Molecular Insight into Propeptide–Protein Interactions in Cathepsins L and O

Cathepsins are mammalian papain-like cysteine proteases
that play
an important role in numerous physiological and pathological processes.
In the present study, various molecular dynamics (MD) simulations
of pro- and mature human cathepsins L and O were performed. This study
is the first to report MD simulations to complement the initial model
structure of (pro‑)­cathepsin O through conformational sampling,
thus offering insight into the maturation of procathepsin O, which
to date has not been described experimentally. The overall fold of
(pro‑)­cathepsin O appears very similar to that of (pro‑)­cathepsin
L. The propeptide binding loop (PBL)–propeptide interface of
both procathepsins is found to form a stable two-stranded β-sheet.
Additional stabilization of the PBL–propeptide interface is
provided by hydrophobic side chain contacts in procathepsin L, whereas
this seems to be due to charge-dipole interactions in procathepsin
O. Introduction of two mutations (L147P and G148P) into procathepsin
O entails a significant loss of hydrogen bonding, disabling formation
of the interfacial β-sheet. Simulations at different protonation
states suggest that procathepsin L is more sensitive to a change in
pH than procathepsin O. Potential differences between the maturation
of procathepsin O and procathepsin L inferred from the MD simulations
might be caused by (i) stronger PBL–propeptide interactions
in procathepsin O due to salt-bridge formation across the interface,
(ii) more limited entropic gain of the propeptide of procathepsin
O upon release into the bulk solvent due to diverse conformational
states sampled in the bound state, (iii) more pronounced entropic
loss of the PBL in procathepsin O upon substrate binding caused by
diverse conformational states sampled in the free, mature enzyme,
and (iv) lower sensitivity of procathepsin O to pH change caused by
the presence of fewer carboxylate groups at the PBL–propeptide
interface.