title: pH-induced co-operative effects in hysteretic enzymes. 2. pH-induced co-operative effects in a cell-wall beta-glucosyltransferase.
abstract: A beta-glucosyltransferase, extracted and purified from the cell walls of isolated soybean cells, displays hysteretic behaviour. The enzyme is monomeric and has a negative co-operative between pH 5.5 and 7.5. Below and above these pH values, the enzyme follows, or approaches, classical Michaelis-Menten kinetics. The free enzyme and the enzyme-glucose complex exhibit, upon pH jumps, conformational transitions which may be followed by monitoring the fluorescence of enzyme-bound toluidinylnaphthalene sulfonate. Taken together these results are consistent with the model of pH-induced co-operativity described in the preceding paper in this journal. This special type of co-operativity relies on a change of the pK value of a strategic ionizable group located outside the active site in a region (or a domain) of the protein which undergoes the conformational transition. The result that at 'low' and 'high' pH values, the enzyme follows or approaches Michaelis-Menten kinetics is explained by assuming that the conformational changes do not affect the active site.
mesh_terms: D000494:Allosteric Regulation; D002473:Cell Wall / Q000201:enzymology; D005947:Glucose / Q000494:pharmacology; D005964:Glucosyltransferases / Q000037:antagonists & inhibitors / Q000378:metabolism*; D006863:Hydrogen-Ion Concentration; D010944:Plants; D011487:Protein Conformation; D013050:Spectrometry, Fluorescence
chemical_list: D005964:Glucosyltransferases; D005947:Glucose