Catalysis of the hydrolysis of internal, alpha-peptide bonds in a polypeptide chain by a mechanism in which the sulfhydryl group of a cysteine residue at the active center acts as a nucleophile.
Source:
https://www.ebi.ac.uk/merops/about/glossary.shtml#ENDOPEPTIDASE,
https://www.ebi.ac.uk/merops/about/glossary.shtml#CATTYPE,
GOC:mah