AMP ACTIVATION OF SNAKE MUSCLE FRUCTOSE 1,6-BISPHOSPHATASE AT ALKALINE PH

AMP, an allosteric inhibitor at neutral pH, activates snake muscle fru ctose 1,6-bisphosphatase at pH 9.2. The activation is virtually unique for the snake muscle enzyme: activation was not observed for the enzy mes from either human and rabbit liver or porcine kidney. The activati on is Mg2+-dependent but was not observed until the concentration of M g2+ reaches 1 mM. It is known that subtilisin, trypsin, or lysosomal p roteases hydrolyse the N-terminal loop of fructose-1,6-bisphosphatase in the vicinity of amino acid residue 60 generating a form of the enzy me with a pH optimum at 9.2. In the presence of AMP, the pH profile of the native snake muscle enzyme resembles that of the alkaline form an d modification of the highly reactive sulfhydryl group abolishes AMP a ctivation. The fact that AMP has a dual function at different pH level s suggests that pH might be an important factor in regulating the acti vity of the enzyme upon binding of AMP at the allosteric site. Indeed, the mode of AMP binding to the allosteric site may differ at neutral and alkaline pH levels. A residue that ionizes with a pK(a) of 8.9 mig ht be involved in this process. (C) 1998 Academic Press.