AMP is an allosteric inhibitor of human muscle and liver fructose-1,6-bisphosphatase (FBPase). Despite strong similarity of the nucleotide binding domains, the muscle enzyme is inhibited by AMP approximately 35 times stronger than liver FBPase: I0.5 for muscle and for liver FBPase are 0.14 microM and 4.8 microM, respectively. Chimeric human muscle (L50M288) and chimeric human liver enzymes (M50L288), in which the N-terminal residues (1-50) were derived from the human liver and human muscle FBPases, respectively, were inhibited by AMP 2-3 times stronger than the wild-type liver enzyme. An amino acid exchange within the N-terminal region of the muscle enzyme towards liver FBPase (Lys20-->Glu) resulted in 13-fold increased I0.5 values compared to the wild-type muscle enzyme. However, the opposite exchanges in the liver enzyme (Glu20-->Lys and double mutation Glu19-->Asp/Glu20-->Lys) did not change the sensitivity for AMP inhibition of the liver mutant (I0.5 value of 4.9 microM). The decrease of sensitivity for AMP of the muscle mutant Lys20-->Glu, as well as the lack of changes in the inhibition by AMP of liver mutants Glu20-->Lys and Glu19-->Asp/Glu20-->Lys, suggest a different mechanism of AMP binding to the muscle and liver enzyme.