title: Inactivation of calcium uptake by EGTA is due to an irreversible thermotropic conformational change in the calcium binding domain of the Ca(2+)-ATPase.
abstract: Calcium uptake by rabbit skeletal sarcoplasmic reticulum (SR) is inhibited with an effective inactivation temperature (TI) of 37 degrees C in EGTA with no effect on ATPase activity. Since the Ca-ATPase denatures at a much higher temperature (49 degrees C) in EGTA, this suggests that a small or localized conformational change of the Ca-ATPase at 37 degrees C results in inability to accumulate calcium by the SR. Using a fluorescent analogue of dicyclohexylcarbodiimide, N-cyclohexyl-N'-[4-(dimethylamino)-alpha-naphthyl]-carbodiimide (NCD-4), the region of the calcium binding sites of the SR Ca-ATPase was labeled. Steady-state and frequency-resolved fluorescence measurements were subsequently performed on the NCD-4-labeled Ca-ATPase. Site-specific information pertaining to the hydrophobicity and segmental flexibility of the region of the calcium binding sites was derived from the steady-state fluorescence intensity, lifetime, and rotational rate of the covalently bound NCD-4 label as a function of temperature (0-50 degrees C). A reversible transition at approximately 15 degrees C and an irreversible transition at approximately 35 degrees C were deduced from the measured fluorescence parameters. The low-temperature transition agrees with the previously observed break in the Arrhenius plot of ATPase activity of the native Ca-ATPase at 15-20 degrees C. The high-temperature transition conforms well with the conformational transition, resulting in uncoupling of Ca translocation from ATP hydrolysis as predicted from the irreversible inactivation of Ca uptake at 31-37 degrees C in 1 mM EGTA.(ABSTRACT TRUNCATED AT 250 WORDS)
authors.affiliation: Department of Physics, Texas Tech University, Lubbock 79409.
authors.forename: K H
authors.initials: KH
authors.lastname: Cheng
authors.forename: J R
authors.initials: JR
authors.lastname: Lepock
mesh_terms: D000818:Animals; D001665:Binding Sites; D002118:Calcium / Q000378:metabolism*; D002122:Calcium Chloride / Q000494:pharmacology; D000252:Calcium-Transporting ATPases / Q000737:chemistry* / Q000378:metabolism; D002234:Carbodiimides; D004533:Egtazic Acid / Q000494:pharmacology*; D005454:Fluorescence Polarization; D005456:Fluorescent Dyes; D006358:Hot Temperature; D009132:Muscles / Q000201:enzymology; D011487:Protein Conformation / Q000187:drug effects; D011817:Rabbits; D012519:Sarcoplasmic Reticulum / Q000187:drug effects / Q000378:metabolism*; D013050:Spectrometry, Fluorescence
chemical_list: D002234:Carbodiimides; D005456:Fluorescent Dyes; D004533:Egtazic Acid; C038088:N-cyclohexyl-N'-(4-dimethylamino-alpha-naphthyl)carbodiimide; D000252:Calcium-Transporting ATPases; D002122:Calcium Chloride; D002118:Calcium