The role of the Cys191-Cys220 disulfide bond in trypsin: new targets for engineering substrate specificity.
The S1 binding site of trypsin is cross-linked by the conserved Cys191-Cys220 disulfide bond. The substitution of Cys191 and Cys220 with Ala decreases the activity of trypsin by 20-200-fold as measured by kcat/K(m) for the hydrolysis of amide substrates; in contrast, ester hydrolysis is decreased by < 10-fold. Similar decreases are observed in the hydrolysis of oligopeptide and single amino acid substrates. This decrease in activity results from a decrease in the acylation rate. The substrate binding and deacylation rate are not affected by the loss of the disulfide bond. C191A/C220A binds BPTI with the same affinity as trypsin, although the affinity of benzamidine is decreased 10-fold and the affinity of leupeptin is decreased 1000-fold. The CD spectrum of C191A/C220A displays significant differences from that of trypsin; these differences most likely result from the loss of the disulfide chromophore, although perturbation of enzyme structure cannot be discounted. The loss of the Cys191-Cys220 disulfide has no effect on the stability of trypsin as measured by urea denaturation. Single and double substitutions of Ser at positions 191 and 220 have a similar activity to C191A/C220A. These results indicate that the Cys191-Cys220 disulfide bond is not essential for the function, structure or stability of trypsin.
Wang
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Graduate Department of Biochemistry, Brandeis University, Waltham, MA 02254, USA.
Hung
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Cahoon
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Hedstrom
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D000215:Acylation; D001665:Binding Sites; D002942:Circular Dichroism; D003545:Cysteine / Q000378:metabolism*; D004220:Disulfides / Q000378:metabolism; D004795:Enzyme Stability; D007700:Kinetics; D016297:Mutagenesis, Site-Directed; D011487:Protein Conformation; D015202:Protein Engineering; D013329:Structure-Activity Relationship; D013379:Substrate Specificity; D014357:Trypsin / Q000235:genetics / Q000378:metabolism*
D004220:Disulfides; D014357:Trypsin; D003545:Cysteine