Effect of chemical modification on struture and activity of glucoamylase from Aspergillus candidus and Rhizopus species.

The histidine, tyrosine, tryptophan and carboxyl groups in the enzyme glucoamylase from Aspergillus Candidus and Rhizopus species were modified using group specific reagents. Treatment of the enzyme with diethylpyrocarbonate resulted in the modification of 0·3 and 1 histidine residues with only a slight loss in activity (10% and 35%) of glucoamylase from Aspergillus candidus and Rhizopus species respectively. Modification of tyrosine either by Nacetylimidazole or [I125]-leads to a partial loss of activity. Under denaturing conditions, maltose did not help in protecting the enzyme against tyrosine modification or inactivation. Treatment with 2-Hydroxy-5-nitro benzyl bromide in the presence of urea, photooxidation at pH 9·0, N-bromosuccinamide at pH 4·8 resulted in a complete loss of activity· However, the results of experiments in the presence of maltose and at pH 4·8 photooxidation and Nbromosuccinamide treatment suggested the presence of two tryptophan residues at the active site. There was a complete loss of enzyme activity when 10 and 28 carboxyl groups from Aspergillus candidus and Rhizopus, respectively were modified. Modification in the presence of substrate maltose, showed at least two carboxyl groups were present at the active site of enzyme and that only one active center seems to be involved in breaking ally 3 types of α-glucosidic linkages namely α-1,4, α-1, 6 and α-l,3.

Fungal glucoamylases.

The purification and properties of glucoamylase (α-l,4-glucan glucohydrolase, EC 3.2.1.3) from different fungal sources have been compared. The studies on the conformation and activity of the native enzyme at a function of pH, temperature, substrate concentration and the effect of denaturants and on the role of carbohydrate moiety on structure and stability have been reviewed. The chemical modification of the active centre, binding kinetics of the substrate and active site and the mechanism of action have been summarized. They differ in their fine structure as revealed by their near ultra-violet circular dichroism spectra and contain 30–35 % α-helix, 24–36 % β-structure and the rest aperiodic structure. The activity of the enzyme is very sensitive to the environment around aromatic aminoacid residues. The glucoamylases are glycoprotein in nature, differ in their content and nature of carbohydrate from different sources. The carbohydrate moiety plays an important role in stabilising the native conformation of the enzyme and is not involved in activity and antigenecity. At the active site of the enzyme, two tryptophan and two carboxyl (glutamate or aspartate) groups are present. It is likely that the histidine and tyrosine residues which are present away from the active site are involved in binding of the substrate. There seems to be seven subsites which are involved in binding of the substrate and the catalytic site is situated in between 1 and 2 subsites. In breaking of α-1,4-, α-1,3-, and α-l,6-bonds only one active centre is involved. Studies on the immobilization of either glucoamylase alone or as a part of a multienzyme system have been reviewed briefly.

Structure and stability of glucoamylase II from Aspergillus niger: A circular dichroism study.

Glucoamylase II (EC 3.2.1.3) from Aspergillus niger has 31 % α-helix, 36 % ßstructure and rest aperiodic structure at pH 4·8 as analysed by the method of Provencher and Glockner (1981, Biochemistry, 20,33). In the near ultra-violet circular dichroism spectrum the enzyme exhibits peaks at 304, 289, 282 and 257 nm and troughs at 285, 277 and 265 nm respectively. The enzyme activity and structure showed greater stability at pH 4·8 than at pH 7·0, were highly sensitive to alkaline pH but less sensitive to acid pH values. The enzyme retained most of its catalytic activity and structure even on partial removal of carbohydrate moieties by periodate treatment but was less stable at higher temperatures and storage at 30°C. Reduction of the periodate treated enzyme did not reverse the loss of stability. Binding of the synthetic substrate,p-nitrophenyl-α-D-glucoside, perturbed the environment around aromatic amino acids and caused a decrease in the ordered structure.

Isolation and characterization of the 7S protein from glanded cottonseed (Gossypium herbacium).

The major protein from glanded cottonseed has been isolated in a homogeneous form. Its S20, w value at 1% protein concentration is 6S in 1 Μ NaCl solution. It contains 1% carbohydrate and is free from phosphorus, gossypol (bound or free) and nucleic acid impurities. It consists of atleast seven non-identical subunits. The protein has an ultraviolet absorption maximum at 278 nm and fluorescence excitation and emission maxima at 280 nm and 325 nm respectively. Optical rotatory dispersion and circular dichroism measurements indicate that the protein consists predominantly of β-structure and random coil. The observed near-ultraviolet circular dichroic bands can be attributed to tyrosine, phenylalanine and tryptophan residues of the protein.