Glutamate residues required for substrate binding and cleavage activity in mitochondrial processing peptidase

Area of Science:

• Biochemistry
• Molecular Biology
• Enzymology

Background:

• Mitochondrial processing peptidase (MPP) is a metalloendopeptidase crucial for protein maturation in mitochondria.
• MPP specifically cleaves N-terminal extension peptides from mitochondrial precursor proteins, requiring basic amino acid residues for recognition.
• Understanding the molecular recognition mechanism of MPP is vital for elucidating mitochondrial protein import and processing.

Purpose of the Study:

• To investigate the role of specific glutamate residues in the active site of the rat MPP beta-subunit in substrate binding and catalytic activity.
• To elucidate the molecular mechanism underlying substrate recognition and cleavage by MPP.

Main Methods:

• Site-directed mutagenesis of glutamate residues (E59, E79, E129, E136) in the rat MPP beta-subunit.
• Analysis of kinetic parameters (kcat, Km), metal binding, and substrate binding of wild-type and mutant enzymes.
• Utilized environmentally sensitive fluorescence probes to study peptide binding interactions.

Main Results:

• Mutations at E79, E129, and E136 significantly reduced cleavage efficiency, while dimer formation with the alpha-subunit remained unaffected.
• E136 appears to be involved in metal binding, and E129 is critical for catalysis (decreased kcat).
• E79 plays a primary role in substrate binding, as mutations led to altered binding site environment and reduced peptide binding (decreased Km, decreased kcat).

Conclusions:

• Specific glutamate residues within the MPP beta-subunit are essential for both substrate binding and catalytic cleavage.
• E136 is implicated in metal coordination, E129 in catalysis, and E79 in substrate recognition.
• These findings provide key insights into the structure-function relationship of MPP and its mechanism of action in mitochondrial protein processing.