Metal-nucleotide structural characteristics during catalysis by beef heart mitochondrial F1

Abstract

This study examined the nature of the metal-nucleotide complexes which serve as substrates, products, and intermediates in the beef heart mitochondrial ATPase reaction. The two methods employed involved the use of phosphorothioate ATP analogs as substrates in the presence of Mg2+ or Cd2+ and the use of substitution inert Cr X ATP complexes (the isolated diastereomers of the bidentate complexes) along with the newly synthesized Cr X ITP complexes as inhibitors of both the F1-ATPase and F1-ITPase activities. Little stereoselectivity was observed in the inhibition of F1-ATPase and F1-ITPase activities by the isolated diastereomers of beta,gamma-bidentate CrATP, while the inhibition by the delta,alpha,beta-bidentate CrADP diastereomer was greater than that of the lambda epimer. gamma-Monodentate CrITP was a weak inhibitor of both the ATPase and ITPase activities, whereas beta,gamma-bidentate CrITP failed to show any inhibition at all up to a concentration of 3.2 mM. When adenosine 5'-O-(2-thiotriphosphate) (ATP beta S) was used as the substrate, (VmSp]/(Vm(Rp] with Mg2+ present was 2.7 at 31 degrees C and 3.5 at 13 degrees C. The (Vm/Km(Sp]/(Vm/Km(Rp] ratios with Mg2+ present were 15.3 at 31 degrees C and 73.3 at 13 degrees C. With Cd2+ present, the (Vm(Sp]/(Vm(Rp] ratios were 0.81 and 0.65 at 31 and 13 degrees C, respectively. The (Vm/Km(Sp]/(Vm/Km(Rp] ratios with Cd2+ present were 1.17 at 31 degrees C and 1.34 at 13 degrees C. The large activation energy observed for the isomers of CdATP beta S was not observed for MgATP beta S, MgATP, or CdATP. The Vm for Cd adenosine 5'-O-thiotriphosphate (ATP gamma S) hydrolysis was the largest of all the metal-phosphorothioate nucleotide complexes, while that for MgATP gamma S was the smallest. The results are interpreted in terms of a catalytic model for F1-catalyzed nucleotide hydrolysis describing metal-nucleotide chelation during the reaction.