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Hemolysins 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. The primary function of peptide hemolysins is pore formation at the cell membranes creating acytolytic effect, and is achieved by the release of cytosolic ions and small molecules through the hydrophilic, transmembrane portion of the beta-barrel pore .

spinqualitylabelsall models Stapholococcal alpha-hemolysin Show:Asymmetric Unit Biological Assembly Drag the structure with the mouse to rotate   Export Animated Image

FunctionFunction

Hemolysins are most commonly proteins found in red blood cells that allow for the rapid release of small molecules and ions across the membrane. or lipid biosurfactants that disrupt membrane composition resulting in cell lysis. Hemolysins act through disruption of the cell membrane. Pore formation 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 . 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.

StructureStructure

Hemolysins have three structural variations: alpha, beta, and gamma. These hemolysin types are comprised of di-, hepta- or octomeric subunits.

• Alpha-hemolysin

Alpha hemolysin causes a partial lysis of red blood cells. The heptameric pore assembles from water-soluble subunits. The alpha subunit, depicted right, consists seven identical monomeric units that exhibit rotational symmetry in oligomerized form. Each distinct subunit is differently colored for easy identification. The beta-barrel transmembrane domain is 50 Å in length.

Hydrophobic

Polar

Pore formationPore formation

Pore formation of hemolysins is believed to be a conserved process across subtypes. 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 proposed mechanism of pore formation in the cell membrane.

Medical ImplicationsMedical Implications

Role in infectionRole in infection

Hemolysin lysis of red blood cells is a marker for many kinds of pathogenic infection characterized by death of red blood cells.

OncologyOncology

Thermostable direct hemoslysin (TDH) is one type of hemolysin, secreted by Vibrio parahaemolyticus, that may be linked to the down regulation of colon carcinoma cell proliferation. The presence of this hemolysin is responsible for the influx of calcium ions from the extracellular space, activating protein kinase C, an inhibitor of tyrosine kinase activity on a key growth factor of this cancer.<ref>http://www.sciencedirect.com/science/article/pii/S030441651200116X<ref>

Hemolytic anemiaHemolytic anemia

Hemolytic anemia occurs when lysis of red blood cells occurs are rates faster than they can be replaced by bone marrow. Hypoxia due to this condition can result in dizziness, shortness of breath, poor maintenance of body temperature, and jaundice. <ref>https://www.nhlbi.nih.gov/health/health-topics/topics/ha<ref>