User:Loganne Wertz/Sandbox1

Caspase-6 is an endopeptidase[1] involved in apoptosis. In terms of its catalytic function, it is a part of the cysteine-aspartate family [2]. Before Caspase-6 becomes functional and active, the enzyme exists as a , also known as a zymogen [3][6]. In solution, two zymogens are associated together, forming a homodimer. Zymogen activation, the process by which Caspase-6 becomes active, is largely conserved across the caspase family.

However, Caspase-6 is unique in that it becomes active through self-cleavage rather than cleavage by a separate enzyme. Each zymogen of the unprocessed enzyme contains a  consisting of two helices and  subunit consisting of three helices, a , as well as an intersubunit linker.  The helices surround a beta sheet core. In order to become active, the intersubunit linker is bound to the active site of Caspase-6, where it is then cleaved.  After cleavage, the four processed subunits, two originating from each zymogen, remain closely associated together through intermolecular forces, forming a dimer of dimers.

Caspase-6 Protein

Specifics of Zymogen Activation

Caspase-6 can be activated by acting as a substrate for other caspases, particularly Caspase-3, as well as other enzymes. It becomes cleaved by these enzymes and proceeding to the same conformation as mentioned above. It was observed that Caspase-6 became active without alternate enzymes present, which suggested that Caspase-6 utilizes a self-cleavage mechanism. Now, self-processing is recognized as the primary mechanism for Caspase-6 activation. In addition to the intersubunit cleavage, cleavage and removal of the pro-domain must also occur for the enzyme to become active. These cleavages are both sequence specific and ordered. First, the site within the pro-domain, following the sequence TETD23, must be cleaved. (The residues of the pro-domain are not visible in the crystallized structure as they have already been removed.) After the pro-domain is removed, cleavage of the intersubunit linker can occur either following the amino acid sequence DVVD179 or TEVD193. Despite the sequence similarities between TETD23 and TEVD193 cleavage sites, the TETD23 cleavage site is always cleaved before TEVD193. It has been proposed that this sequence of cleavage is due to the structure of Caspase-6's zymogen, which allows the pro-domain to be more readily available to enter the active site. To some extent, the pro-domain inhibits Caspase-6's ability to cleave the intersubunit loop and self-activate, but this happens in a currently unknown mechanism. The result of the TETD23 cleavage site priority is that the prodomain acts as a “suicide protector”, which protects the TEVD193 cleavage site from self-cleavage[3]. This protection is necessary when there are low levels of inactive proteins, which must be preserved, in the tissue. The intramolecular cleavage of TETD23 and DVVD179 or TEVD193 are essential for the initiation caspase-6 activation without other caspases present. After both cleavages occur, the processed Caspase-6 can be found in solution as a dimer of dimers.