Rabbit muscle dissociation constants

In all the above-mentioned studies, only the tetramer has catalytic activity. The only claims for an active dimer 106,107) are not supported by satisfactory experimental evidence. Deal and co-workers 104, 108) have, on the other hand, presented extensive studies of the ATP-induced dissociation of GPD to inactive dimers and monomers at low temperatures. Furthermore, these subunits display very little unfolding, which has been taken to imply that dissociation is the major factor in the activity loss. These results were confirmed 24) with the rat skeletal muscle enzyme, which dissociates at 0° to inactive dimers in the absence of ATP. In addition, the activity transport studies of Hoagland and Teller 109) have given strong evidence that only the tetrameric form is active and that the presence of all three substrates promotes tetramer formation. It was also shown that rabbit muscle enzyme exists in a dimer-tetramer equilibrium in dilute aqueous solution at 5°, with an association constant... 

The enzyme is also subject to allosteric inhibition. When glucose binds at the active site, it stabiles the T-state conformation of the enzyme. The T state is also stabilised by bi- or tricyclic aromatic compounds such as caffeine or flavins, which bind at the entrance to the active site tunnel,and by acylated p-glucopyranosylamine derivatives, which bind similarly to glucose, but more tightly. A third allosteric site, formed at the interface of two subunits and normally an internal pool of water molecules , has recently been discovered in rabbit muscle phosphorylase b " and human liver phosphorylase a. Occupancy of this site freezes the enzyme in the T state and inhibitors with 10 M dissociation constants from the site are being investigated in the treatment of diabetes. 

Dissociation Constants foe Coenzyme Compounds op Rabbit Muscle Glyceraldehyde-3-Phosphate Dehydrogenase... 

Figure 1. (Top) FDP Aldolase from rabbit muscle and E. ColL (Bottom) Activity, stability and dissociation constants of Zn" - and Co -FDP aldolase from .Co/i. at 20°C, pH 8.

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