosis leads to increase of shear stress on the blood- increased vwf unravelling - increased proteolysis

Leads to angiodysplasia and epistaxis

Question 22

How does TTP occur?

What is congenital TTP called?

Answer 22

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

Question 23

What is aquired TTP?

Answer 23

Autoimmune TTP

caused by production of IgG against ADAMTS13 leading to inhibition and clearance