A pH-induced change in state around active-site tryptophan residues of Rhizopus niveus glucoamylase, detected by stopped-flow studies of chemical modification with N-bromosuccinimide
✍ Scribed by Masatake Ohnishi; Yuki Nakamura; Masako Murata-Nakai; Keitaro Hiromi
- Publisher
- Elsevier Science
- Year
- 1990
- Tongue
- English
- Weight
- 541 KB
- Volume
- 197
- Category
- Article
- ISSN
- 0008-6215
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✦ Synopsis
At pH 4.5 (the optimum), stopped-flow studies of the chemical modification with N-bromosuccinimide (NBS) of the tryptophan residues of glucoamylase from Rhizopus niveus clearly show reaction curves having two phises, fast and slow, in which two separate types of Trp residues are respectively involved. At pH values higher than 4.5 (to 7.0), however, separation of the slow phase becomes less clear. The pH profile of the A&2@ (caused by NBS) gave a pK -5.8, which corresponds with pK 5.9 for the catalytic residue. At pH 7.5 (no catalytic activity), Trp residues in the slow phase are not modified, but those in the fast are. Trp residues do not have ionizable groups and NBS modification of Trp does not show pH dependency in this pH range. Hence, some change in the environment of Trp residues may be linked to some ionizable residue(s) which has a pK of 5.8 and participates importantly in maltosaccharide hydrolysis.