Page 69 - Read Online

Page 4 of 17 Stojkovska Docevska et al. Rare Dis Orphan Drugs J 2023;2:14 https://dx.doi.org/10.20517/rdodj.2023.09

PROCESSING AND ACTIVATION OF CATHEPSIN C
As mentioned in the previous section, cathepsin C is synthesized as an inactive single-chain precursor of
~ 60 kDa per subunit. At this stage, the unique exclusion domain presumably functions as an
intramolecular chaperone that aids in the folding of the protein . Evidence shows that human
[27]
procathepsin C is a homodimer [28,29] . It contains four N-glycosylation sites, one in the heavy chain region of
the peptidase domain (Asn252) and three in the exclusion domain (Asn5, Asn29, and Asn95), all of which
are retained in the mature form of the enzyme. Proper N-glycosylation is critical for the proper processing
[30]
and transport of procathepsin C , as well as for the assembly of the oligomeric structure, since
recombinant non-glycosylated human cathepsin C produced in E. coli is a monomer . There is also some
[31]
[4]
interspecies variability in the number of N-glycosylation sites between mammals and in the oligomeric
states of cathepsin C orthologs, as rat cathepsin C was reported to be a dimer .
[32]
Procathepsin C is transported from the endoplasmic reticulum to lysosomes via the mannose-6-phosphate
pathway [33,34] . Unlike other cysteine cathepsins, procathepsin C is incapable of autocatalytic processing and
must be activated by other lysosomal peptidases. Cathepsins L and S were identified as the first cathepsin C
[28]
activating peptidases . The maturation of procathepsin C is a multistep process that involves the removal
of the internal propeptide segment and the cleavage of the catalytic domain into a heavy chain and a light
[28]
chain. Full processing is required to obtain the final active conformation . However, it has been shown
that cathepsins L and S are not required for the activation of procathepsin C in mice . Similarly, a study on
[35]
human neutrophil progenitor cell lines PLB-985 and HL60 revealed that complete inhibition of CatS was
not sufficient to completely block the activation of procathepsin C, suggesting that other peptidases are
[36]
involved in this process . Recently, cathepsins F, K, and V were shown to activate procathepsin C in vitro
[29]
via the same intermediate species as cathepsins L and S . These results suggest that the maturation of
procathepsin C is a redundant process that can be carried out in vivo by different peptidases in tissue-
specific patterns.

The purpose of the tetrameric structure of cathepsin C, in contrast to the monomeric forms of other related
peptidases, has remained largely unexplained. Olsen et al. suggested that the tetrameric structure merely
stabilizes the interaction of the exclusion domain with the peptidase domain, thus maintaining the
dipeptidyl-peptidase activity . An early report also indicated that cathepsin C is an allosteric enzyme that
[37]
exhibits pH- and cofactor-dependent cooperative effects . However, these results were later disputed by
[23]
[14]
others . A partial inhibitor of cathepsin C, i.e., one that enables the enzyme to retain part of its activity in
the enzyme/inhibitor complex, has been described that could presumably act via allosteric mechanisms at
the level of individual subunits , a definitive answer to the question of cooperativity between subunits in
[31]
cathepsin C is still pending.

The non-covalent interactions of the exclusion domain with the peptidase domain and between subunits are
strong, and the tetramer does not dissociate in the presence of 2 M guanidinium chloride, but with further
increased concentration, the oligomer is completely destroyed . The recombinant peptidase domain alone
[38]
[39]
has been shown to have similar endoproteolytic activity to other papain-like peptidases . In addition,
canine and rabbit cathepsins C have been shown to cleave substrates typical of PLP endopeptidases under
certain conditions [40,41] , suggesting that the exclusion domain can be removed.

PHYSIOLOGICAL AND PATHOLOGICAL FUNCTIONS OF CATHEPSIN C
As pointed out in the introduction, a major physiological role of cathepsin C is the activation of latent
zymogens of effector serine peptidases in immune cells. These include four neutrophil serine peptidases
(NSPs),i.e., neutrophil elastase, proteinase 3, cathepsin G , and the recently discovered neutrophil serine
[42]