Halbseiten-Reaktivität der Glycerinaldehyd-3-phosphat Dehydrogenase aus Kaninchen-Skelettmuskel mit Strukturanalogen von NAD / Half-of-the-Sites Reactivity of Glyceraldehyde-3 Phosphate Dehydrogenase from Rabbit Muscle with Structural Analogs of NAD

: Omega-(3-Bromoacetylpyridinio)alkyldiphosphoadenosines with alkyl chain lengths of 2-6 methylene groups inactivate glyceraldehyde-3 phosphate dehydrogenase from rabbit muscle. Half-of-the-Sites reactivity is observed in each case: The analogs are covalently bound to highly reactive cysteine residues in two of the four subunits. The remaining two subunits still bind NAD and the reactive SH-groups, although modified by SH-reagents of low molecular weight are not labeled by any of the brominated coenzyme models. This behaviour may be explained by the assumption, that the modification of 2 subunits induces structural changes in the neighboured unoccupied subunits which prevent any attack on reactive cysteine residues caused by fixation and orientation of the bromoketo-coenzyme analog when bound to the active center. Structural similarities of the covalently bound coenzyme analogs in the active center and the native ternary GAPDH-NAD-substrate complex suggest that half-of-the-sites reactivity is a natural characteristic of the enzymes catalytic mechanism.