Trypsin is a serine protease that works enzymatically by using a mixture of base, acid, and covalent catalysis. The protein uses serine in its active site to interact covalently with the substrate. To create a nucleophilic attack, the histidine 57 group () activates the serine 195 group via base catalysis and covalent catalysis follows. To complete the formation of a nucleophile, aspartic acid 102 pulls positive charge from histidine 57, completing the and forming an effective nucleophile. This forms a , in which the anionic carbonyl oxygen moves into the active site to a location known as the [1]. The tetrahedral intermediate is followed by acid catalysis from the -NH2 of the of the c-terminus, resulting in a broken peptide bond in the substrate.The transition state from the tetrahedral intermediate is stabilized by Asp 189 interacting with Gly 219 to create a stable . The acyl-enzyme intermediate is present, and hydrolysis occurs which ultimately releases the c-terminal chain of the substrate and forms a new bond between water and the carbonyl carbon of the enzyme-substrate complex. The covalent C-O bond of the substrate-enzyme complex is broken, and the enzyme is reformed as the product is released [2]
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This depicts the amino acid residues in mutation sites that affect the conductance of CFTR in cystic fibrosis. These residues are located near the connection of the two amino acid chains that form the quaternary structure of the protein. A possible reason why these mutations affect the ability of CFTR to conduct chloride ions through the channel is because these residues are important in the opening of the channel. If mutations occur at R117, R334, and R347, then the ion gate is no longer able to open as effectively due to the changes in residue properties.
This also highlights the residues that mutate in cystic fibrosis to affect the regulation of CFTR. These residues are located on both ends of the amino chains which are opposite the ends with the amino acid residues discussed in conductance mutations. A possible reason why mutations at these specific residues cause regulation issues is that these residues are involved in a phosphorylation reaction that provides the energy necessary to move chloride ions though the channel. Mutations at these sites would prevent phosphorylation from supplying the energy needed for the functioning of the ion channel.
- ↑ Fundamentals of Biochemistry 3rd Ed.
- ↑ Radisky ES, Lee JM, Lu CJ, Koshland DE Jr. Insights into the serine protease mechanism from atomic resolution structures of trypsin reaction intermediates. Proc Natl Acad Sci U S A. 2006 May 2;103(18):6835-40. Epub 2006 Apr 24. PMID:16636277
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