subtilisin

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Subtilisin
Crystal structure of Subtilisin
Gene code:
Structure:
Recent publications:
protein type:	Serine protease
Functions:
Domains:
Diseases:

Subtilisin (serine endopeptidase) is a protease (a protein-digesting enzyme) initially obtained from Bacillus subtilis.

Subtilisins belong to the group of serine proteases which initiate the nucleophilic attack on the peptide (amide) bond through a serine residue at the active site. They are physically and chemically well characterized enzymes. Subtilisins typically have molecular weights of about 20,000 to 45,000 dalton. They can be obtained from soil bacteria, for example Bacillus amyloliquefaciens. Subtilisins are secreted in large amounts from many Bacillus species.

Subtilisins are widely used in commercial products, for example in laundry and dishwashing detergents, cosmetics, food processing^[1], skin care ointments^[2], contact lens cleaners, and for research purposes in synthetic organic chemistry.

The structure of subtilisin has been determined by X-ray crystallography. It is a 275 residue globular protein with several alpha-helices, and a large beta-sheet. It is structurally unrelated to the chymotrypsin-clan of serine proteases, but uses the same type of catalytic triad in the active site. This makes it the classic example of convergent evolution.

Charge-relay Site of Subtilisin

The active site features a charge-relay network involving Asp-32, His-64, and active site Ser-221 arranged in a catalytic triad. The charge-relay network functions as follows: The carboxylate side chain of Asp-32 hydrogen bonds to a nitrogen-bonded proton on His-64's imidazole ring. This is possible because Asp is negatively charged at physiological pH. The other nitrogen on His-64 hydrogen bonds to the O-H proton of Ser-221. This last interaction results in charge-separation of O-H, with the oxygen atom being more nucleophilic. This allows the oxygen atom of Ser-221 to attack incoming substrates (ie. peptide bonds), assisted by a neighboring carboxyamide side chain of Asn-155.

Even though Asp-32, His-64, and Ser-221 are sequentially far apart, they converge in the 3D structure to form the active site.

To summarize the interactions described above, Ser-221 acts as a nucleophile and cleaves peptide bonds with its partially negative oxygen atom. This is possible due to the nature of the charge-relay site of subtilisin.

References

Deber, C.M. (Lecturer). (2006, Sep. 29). BCH210H1F. [Lecture]. Toronto: University of Toronto.