Chymotrypsinogen structure

Figure 11.7 Schematic diagram of the structure of chymotrypsin, which is folded into two antiparallel p domains. The six p strands of each domain are red, the side chains of the catalytic triad are dark blue, and the disulfide bridges that join the three polypeptide chains are marked in violet. Chain A (green, residues 1-13) is linked to chain B (blue, residues 16-146) by a disulfide bridge between Cys 1 and Cys 122. Chain B is in turn linked to chain C (yellow, residues 149-245) by a disulfide bridge between Cys 136 and Cys 201. Dotted lines indicate residues 14-15 and 147-148 in the inactive precursor, chmotrypsinogen. These residues are excised during the conversion of chymotrypsinogen to the active enzyme chymotrypsin.

Wang, D., Bode, W., Huber, R. Bovine chymotrypsinogen A. X-ray crystal structure analysis and refinement of a new crystal form at 1.8 A resolution. /. Mol. Biol. 

Chymotrypsinogen and related proenzymes have extremely low catalytic activity even though a major part of the substrate binding site as well as the catalytic triad system are already in place. However, the oxyanion hole is created during activation of the proenzyme by a subtle conformational change197,262,271 that involves the chain segment containing Gly 193 (Fig. 12-12). This is further evidence of the importance of this part of the active site structure. 

Chymotrypsinogen 480, 481 Chymotrypsin inhibitor 2 (CI2) folding kinetics 544-577, 577 GroEL binding 605 fragments 577, 578, 587, 588, 595 mechanism of folding 576-588 structure 576, 577 Circular dichroism (CD) 193-195 optimal absorbance for signal to noise 212-214... 

The inactive precursors are called trypsinogen, pepsinogen, chymotrypsino-gen, and procarboxypeptidase. These precursors are converted to the active enzymes by hydrolytic cleavage of a few specific peptide bonds under the influence of other enzymes (trypsin, for example, converts chymotrypsinogen to chymotrypsin). The digestive enzymes do not appear to self-destruct, probably because they are so constructed that it is sterically impossible to fit a part of one enzyme molecule into the active site of another. In this connection, it is significant that chymotrypsin attacks denatured proteins more rapidly than natural proteins with their compact structures of precisely folded chains. 

Chymotrypsinogen, a single polypeptide chain of 245 amino acid residues, is converted to a-chymotrypsin, which has three polypeptide chains linked by two of the five disulfide bonds present in the primary structure of chymotrypsinogen. tt- and S-chymotrypsin also have proteolytic activity. In contrast, the conversion of procarboxypeptidase to carboxypeptidase involves the hydrolytic removal of a single amino acid. 

The second, anionic bovine chymotrypsinogen (chymotrypsinogen B), crystallized by Laskowski et al. (46), was initially thought to be of minor importance. This impression probably arose because of heavy losses during purification. Direct chromatography (1) proved later that the amounts of chymotrypsinogens A and B in bovine pancreatic juice were in fact very similar. Therefore it became important to compare as closely as possible the structures and modes of activation of two chymotrypsinogens synthesized by the same species. 

It is evident that monochromatic rotations are a useful adjunct to any study of protein structure, but a final illustration of the insight that rotatory dispersion can provide may be seen in the conformational analysis of ehy-motrypsinogen activation. Neurath et al. (1956) have found an exact correlation between the rate of activation of both chymotrypsinogen and trypsinogen as measured by the enzymatic activity of their products and the rate at which their specific rotations become more positive, a change which Neurath and Dixon (1957) suggest represents an increase in helical content. By dispersion measurements on chymotrypsinogen and x-chymo-trypsin, Imahori et al. (1960) have demonstrated that a twofold increase in helical content, from 12 to 24 %, indeed occurs. They are thus able to esti-... 

How does cleavage of a single peptide bond activate the zymogen Key conformational changes, which were revealed by the elucidation of the three-dimensional structure of chymotrypsinogen, result from the cleavage of the peptide bond between amino acids 15 and 16. 

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