Subunit conformation and catalytic function in rabbit-muscle glyceraldehyde-3-phosphate dehydrogenase

This paper describes the functional properties of glyceraldehyde-3-phosphate dehydrogenase which has been chemically modified at the active site. Using the fact that in the tetrameric enzyme only two of the four presumably equivalent thiol groups are readily acylated by substrate, a diacylated dicarboxymethylated enzyme was prepared by reaction of acyl-enzyme with iodoacetate. The acyl groups were then catalytically completely removed. The resulting carboxymethyl-enzyme is presumably a single homogeneous species, potentially distinguishable from carboxymethyl-enzyme prepared by limited alkylation of native (non-acylated) enzyme. However, the specific enzymic activity and number of acylatable active sites were independent of the method of preparation of carboxymethyl-enzyme. Two acyl groups could be removed from diacyl-dicarboxymethyl-enzyme but only one could be re-introduced. These and other properties of the enzyme are explainable in terms of an intramolecular subunit rearrangement within the tetrameric molecule. It is concluded that those models which assume that the unmodified enzyme contains four equivalent sites are inconsistent with the acylation and alkylation results.