Cells undergo apoptosis due to a responsible enzyme. It is called, Caspase. It is a Cysteine aspartic arotease This acronym forms the name caspase. The reason this acronym is used is because Caspase is a cysteine protease ( protease that uses cysteine as catalytic residues ) that is directed with aspartate amino acids. In other words, Caspase cuts polypeptides where there are aspartate amino acids, using its catalytic cysteine amino acid residues.
There are a total of 12 caspases in humans. They both have active forms and inactive forms called procaspes. The reason there are procaspases is because activated caspases can be dangerous in its active form when it's not needed.
Caspase 1 is a fairly small heterodimeric enzyme, made up of two different subunits. It activates other proteins and enzymes. It plays a role a little bit in apoptosis and prefers more action in the immune system, and is an imflammatory caspase.
Caspase 2 is a bigger enzyme made up of four different subunits forming a heterotetramer because the 2 same subunits are made up of 2 smaller subunits that are different from eachother. This in total is 4 subunits but 2 are same and 2 are different. This therefore means caspase 2 is a heterotetramer. It interacts with many proteins and Caspase 8.
Caspase 3 is similar in structure to caspase 1, slightly different. It is also a small heterodimer.
Caspase 4 does not really play a role in apoptosis. But more of the immune system. It is found to be involved in the imflammatory response.
Caspase 5 is also an imflammatory caspase like caspase 1 and 4.
Caspase 6 is a bigger enzyme, a homotetramer, that prefers apoptosis activity. It is activated by Caspase 7,8, and 10. It interacts with Caspase 8.
Caspase 7 is a big enzyme and is a homotetramer. It interacts with caspase 8. It is activated by caspase 8 and 9.
Caspase 8 also cleaves proteins in apoptosis. It mostly activates caspase 3. It is a heterodimer, again small and similar in structure to casp. 1 and 3.
Caspase 9 is a homotetramer that is big, and is in apoptosis. It is activated by the apoptosome. This enzyme activates casp 3 and 7.