Conformational changes in the vicinity of the N-iodoacetyl-N'-(5-sulfo-1-naphthyl)ethylenediamine attached to the specific thiol of sarcoplasmic reticulum Ca2+-ATPase throughout the catalytic cycle.
In the previous experiment (Suzuki, H., Obara, M., Kuwayama, H., and Kanazawa, T. (1987) J. Biol. Chem. 262, 15448-15456), the Ca2+-ATPase of sarcoplasmic reticulum vesicles was labeled with N-iodoacetyl-N'-(5-sulfo-1-naphthyl)ethylenediamine without a loss of the catalytic activity. The main labeled site was Cys674. A large monophasic fluorescence drop occurred upon ATP binding to the catalytic site of the Ca2+-activated enzyme in the presence of K+. The present results show that this fluorescence drop is biphasic in the absence of K+. The first and rapid phase of this drop accounts for most of the fluorescence drop. This phase reflects a conformational change in the enzyme.ATP complex. The second and slow phase, being much smaller than the first phase, coincides with phosphoenzyme (EP) isomerization from the ADP-sensitive form to the ADP-insensitive form. This phase disappears when accumulation of ADP-insensitive EP is inhibited by K+ or when EP isomerization is prevented by the N-ethylmaleimide treatment. These results show that this phase reflects a conformational change upon EP isomerization. When free Ca2+ is chelated after EP formation from ATP, the fluorescence intensity is restored to the initial level without Ca2+. This restoration coincides with EP decomposition. This suggests that the fluorescence restoration reflects a conformational change upon hydrolysis of ADP-insensitive EP. This probability is supported by the concurrent occurrence of the Pi-induced fluorescence drop and EP formation from Pi. The results demonstrate that the fluorescence drop upon ATP binding is predominant in the fluorescence change throughout the catalytic cycle.
Obara
M
M
Department of Biochemistry, Asahikawa Medical College, Japan.
Suzuki
H
H
Kanazawa
T
T
D000255:Adenosine Triphosphate / Q000378:metabolism; D000818:Animals; D001665:Binding Sites; D000252:Calcium-Transporting ATPases / Q000378:metabolism*; D005456:Fluorescent Dyes; D007700:Kinetics; D009132:Muscles / Q000201:enzymology; D009282:Naphthalenesulfonates / Q000494:pharmacology*; D011485:Protein Binding; D011487:Protein Conformation; D011817:Rabbits; D012519:Sarcoplasmic Reticulum / Q000201:enzymology*; D013050:Spectrometry, Fluorescence; D013438:Sulfhydryl Compounds / Q000378:metabolism
D005456:Fluorescent Dyes; D009282:Naphthalenesulfonates; D013438:Sulfhydryl Compounds; C048876:1,5-AEDANS; C006421:1,5-I-AEDANS; D000255:Adenosine Triphosphate; D000252:Calcium-Transporting ATPases