Tertiary structure
Chymotrypsin is initially synthesized as a 245 amino acid inactive precursor (a zymogen) termed chymotrypsinogen. Activation of chymotrypsinogen involves proteolytic cleavage at two sites along the chain and removal of two amino acids at each cleavage site. The resultant are shown here (chain 1 = 1-13 in green; chain 2 = 16-146 in red; chain 3 = 149-24 in blue). Note, some amino acids at the temini of these chains are not shown in this representation (e.g. 11-13, 149, ). This is because these residues show too much flexibility in the crystal structures to give X-ray diffraction patterns which would locate them in space.
The three chains are held together by five . Can you identify the specific cys residues linked in each disulfide bond? Why do you think is it very difficult to obtain active chymotrypsin after denaturation and renaturation?
Beta Barrels, Protein Domains and the Active Center
The chymotrypsin molecule is folded into two , each containing six beta strands (orange) arranged as anti-parallel sheets which form a circular structure known as a beta barrel. Rotate the molecule so that you can see down through each of the two beta barrels in turn.
The (Ser-195, His-57 and Asp-102 shown here in spacefill representation), are far apart in the primary sequence but are brought together in a crevice formed between the two beta barrel protein domains.
The Active Site Triad
The of chymotrypsin consists of Asp102 positioned close to His 57 and Ser 195. The precise mechanism of action is still debated, but it appears that a hydrogen on the his imidazole ring is transferred to the Asp 102 carboxylate (either via a "charge relay system" or via a "low barrier H-bond"). This shift results in the histidine ring being able to accept the serine 195 hydroxyl hydrogen, forming a very nucleophilic serine alkoxide ion.
