C3-degrading proteinase enzyme. Molecular model of a proteinase enzyme from the TIGR4 strain of the bacterium Streptococcus pneumoniae. This enzyme is named cppA, after the gene that regulates its production. Proteinase enzymes act to degrade proteins, and this one degrades the C3 protein, also called complement component 3. This protein is part of the innate, or complement, immune system, named after the C3 gene that encodes it. Those with a deficiency in C3 are more susceptible to bacterial infection. This enzyme consists of four protein chains (pink, orange, blue, green).