SSI protease inhibitor

In molecular biology the protein SSI is a Subtilisin inhibitor-like which stands for Streptomyces subtilisin inhibitor. This is a protease inhibitor. These are often synthesised as part of a larger precursor protein, either as a prepropeptide. The function of this protein domain is to prevent access of the substrate to the active site. It is found only in bacteria.

PDB 5sic EBI.jpg
Molecular recognition at the active site of subtilisin BPN': crystallographic studies using genetically engineered protease inhibitor SSI (Streptomyces subtilisin inhibitor)


SSI is a protease inhibitor, it prevents enzymes from acting on a substrate. Some SSI's also inhibit trypsin, chymotrypsin and griselysin.[1][2] Commercially, SSI's have huge potential in the commercial market, they help stabilise proteases in products such as laundry detergents to prevent autolysis of biological washing powders.[3] This means that the enzymes in the washing powder are kept in optimum performance.


SSI is a homodimer, in other words, it is made of two subunits which are exactly the same as each other. Each monomer contains 2 antiparallel beta-sheets and 2 short alpha-helices. Protease binding induces the widening of a channel-like structure, in which hydrophobic side-chains are sandwiched between 2 lobes.[4]

Studies have shown that the loss of the C-terminal domain reduces the inhibitory effect of the proteins. This implies that the C-terminal domain is responsible for maintaining the correct 3D fold.[5]

Structural similarities between the primary and secondary contact loops of SSI, and the ovomucoid and pancreatic secretory trypsin inhibitor family suggest evolution of the 2 families from a common ancestor.[4]


  1. ^ Rawlings ND; Tolle DP; Barrett AJ (March 2004). "Evolutionary families of peptidase inhibitors". Biochem. J. 378 (Pt 3): 705–16. doi:10.1042/BJ20031825. PMC 1224039. PMID 14705960.
  2. ^ Kojima S; Nishiyama Y; Kumagai I; Miura K (March 1991). "Inhibition of subtilisin BPN' by reaction site P1 mutants of Streptomyces subtilisin inhibitor". J. Biochem. 109 (3): 377–82. doi:10.1093/oxfordjournals.jbchem.a123389. PMID 1908859.
  3. ^ Ganz PJ, Bauer MD, Sun Y, Fieno AM, Grant RA, Correa PE, et al. (2004). "Stabilized variant of Streptomyces subtilisin inhibitor and its use in stabilizing subtilisin BPN'". Protein Eng Des Sel. 17 (4): 333–9. doi:10.1093/protein/gzh045. PMID 15187224.
  4. ^ a b Hirono S; Akagawa H; Mitsui Y; Iitaka Y (September 1984). "Crystal structure at 2.6 A resolution of the complex of subtilisin BPN' with streptomyces subtilisin inhibitor". J. Mol. Biol. 178 (2): 389–414. doi:10.1016/0022-2836(84)90150-5. PMID 6387152.
  5. ^ Sakai M; Odani S; Ikenaka T (March 1980). "Importance of the carboxyl-terminal four amino acid residues in the inhibitory activity of Streptomyces subtilisin inhibitor (with a revision of its carboxyl-terminal sequence)". J. Biochem. 87 (3): 891–8. doi:10.1093/oxfordjournals.jbchem.a132819. PMID 6993452.
This article incorporates text from the public domain Pfam and InterPro: IPR000691