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== Structure ==

Isolated from human liver, the protein Cathepsin B has a very unique structure. Human Cathepsin B contains 14 cysteine residues, with Cys29 representing the active site cysteine, and 12 other cysteine residues found to be involved in [http://en.wikipedia.org/wiki/Disulfide_bond disulfide bridges].  Cathepsin B contains six dilsulfide bridges which are enclosed to the amino-terminal half of the molecule. Two of the cysteine residues (Cys 29 and Cys 240) are unpaired. The crystal structure of Cathepsin B consist of a double-chain form, with one light chain and one heavy chain composed of residues Leu1-Asn47 and Val50-Asp254. Some other features of Cathepsin B is that it has a diameter of 50 angstroms and a thickness of 30 angstroms. Also, you can see that there is a marked incision on the structure which represents the active site cleft. The polypeptide chain is described as being folded into two distinct domains which are known to interact with one another through an extended polar interface which opens to the V-shaped active site cleft. The left-hand” L” domain is formed by the amino-terminal half of the polypeptide chain up to about Tyr 148 and by the last carboxy-terminal residues. The right hand “R” domain is composed of the first ten residues and the carboxy-terminal half of the polypeptide chain. The amino and the carboxy-terminal ends act as ‘straps’ which help in clamping both domains together.  The occluding loop of the structure has a 20-residue insertion that blocks primed terminus of the [http://en.wikipedia.org/wiki/Active_site active site cleft].  A critical feature of the structure is that it contains two histidine residues (His110 and His111) in the occluding loop of the structure and this allows positively charged anchors for the C-terminal carboxylate group of  polypeptide substrates. These structural features give reasons for the dipeptidyl carboxypeptidase activity of cathepsin B.  The structure of human Cathepsin B proteins have been solved using X-ray crystallography.