Effects of High-Affinity Inhibitors on Partial Reactions, Charge Movements, and Conformational States of the Ca²⁺ Transport ATPase (Sarco-Endoplasmic Reticulum Ca²⁺ ATPase)

Abstract

The inhibitory effects of thapsigargin, cyclopiazonic acid, and 2,5-di(tert-butyl)hydroquinone, and 1,3-dibromo-2,4,6-tri(methylisothiouronium)benzene on the Ca²⁺ ATPase were characterized by comparative measurements of sequential reactions of the catalytic and transport cycle, including biochemical measurements and detection of charge movements within a single cycle. In addition, patterns of ATPase proteolytic digestion with proteinase K were derived to follow conformational changes through the cycle or after inhibitor binding. We find that thapsigargin, cyclopiazonic acid, and 2,5-di(tert-butyl)hydroquinone inhibit Ca²⁺ binding and catalytic activation as demonstrated with isotopic tracers and lack of charge movement upon addition of Ca²⁺ in the absence of ATP. It has been shown previously that binding of these inhibitors requires the E2 conformational state of the ATPase, obtained in the absence of Ca²⁺. We demonstrate here that E2 state conformational features are in fact induced by these inhibitors on the ATPase even in the presence of Ca²⁺. The resulting dead-end complex interferes with progress of the catalytic and transport cycle. Inhibition by 1,3-dibromo-2,4,6-tri(methylisothiouronium)benzene, on the other hand, is related to interference with a conformational transition of the phosphorylated intermediate (E1∼P · 2Ca²⁺ to E2-P · 2Ca²⁺ transition), as demonstrated by increased phosphoenzyme levels and absence of bound Ca²⁺ translocation upon addition of ATP. This transition includes large movements of ATPase headpiece domains and transmembrane segments, produced through utilization of ATP-free energy as the “conformational work” of the pump. We conclude that the mechanism of high-affinity Ca²⁺ ATPase inhibitors is based on global effects on protein conformation that interfere with ATPase cycling.