Mechanism of the calcium pump in the endoplasmic reticulum of liver: phosphoproteins as reaction intermediates.
Microsomal fractions, highly enriched with endoplasmic reticulum of rat and human liver exhibit Ca2+ uptake catalyzed by a Ca2+-pumping ATPase. The mechanism of Ca2+-translocation involves: (i) reversible Ca2+-dependent formation of an acyl-phosphoenzyme intermediate (Mr 116,000 to 118,000) with bound Ca2+, which in the reversed reaction can transphosphorylate its Pi to ADP to re-synthesize ATP; (ii) reversible transition of the ADP-reactive phosphoenzyme into an isomer without bound Ca2+, not further reactive to ADP; (iii) hydrolytic cleavage, stimulated by Mg2+, K+, and ATP of the ADP-unreactive phosphoenzyme with liberation of Pi. By analogy to a mechanism proposed for the Ca2+ pump of sarcoplasmic reticulum, the translocation of Ca2+ to and dissociation from the inner side of the membrane is suggested to occur by a conformational change, coupled with a decrease in Ca2+-affinity of the phosphoenzyme during its transition into the ADP-unreactive isomer. With CaATP as the effective substrate the reactions proceed normally but at a considerably slower rate.
Heilmann
C
C
Department of Gastroenterology, University of Freiburg, Federal Republic of Germany.
Spamer
C
C
Gerok
W
W
D000255:Adenosine Triphosphate / Q000378:metabolism; D000818:Animals; D001693:Biological Transport, Active; D002118:Calcium / Q000378:metabolism*; D000252:Calcium-Transporting ATPases / Q000378:metabolism; D004591:Electrophoresis, Polyacrylamide Gel; D004721:Endoplasmic Reticulum / Q000378:metabolism*; D006801:Humans; D007700:Kinetics; D008099:Liver / Q000378:metabolism*; D008274:Magnesium / Q000494:pharmacology; D008297:Male; D010750:Phosphoproteins / Q000096:biosynthesis; D010766:Phosphorylation; D051381:Rats; D011919:Rats, Inbred Strains; D012519:Sarcoplasmic Reticulum / Q000378:metabolism
D010750:Phosphoproteins; D000255:Adenosine Triphosphate; D000252:Calcium-Transporting ATPases; D008274:Magnesium; D002118:Calcium