Structures of the mono- and divalent metal nucleotide complexes in the myosin ATPase

The structure of both the mono- and the divalent metal nucleotide complexes active in the myosin subfragment 1 ATPase has been determined using the phosphorothioate analogs of ATP in the presence of various cations. Both the Sp and the Rp diastereomers of adenosine 5'-O-(1-thiotriphosphate) (ATP alpha S) were substrates in the presence of Mg2+, Ca2+, Mn2+, Co2+, Zn2+, and Cd2+ as well as with NH4+ and T1+. The Sp/Rp activity ratios obtained were largely independent of the cation. The simplest explanation of these results is that both mono- and divalent cations do not coordinate to the alpha-phosphate group. With adenosine 5'-O-(2-thiotriphosphate) (ATP beta S), essentially only the Sp diastereomer was active with Mg2+ with Sp/Rp ratio of greater 3000. As the divalent metal ion was varied in the series given above, this ratio was progressively lowered to the value of 0.2 found with Cd2+. Similar changes in stereoselectivity were seen with monovalent cations. Thus, with NH4+, an Sp/Rp ratio of 8 was observed, whereas with T1+, this figure was reduced to 0.04. These data indicate that both mono- and divalent cations coordinate to the beta-phosphate group of the nucleoside triphosphate substrate. These results obtained with ATP alpha S and ATP beta S suggest that myosin uses the mono- or divalent cation delta, beta, gamma-bidentate nucleotide chelate as substrate.