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Complement System01:27

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The complement system is a group of approximately 20 plasma proteins that strengthen the body's defenses against infections through opsonization, inflammation, and cell lysis. Opsonization involves coating pathogens with complement proteins, making them more recognizable and facilitating phagocyte engulfment. Certain complement proteins induce inflammation that attracts immune cells to the site of infection. Cell lysis involves the destruction of pathogens through the formation of a...
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Extrinsic and Intrinsic Pathways of Hemostasis01:20

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Blood clotting or coagulation involves extrinsic and intrinsic pathways, which ultimately merge into the common pathway, forming a fibrin clot.
The Extrinsic Pathway
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After a fibrin clot is formed, the next step is clot retraction, a vital process facilitated by platelet contractile proteins, such as actin and myosin. These proteins pull the fibrin strands closer together and condense the clot. This action reduces the size of the clot, creating a smaller, denser structure that effectively seals off the damaged vessel. Clot retraction consolidates the clot and helps with wound healing by bringing the edges of the damaged blood vessel closer together.
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Formation of the Platelet Plug01:22

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The platelet phase, the second stage of hemostasis, commences around 15-20 seconds after an injury. It follows and overlaps with the vascular phase, during which blood vessels constrict to minimize blood loss.
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Hemostasis is a crucial process that prevents excessive blood loss from damaged blood vessels. It involves various mechanisms such as vasoconstriction, platelet adhesion and activation, and fibrin formation. The importance of each mechanism depends on the type of vessel injury. In contrast, thrombosis is the abnormal formation of a blood clot within the blood vessels, leading to potential complications if the clot obstructs blood flow. Thrombosis can be caused by increased coagulability of the...
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When a ligand binds to a cell-surface receptor, the receptor's intracellular domain changes shape, which may either activate its enzyme function or allow its binding to other molecules. The initial signal is amplified by most signal transduction pathways. This means that a single ligand molecule can activate multiple molecules of a downstream target. Proteins that relay a signal are most commonly phosphorylated at one or more sites, activating or inactivating the protein. Kinases catalyze...
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Related Experiment Video

Updated: Mar 6, 2026

Measuring the 50% Haemolytic Complement CH50 Activity of Serum
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Functional and structural insight into properdin control of complement alternative pathway amplification.

Dennis V Pedersen1, Lubka Roumenina2,3,4, Rasmus K Jensen1

  • 1Department of Molecular Biology and Genetics, Center for Structural Biology, Aarhus University, Aarhus, Denmark.

The EMBO Journal
|March 8, 2017
PubMed
Summary

Properdin

Keywords:
alternative pathwaycomplementdeficiencyproperdinstructure

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Area of Science:

  • Immunology
  • Structural Biology

Background:

  • Properdin (FP) is crucial for the complement alternative pathway (AP).
  • FP's oligomeric structure complicates detailed structural analysis.
  • Understanding FP's structure-function relationship is key for complement modulation.

Purpose of the Study:

  • To investigate the role of properdin oligomerization in complement alternative pathway function.
  • To characterize a novel monomeric properdin deficiency (E244K) and its impact on AP activity.
  • To gain structural insights into properdin-convertase interactions.

Main Methods:

  • Site-directed mutagenesis to create recombinant FP E244K.
  • Biochemical assays to assess AP activity and bacteriolysis.
  • Crystal structure determination of FP-convertase complex.
  • Small-angle X-ray scattering (SAXS) to analyze FP conformation.

Main Results:

  • A novel FP deficiency (E244K) mutation leads to monomeric FP with severely reduced AP activity.
  • Monomeric FP, both E244K and excised units, are dysfunctional in bacteriolysis.
  • Structural analysis reveals key interactions between FP thrombospondin repeat and C3b in convertase stabilization.
  • FP E244K is conformationally locked in a monomeric state, preventing oligomerization.

Conclusions:

  • Properdin oligomerization is essential for its function in the complement alternative pathway in vivo.
  • Monomeric FP provides valuable structural insights into complement regulation.
  • These findings pave the way for developing novel complement modulators.