title: Modification of the cysteine residue of streptococcal dihydrofolate reducatse.
abstract: Modification of the single cysteine residue of streptococcal dihydrofolate reducatase with 5,5'-dithiobis (2-nitrobenzoic acid) results in virtually complete inactivation of the enzyme. Reduction of the enzyme-S-nitrobenzoate mixed disulfide with dithiothreitol indicates that 1.0 cysteine has been modified and approximately 70 percent of the original enzyme activity restored. Circular dichroic and fluorescence studies suggest that a localized conformational change involving aromatic residues, possibly tryptophan, has occurred following modification. In addition, nearly stoichiometric amounts of p-hydroxymercuribenzoate lead to complete inactivation of the reducatas. It is suggested that the S-nitrobenzoate moiety bound to the cysteine residue of the enzyme may perturb one or more aromatic side chains and lead to a distortion of the hydrophobic substrate binding site.
mesh_terms: D001565:Benzoates; D001665:Binding Sites; D002942:Circular Dichroism; D003545:Cysteine; D005493:Folic Acid Antagonists; D007700:Kinetics; D009574:Nitro Compounds; D011485:Protein Binding; D011487:Protein Conformation; D013050:Spectrometry, Fluorescence; D013056:Spectrophotometry, Ultraviolet; D013291:Streptococcus / Q000201:enzymology; D013762:Tetrahydrofolate Dehydrogenase; D013997:Time Factors
chemical_list: D001565:Benzoates; D005493:Folic Acid Antagonists; D009574:Nitro Compounds; D013762:Tetrahydrofolate Dehydrogenase; D003545:Cysteine