The complement component C1s catalysed hydrolysis of peptide 4-nitroanilide substrates

Area of Science:

• Biochemistry
• Enzymology
• Immunology

Background:

• Complement component C1s is a key enzyme in the classical complement pathway.
• Understanding C1s substrate specificity is crucial for deciphering its role in immune responses and developing therapeutic strategies.
• Previous studies have identified peptide substrates for C1s, but detailed kinetic analysis of sequence variations is ongoing.

Purpose of the Study:

• To determine the kinetic parameter kcat/Km for the hydrolysis of various peptide 4-nitroanilide substrates catalyzed by complement component C1s.
• To investigate the impact of specific amino acid substitutions on C1s substrate reactivity.
• To identify the most reactive peptide 4-nitroanilide substrate for C1s.

Main Methods:

• Synthesis of peptide 4-nitroanilide substrates based on the C-terminal sequence of human C4a (Leu-Gln-Arg).
• Enzymatic assays to measure the hydrolysis rates of synthesized substrates catalyzed by purified C1s.
• Determination of kinetic parameters, specifically kcat/Km, for each substrate.

Main Results:

• Replacement of glutamine with glycine or serine in the peptide substrate increased kcat/Km.
• Substitution of valine for leucine at the N-terminal position enhanced kcat/Km.
• D-Val-Ser-Arg 4-nitroanilide emerged as the most reactive substrate identified to date for C1s.

Conclusions:

• Amino acid sequence variations significantly influence the catalytic efficiency of C1s towards peptide substrates.
• The findings provide insights into the substrate recognition and cleavage mechanisms of C1s.
• The identified highly reactive substrate can serve as a valuable tool for further biochemical and structural studies of C1s.