Active site-directed inhibitors of Rhodococcus 20 S proteasome. Kinetics and mechanism

Area of Science:

• Biochemistry
• Enzymology
• Molecular Biology

Background:

• The Rhodococcus proteasome is a key enzyme with a single active site type.
• Understanding proteasome inhibition is crucial for drug development.
• Small molecule inhibitors offer targeted approaches to enzyme modulation.

Purpose of the Study:

• To investigate the mechanism of inhibition of the Rhodococcus proteasome by various small molecule inhibitors.
• To identify the specific modification sites of inhibitors on the proteasome.
• To elucidate the cross-reactivity and sequential inhibition patterns.

Main Methods:

• Enzyme kinetics studies using Suc-Leu-Leu-Val-Tyr-AMC substrate.
• Peptide mapping of tryptic digests.
• Liquid Chromatography-Mass Spectrometry (LC/MS).
• Amino acid sequence analysis.
• Active site titrations.
• High-performance liquid chromatography (HPLC).

Main Results:

• Clasto-lactacystin beta-lactone is a time-dependent inhibitor (kinact/[I] = 1,700 M-1 s-1) that modifies the Ogamma of the N-terminal threonine hydroxyl group on the beta-subunit.
• This beta-lactone modification completely blocks the binding of peptidyl boronic acid inhibitors.
• Peptidyl vinyl sulfone modifies a residue within the N-terminal 20 amino acids, and this modification is also prevented by prior beta-lactone treatment.

Conclusions:

• The N-terminal threonine hydroxyl group is a critical site for inhibition by multiple classes of small molecule inhibitors targeting the Rhodococcus proteasome.
• The mechanism of inhibition involves covalent modification of this specific residue.
• Sequential application of inhibitors reveals distinct but overlapping modification sites and mechanisms.