Sandbox WWC6: Difference between revisions

From Proteopedia
Jump to navigationJump to search
← Older editNewer edit →
No edit summary
No edit summary
Line 46: Line 46:


== Mechanism of action ==
== Mechanism of action ==
The S1 through S4 segments make up the gating mechanism. <ref name= "crystal"/> In particular, the S4 segments consist of repeated motifs of a positively charged residue (usually Arg) followed by two hydrophobic residues. <ref name= "crystal"/> This alpha helix is exposed to the membrane electric field, and in response to a depolarization, it is displaced outwards, causing the channel to open.<ref name= "crystal"/> S4 segments can be seen in red <scene name='69/696300/S4/1'>here</scene>. Surprisingly, each S4 chain does not contribute equally to the gating of this channel. A mutation in one chain may have a much larger effect than a mutation in another S4 chain. <ref name= "crystal"/> However, these S4 segments are merely sensors. The actual gating mechanism is still up for debate, with the leading contender being the S6 segment. This mechanism would be similar to the gating of the K<sup>+</sup> channel. <ref name = "struct"/> The outward movement of the S4 segment moves the S4-S5 linker, which pulls the S5-S6 segments and opens the pore. <ref name= "crystal"/> Overall, the voltage sensing domain makes a rolling motion of the S4-S5 linker around the pore. <ref name= "crystal"/> The S4-S5 linker can be seen <scene name='69/696300/S4_s5_linker/2'>here</scene>.


===Pore formation===
This channel conducts sodium at nearly the rate of free diffusion. <ref name= "crystal"/> This pore consists of an outer funnel-like vestibule, a selectivity filter, a central cavity, and an intracellular activation gate. <ref name= "crystal"/> Unlike the K<sup>+</sup> channel, the Na<sup>+</sup> channel conducts sodium ions that are hydrated with four water molecules. <ref name= "crystal"/> Much like with the gating mechanism, each domain does not contribute equally to the selectivity of the pore, but the channel selects for sodium 100x greater than it selects for any other ion. <ref name= "struct"/>
It is likely that the sodium ion can get much closer to the channel entry than a larger K<sup>+</sup> ion. When the ion reaches this distance, there is a more efficient removal of water and the sodium interacts with the Glu117 side chains. Two side chains interact directly with the sodium ion and two additional side chains form hydrogen bonds with the water molecules <ref name= "crystal"/>. The pore is made up of T175, L176, E177 and S178. <ref name= "crystal"/> The Glu side chains act as hydrogen bond acceptors two in-plane molecules surrounding the sodium ion. <ref name= "crystal"/> The other two water molecules would be axial to the sodium ion. <ref name= "crystal"/> Full rehydration would occur when the ion moves next to the Leu and Thr residues. <ref name= "crystal"/> Full rehydration then allows the sodium ion to enter the cytoplasm. <ref name= "crystal"/>


To see the selectivity filter, click <scene name='69/696300/Selectivity_pore/2'>here</scene>. T175 is blue, L176 is yellow, E177 is red and S178 is black.


[[Image:Nature10238-f3.2.jpg]]




Four of each of the two subunits assemble in an alternating, circular pattern in the γ-HL pore, whereas seven ​distinct α-HL protomers assemble in a circular arrangement in the ​α-HL pore. These typically are comprised of three domains: the cap, rim and stem domains, named for the structural resemblance to a mushroom. The cap domain contains β-sandwiches from each protomer, while just below, the rim domain contains four looping β-strands. The stem domain takes on the antiparallel β-barrel, a portion of which becomes the transmembrane structure.
===Pore formation===
Studies suggest that pore formation is achieved through a nonlytic intermediate oligomer, known as a prepore.  The prepore model proposal suggests that the monomeric components assemble on the cell membrane surfacte into a prepore with prestem subunits packed inside. The formed prepore then goes through a conformational change prestem, forming the β-barrel pore. Several issues with the proposed pore formation mechanism have been identified including steric hindrance of the packed prestem structure.


This image shows the crystal structure of the selectivity filter.  <ref name= "crystal"/>
This image shows the crystal structure of the selectivity filter.  <ref name= "crystal"/>

Revision as of 10:54, 13 May 2016

Hemolysins [1] are a lipid or protein toxins secreted by pathogens that lyse erythrocyte and some bacterial cell membranes. These toxins belong to a family of microbial exotoxins called cytolysins, which act on a broad number of cells[2]. The primary function of peptide hemolysins is pore formation at the cell membranes creating acytolytic effect, and is achieved by the release of cytosolic K+ ions through the hydrophilic, transmembrane portion of the beta-barrel pore[3].

Stapholococcal alpha-hemolysin

Drag the structure with the mouse to rotate

Function

Hemolysins are most commonly proteins found in red blood cells that selectively allow for the diffusion of potassium ions across the membrane. [1] or lipid biosurfactants that disrupt membrane composition resulting in cell lysis. These proteins are important for some erythrocyte nutrient accession, but cause massive erythrocyte destruction in bacterial infection, specifically responsible forhemolytic anemia, which causes fatigue, pain, arrythmias, and even heart failure in affected individuals.[2] Each hemolysin pore is composed of three subunits: the alpha subunit, which is the transmembrane ion channel, and two beta subunits that modulate channel gating and regulate the channel expression in the membrane. [3] [4] The alpha subunit is able to function independently of the beta subunit. [3]


Hemolysins act through disruption of the cell membrane. Two main functions destroy phospholipid membranes: pore formation and phosphilipid hydrosysis. [5] Pore formation, the most common mechanism of hemolysin cell [4] is the olgomerization of the pore sunbunits within the membrane. The pore is quickly filled with water, ions, and small molecules that rapidly exit the cell, dissipating ionic gradients and membrane potential. Osmotic pressure causes a rapid swelling of the cell, leading to total rupture of the membrane Cite error: Closing </ref> missing for <ref> tag These structures consist of six transmembrane alpha helices named S1 through S6. [6] Interestingly, each repeating subunit resembles a bacterial K+ channel. [6] These subunits fold together to form a central pore, and this complete structure resembles a bacterial Ca2+ channel. [6]


Alpha-hemolysin

Alpha-hemolysin

Alpha hemolysins cause a partial lysis of red blood cells. The heptameric pore assembles from water-soluble subunits The transmembrane domain of this water-filled pore is primarily comprised of an anti-parallel beta-barrel


Beta-hemolysin

Beta-hemolysin

Beta-hemolysins cause a total lysis of red blood cells.



Gamma-hemolysin

Gamma-hemolysin


Gamma-hemolysin is both hemolytic and leukotoxic.

Pathogenic Microorganisms

Pore-forming toxins have been shown to closely relate to the pathogenicity of the toxin-producing organism Cite error: Closing </ref> missing for <ref> tag

Oncology

This disease causes seizures, fainting or sudden death from cardiac arrhythmias and is caused my a mutation in the SCN5A gene, or the gene that encodes the NaV1.5 alpha subunit. [7][8] It was found that this deletion includes residues 1505-1507 (KPQ).[7] These residues occur in the cytoplasmic linker between domain III and domain IV. [7]

Hemolytic anemia

Hyperkalemic period paralysis is caused by the mutations T704M, S906T, A1156T, M1360V, A1448C and/or M1592V. [9] These mutations cause periodic or permanent weakness. [9] Physiologically, this is a gain of function mutation. During rest after exercise, or after eating foods rich in K+, the extracellular K+ increases, which mildly depolarizes the membrane.[9] This causes abnormal Na+ channels to open, and they are unable to inactivate. [9] This sustained depolarization of the membrane causes even more abnormal Na+ channels to open and ultimately this leads to loss of excitability and weakness. [9] This symptom usually appears within the first decade of life and can be aggravated by exercise, cold, potassium loading, fasting or pregnancy. [9] Attacks are usually brief and do not need treatment. [9]


Marker for fungi exposure

Many indoor fungi have been shown to produse both alpha and beta-hemolysins. The treatment of blood samples with




Treatment

References

  1. ↑ https://en.wikipedia.org/wiki/Hemolysin#cite_note-pmid20692229-3
  2. ↑ https://www.nhlbi.nih.gov/health/health-topics/topics/ha/
  3. ↑ 3.0 3.1 Isom LL. Sodium channel beta subunits: anything but auxiliary. Neuroscientist. 2001 Feb;7(1):42-54. PMID:11486343
  4. ↑ 4.0 4.1 Cite error: Invalid <ref> tag; no text was provided for refs named sod
  5. ↑ https://www.sciencedirect.com/science/article/pii/S0005273610002610
  6. ↑ 6.0 6.1 6.2 Cite error: Invalid <ref> tag; no text was provided for refs named struct
  7. ↑ 7.0 7.1 7.2 doi: https://dx.doi.org/10.1016/0092-8674(95)90359-3
  8. ↑ https://www.mayoclinic.org/diseases-conditions/long-qt-syndrome/basics/definition/con-20025388
  9. ↑ 9.0 9.1 9.2 9.3 9.4 9.5 9.6 https://neuromuscular.wustl.edu/mother/activity.html#hrpp
Retrieved from "https://proteopedia.org/index.php?title=Sandbox_WWC6&oldid=2599133"