The ubiquitin proteasome system is the major cellular system for the regulated degradation
of short-lived, damaged or defectively folded cytoplasmic proteins. It consists of the
ubiquitination system, which selects and targets proteins, and the proteasome that performs
the degradation. The immunoproteasome is a variant form of the standard proteasome.
Immune system cells express a higher level of immunoproteasome, while in other cells,
oxidative stress and proinflammatory cytokines are the stimuli that lead to elevated
production of immunoproteasome. Upon induction, the immunoproteasome catalytic
subunits: caspase-like, trypsin-like and chymotrypsin-like subunit, are synthesized and
incorporated, replacing their standard proteasome counterparts β1, β2 and β5, respectively.
Although the immunoproteasome is thought to be specialized in the generation of major
histocompatibility complex class I antigenic peptides with hydrophobic C-terminal residues,
increasing evidence indicates additional non-immune functions. Elevated levels of
immunoproteasome catalytic subunits have been also observed in certain disease states, such
as cancer, inflammatory bowel disease and neurodegenerative diseases. For all these reasons
immunoproteasome is becoming an interesting pharmacological target.
In the presented work, we successfully designed, synthesized and structurally characterized
twelve new compounds with inhibitory activity on the chymotrypsin-like catalytic subunit
of the immunoproteasome. All of the synthesized compounds were based on a psoralen
scaffold, on which structural changes were introduced at the positions 3 and 4'.
The synthesis of final products was performed in several consecutive reaction steps. First,
we synthesized 7-hydroxycoumarin ring by reacting resorcinol with diethyl 2-
acetylsuccinate and concurrently performed the alpha bromination of selected heterocyclic
ketone derivates. Then, the alpha brominated ketones were introduced to properly modify
the hydroxyl group of the 7-hydroxycoumarin ring. In the next stage, the condensation
reaction was performed leading to the formation of the psoralen-3-acetic acid derivatives.
Finally, the carboxylic acid of the prepared compounds was transformed into activated esters and N-cyanomethylamides. These can form a covalent interaction with the threonine in the
active site of the immunoproteasome.
At the Department of the Clinical Biochemistry, the inhibitory activity of the synthesized
products was evaluated. This was performed by determination of the residual activity of the
chymotrypsin-like activity of the immunoproteasome in the presence of inhibitors. We have
discovered that compounds with 2,5-dimethylthiophene heteroaromatic system at the
position 4' of the psoralen ring and an activated succinimidyl ester show the best inhibitory
activity. The most potent inhibitor in the presented series was compound, which possessed
both above-mentioned fragments in its structure.