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  • Title: Relationship between the ATPase activity and the ATP-induced fluorescence enhancement of SH-modified heavy meromyosin during its fractional inactivation by vanadate plus ADP: evidence for heterogeneity in the active sites.
    Author: Kawamura T, Higuchi W, Emoto Y, Tawada K.
    Journal: J Biochem; 1985 Jun; 97(6):1583-93. PubMed ID: 3161876.
    Abstract:
    We have examined whether heavy meromyosin (HMM) consists of a single kind of active site by analyzing the changes in the relative MgATPase activity and the relative amplitude of the ATP-induced fluorescence enhancement of the protein when the fraction of HMM "affinity"-labeled by vanadate plus ADP was varied. The analysis is based on a prediction that these two changes should be proportional to each other if myosin consists of a single kind of active site and generates the rate-limiting myosin**product complex emitting enhanced fluorescence. Although the difference between these two changes was very small with native HMM, it was large with HMM in which 5 fast-reactive sulfhydryl-groups per head were pre-modified with thimerosal. The difference indicated the existence of heterogeneous active sites in the SH-modified HMM. The results were best explained in terms of the hypothesis that fifty percent of the active site splits MgATP by a mechanism giving a fluorescence enhancement whereas the other fifty percent splits MgATP by another mechanism giving no fluorescence enhancement. Two possible explanations for the existence of heterogeneous active sites in the SH-modified HMM are discussed. One assumes the pre-existence of some sort of 1:1 heterogeneity in the micro-environment of the active sites and the other, which is considered less likely, assumes the introduction of the heterogeneity as a result of the SH-modification.
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