Potassium activation and its relationship to a highly reactive cysteine residue in fructose 1,6-bisphosphatase

The specific chemical modification by sodium cyanate of highly reactive cysteine residues at pH 7.5 in pig kidney fructose 1,6-bisphosphatase results in the reversible loss of activation of the enzyme by monovalent cations. No loss of activation by potassium ions occurs when modification is carried out in the presence of fructose 2,6-bisphosphate. The effect of Mg2+ on native and cyanate-modified enzyme activities implicates the above cysteine residue as being directly linked to the inhibition by both the divalent cation and fructose 2,6-bisphosphate. Incorporation of [14C]cyanate to the enzyme shows that the blockage of two reactive residues per tetramer is sufficient to eliminate the activation of the enzyme by K+.