Ion channel (Darnell) - \) ~R r v E\.- \._ E TA k ....

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542 \) ~R rv E\.- \._ ETA k. • ~1'\\ol~1.: u..\l.'" Co<\ \ ~\ eXc'(j(:)I ~ ~(,{ ~ Q \ \(\(\0 CHAPTER 14 • TRANSPORT ACROSS CELL MEMBRANES (a) (b) Exterior I ,I .~ t' ~. -.~. . ;.~ E ~ .... ... Transmembrane hydrolysis of the aspartyl-phosphate bond is reduced, de-., noted E2 - P. In step 4, the Ca2+ ions are released to the a- ;:- terior, followed (step 5) by the hydrolysis of the aspartyl- T" phosphate bond. This regenerates the El form of the prottin.::~: (b) Model for the structure of the Ca2+ ATPase in the sarco-L plasmic reticulum, deduced from the amino acid sequence t~, and biochemical data. Ten transmembrane a helices are: h thought to form the Ca 2 + channel. The bulk of the protein'.f forms globular domains on its cytoplasmic surface. Trypsin 1': digestion of this segment releases the three domains: one $; functions in ATP binding; a second bears the phosphorylated / aspartate; and the third is involved in energy transduction. ~". At least part of the Ca2+-binding domain is in the stalk. (Part I: (a) see W. P. Jencks, 1980, Adv. Enzymology 51:75-106; ~" W. P. Jencks, 1989,]. Bioi. Chem. 164:18855-18858.] Part k (b) from D. H. MacLennan et al.• 1985, Nature 316:696 .. K F Exterior (Sft lumen) Step 1 Ca Z ,. binding sites Step 2 ATPADP ATP CI helix Globular { . Energy domains transduction C.'·ob'nd'n g :"" { Phosphorylation of aspartate Cytosol A Figure 14-10 (a) The mechanism of action of the sarco- plasmic reticulum Ca2+ ATPase. El and El are alternate conformational forms of the ATPase in which the Ca 2 +- binding sites are on the cytoplasmic and exoplasmic faces, respectively. The Ca 2 + -pumping reaction follows an ordered bnetic mechanism; this cycle is essential for coupling be- tween Ca 2 + transport and ATP hydrolysis. In step 1, two
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Ion channel (Darnell) - \) ~R r v E\.- \._ E TA k ....

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