Glucose 6-Phosphate Dehydrogenase from Brewers' Yeast (Zwischenferment)

Abstract

Abstract Sedimentation equilibrium studies of the glucose 6-phosphate dehydrogenase apoprotein (mol wt 101,600 g per mole), conducted in the presence of guanidinium chloride, permit the conclusion that it is composed of two polypeptide chains of 51,000 molecular weight with similar, if not identical, physical properties. Elemental metal analysis on the apoprotein for nine different metals has failed to reveal significant concentrations of a metal constituent. Contrary to previous reports, this enzyme is not a zinc metalloprotein. The observations of Noltmann and Kuby (in P. D. Boyer, H. Lardy, and K. Myrback (Editors), The enzymes, Vol. 7, Academic Press, New York, 1963, p. 223) on the triphosphopyridine nucleotide-induced association phenomenon of the protein have been confirmed by sedimentation equilibrium measurements conducted in the presence of excess TPN+. The TPN-enzyme compound is the dimer of the apoprotein; i.e. it consists of four subunits. The apoenzyme may be titrated with TPN+ to alter successively its hydrodynamic properties, and sedimentation velocity studies in the presence of appropriate concentrations of TPN+ reveal this TPN-enzyme, with a sedimentation coefficient larger than that of the apoenzyme. Values for its s20,w0 and D20,w0 have been deduced, and a prolate model for the tetra-chain species is proposed. Ethylenediaminetetraacetate, if present at significant concentrations, will completely inhibit the TPN-induced association reaction. This has been interpreted as an effect of the EDTA3- ion on solvent-macromolecular interactions, modifying the dissociation-association equilibria, rather than an effect due to its chelation properties. In the absence of interacting ionic species, successful ultracentrifugal titrations have been conducted and interpreted quantitatively in terms of a set of equilibria involving the successive binding of TPN+ to its subunits. The controlled use of EDTA will permit a simplified approach to studies on the catalytic mechanism (mechanisms) through direct steady state kinetic measurements on the active two-chain species.