Trypsin: Difference between revisions

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Serine proteases played a central role in the discovery and study of enzymes. This is because they are particularly easy to study. They are plentiful in digestive juices and very stable, so they are relatively easy to collect and purify. It is also easy to study their function: you just toss in some protein and see how fast it is digested. Chymotrypsin was among the first proteins to be studied by X-ray crystallography, revealing its complex machinery for holding the protein targets and performing a precise atomic change. Today, there are hundreds of structures of serine proteases available in the PDB, waiting to be explored.

==The Perils of Proteases==

As you might imagine, the digestion of proteins in your body is a delicate business. Protein makes up about one fifth of the material in each of your cells, so you must be careful when creating protein-cutting machines. For digestive enzymes, the trick is to create the enzyme in an inactive form (termed a zymogen), and then to activate it once it is in the intestine. Trypsin is built with an extra piece of protein chain, colored in green in the structure on the left (PDB entry 1tgs). Actually, only two amino acids of this extra bit are seen in crystal structure, so you have to imagine the rest flopping around away from the protein. This longer form of trypsin, called trypsinogen, is inactive and cannot cut protein chains. Then, when it enters the intestine, the enzyme enteropeptidase makes one cut in the trypsin chain, clipping off the little tail. This allows the new end of the chain, colored here in purple, to tuck into the folded protein and stabilize the active form of the enzyme, as shown on the right (PDB entry 2ptc). As extra insurance, the pancreas also makes a small protein, trypsin inhibitor (shown in red), that binds to any traces of active trypsin that might be present before it is secreted into the intestine. It binds to the active site of trypsin, blocking its action but not itself being cut into tiny pieces.

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 Serine proteases played a central role in the discovery and study of enzymes. This is because they are particularly easy to study. They are plentiful in digestive juices and very stable, so they are relatively easy to collect and purify. It is also easy to study their function: you just toss in some protein and see how fast it is digested. Chymotrypsin was among the first proteins to be studied by X-ray crystallography, revealing its complex machinery for holding the protein targets and performing a precise atomic change. Today, there are hundreds of structures of serine proteases available in the PDB, waiting to be explored.
+==The Perils of Proteases==
+As you might imagine, the digestion of proteins in your body is a delicate business. Protein makes up about one fifth of the material in each of your cells, so you must be careful when creating protein-cutting machines. For digestive enzymes, the trick is to create the enzyme in an inactive form (termed a zymogen), and then to activate it once it is in the intestine. Trypsin is built with an extra piece of protein chain, colored in green in the structure on the left (PDB entry 1tgs). Actually, only two amino acids of this extra bit are seen in crystal structure, so you have to imagine the rest flopping around away from the protein. This longer form of trypsin, called trypsinogen, is inactive and cannot cut protein chains. Then, when it enters the intestine, the enzyme enteropeptidase makes one cut in the trypsin chain, clipping off the little tail. This allows the new end of the chain, colored here in purple, to tuck into the folded protein and stabilize the active form of the enzyme, as shown on the right (PDB entry 2ptc). As extra insurance, the pancreas also makes a small protein, trypsin inhibitor (shown in red), that binds to any traces of active trypsin that might be present before it is secreted into the intestine. It binds to the active site of trypsin, blocking its action but not itself being cut into tiny pieces.