The Space External to the Outermost Structure of a Cell. For Cells Without External Protective or External Encapsulating Structures This Refers to Space Outside of the Plasma Membrane. This Term Covers the Host Cell Environment Outside an Intracellular Parasite.
Catalysis of the Hydrolysis of Internal Alpha Peptide Bonds in a Polypeptide Chain by a Catalytic Mechanism That Involves a Catalytic Triad Consisting of a Serine Nucleophile That Is Activated by a Proton Relay Involving an Acidic Residue (e.g. Aspartate or Glutamate) and a Basic Residue (usually Histidine).