Reaction mechanisms of Trp120-->Phe and wild-type glucoamylases from Aspergillus niger. Interactions with maltooligodextrins and acarbose.
Interactions of wild-type and Trp120-->Phe glucoamylase with maltooligodextrin (Gx) substrates and the tight-binding inhibitor acarbose (A) were investigated here using stopped-flow fluorescence spectroscopy and steady-state kinetic measurements. All wild-type and Trp120-->Phe glucoamylase reactions followed the three-step model E + Gx(or A) (k1) <==> (k-1) EGx (or A) (k2) <==> (k-2) E*Gx(or A) (k3) --> E + P or E-A, previously shown to account for the glucoamylase-maltose system [Olsen, K., Svensson, B., & Christensen, U. (1992) Eur. J. Biochem. 209, 777-784]. K1 = k-1/k1, k2, and k-2, and the catalytic constant, k3, are determined. Binding of maltooligodextrins in the first reaction step is weak, with little difference between wild-type and Trp120-->Phe glucoamylase. The second step, involving a conformational change, in contrast, is strongly influenced by the mutation and by the substrate length. Here wild-type glucoamylase reacts faster and forms more stable intermediates the longer the substrate. In contrast, Trp120-->Phe reacts slower the longer the substrate. The effect of the mutation is thus smallest on maltose. The Trp120-->Phe substitution reduces the fluorescence signal only by 12-20%, indicating that other tryptophanyl residues are important in reporting the conformational change. Trp120 also strongly influences the actual catalytic step, since the mutation decreases the kc values 30-80-fold. Acarbose behaves similar to maltotetraose in the first and the second steps with wild-type but not the Trp120-->Phe glucoamylase. Also, a third step in the acarbose reaction which parallels the catalytic step is strongly affected by the mutation. The rate constant k3 increases 200-fold.
Olsen
K
K
Chemical Laboratory IV, University of Copenhagen, Denmark.
Christensen
U
U
Sierks
M R
MR
Svensson
B
B
D020909:Acarbose; D001234:Aspergillus niger / Q000201:enzymology*; D002240:Carbohydrate Sequence; D003912:Dextrins / Q000378:metabolism*; D005087:Glucan 1,4-alpha-Glucosidase / Q000737:chemistry / Q000378:metabolism*; D007700:Kinetics; D008024:Ligands; D008969:Molecular Sequence Data; D016297:Mutagenesis, Site-Directed; D010649:Phenylalanine; D013050:Spectrometry, Fluorescence; D013329:Structure-Activity Relationship; D014312:Trisaccharides / Q000378:metabolism*; D014364:Tryptophan
D003912:Dextrins; D008024:Ligands; D014312:Trisaccharides; D010649:Phenylalanine; D014364:Tryptophan; D005087:Glucan 1,4-alpha-Glucosidase; D020909:Acarbose