Apoptosome - The Gatekeeper of Apoptosis
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The apoptosome is a quaternary protein - an enzyme - that forms during apoptosis (or programmed cell death). Its function is not to directly keep cellular homeostasis nor disassemble damaged, infected or cancerous cells. Rather, this enzymes' function is to activate specific procaspases that will carryout disassembly under the aforementioned conditions.
The process of programmed cell death is completed in a cascade of events. These events involve both extrinsic and intrinsic pathways of apoptosis. While both pathways involve the activation of procaspases (that eventually become active caspases), the apoptosome is found to aid the process of the intrinsic pathway, in the mitochondria.
Caspases are cystine proteases and are considered the executioners of apoptosis. As the caspases are activated, the sulfur atom reacts, in the cystine, to systematically cleave the amino acid aspartate in targeted proteins. The caspase that interacts with the apoptosome in the intrinstic pathway is caspase-9, the apoptosis initiator. In the active form, the apoptosome also cleaves procaspase-3 and -7 dimers.
Pro-apoptotic molecules are released from the mitochondria as a result of an interaction between the mitochondria and cell death signals. Cytochrome c, a protein involved in energy metabolism, is one of the most important pro-apoptotic factors in the formation of the apoptosome. Cytochrome c crosses the mitochondrial membrane into the cytoplasm where it interacts with the β-propellers located on the Apaf-1 subunit, thus, regulating apoptosome assembly.
The Apaf-1 (apoptosis protease activating factor 1) exists in the cytoplasm of cells and remains inactive until cytochrome c is present, binds to Apaf-1 and nucleotide exchange occurs. During this exchange, Apaf-1 is altered from its inactive form (ADP bound) to its active ATP bound form. Going from inactive to active also changes its conformation.
The heptameric structure of the apoptosome is formed as seven Apaf-1 subunits arrange around a central point. Cytochrome c is bound to Apaf-1, ATP is formed and the holo-apoptosome is now formed.
During activation of the holo-apoptosome, an acentric disk, or N-terminal caspase recruiting domain (CARD) is assembled. The card is the central linking point of the four Apaf-1 monomers and four procaspase-9 monomers. Procaspase-9 is thought to be activated upon interaction with the this CARD. However, it is also theorized that procaspase-9 is activated upon interaction with another domain on the enzyme: pc-9 catalytic domain.
Activated Apotosome and Procaspase-9 CARD complexThe apoptosome is a large enzyme made of of multiple parts. Once assembled, and excluding the CARD, this enzyme has 14 polypeptide chains: seven Apaf-1 and seven cytochrome c.
14 Polypeptide ChainsWhen examining the secondary structure, the Apaf-1 subunit is made up of 32 helices and 69 strands, making it 21 percent helical and 26 percent beta sheet. Cytochrome c chain is made up of five helices and two strands, making it 41 percent helical and one percent beta sheet.
Secondary StructureTwo ligands are found in the apoptosome: Heme and ATP. The heme ligand is bound to cytochrome c within the regulatory region, clamped between the β-7 and β-7 propellers of the Apaf-1. The ATP ligand is bound within the Apaf-1 itself and is located in the NBD (nucleotide binding domain).
LigandsThere are five domains within the Apaf-1 subunit: CARD, NBD, HD1 (helical domain 1), WHD (winged helix domain), and HD2 (helical domain 2). The NBD, as mentioned before, is the binding site for ATP. NBD is attached to the WHD and HD1, which makes up the hub of the apoptosome. The HD2 attaches to the β-7 and -8 propellers and forms the arm of the apoptosome Apaf-1 subunit.
Quaternary StructureBeing that the apoptosome is such an important part in apoptosis, an ineffective assembly or activation of this enzyme can result in uncontrolled cell growth. An ineffective cell death pathway can lead to many diseases: autoimmune, neurodegenerative, and cancer.
An abnormal apoptosis pathway can lead to cancer due to the un-mediated proliferation of cells, resulting in tumor growth. The homeostasis that is usually kept is not regulated nor is the removal of defective cells.
Cancerous cells can cause suppression of apoptosis through the secretion of proteins that inhibit the cell death process and/or the down-regulation of pro-apoptotic factors. As a result, the assembly of the apoptosome is not carried out due to the lack of initiation and procapase-9 is never activated.
Likewise, much research is being done in the link between the apoptosome and cancer and the possibility of reversing the formation of tumors.