Second, they have a unique quaternary
structure compared to other cysteine proteases.
18,19
It appears that each active
caspase is an
α
2
β
2
tetramer composed of two 17- to 20-kDa and two 10- to
12-kDa subunits, with each active site involving one
α
and one
β
subunit.
18-20
Third, the caspases are all synthesized as zymogens, with each precursor
polypeptide giving rise to one large subunit and one small subunit during
an activation process that involves proteolytic cleavage at Asp-X cleavage
sites.
18,20-22
The fact that these precursors are activated by cleavage at the same
type of bond they themselves cleave raises the possibility that the various
caspases activate each other
23-25
and are also capable of autoactivation.
26-29
This concept will be elaborated upon in subsequent Sections.
Despite these similarities, there are also differences among the