MEROPS

MEROPS is an online database for peptidases (also known as proteases, proteinases and proteolytic enzymes) and their inhibitors. The classification scheme for peptidases was published by Rawlings & Barrett in 1993, and that for protein inhibitors by Rawlings et al. in 2004. The most recent version, MEROPS 12.4, was released in late October 2021. Overview The classification is based on similarities at the tertiary and primary structural levels. Comparisons are restricted to that part of the sequence directly involved in the reaction, which in the case of a peptidase must include the active site, and for a protein inhibitor the reactive site. The classification is hierarchical: sequences are assembled into families, and families are assembled into clans. Each peptidase, family, and clan has a unique identifier. Classification Family The families of peptidases are constructed by comparisons of amino acid sequences. A family is assembled around a type example, the sequence of a well-characterized peptidase or inhibitor. All other sequences in the family must be related to the family type example, either directly or through a transitive relationship involving one or more sequences already shown to be family members. Typically, FastA or BlastP is used to establish sequence relationships, with an expect value of 0.001 or lower taken to be statistically significant. HMMER or psi-blast searches are used for adding sequences which are distantly related to a family. Each family is identified by a letter representing the catalytic type of the peptidases it contains followed by an arbitrary unique number.