Calcium-Cellular Signalling - Biological Inorganic Chemistry - page 220

characterised by the presence of a phosphorylated intermediate (hence the name P-type), bound to an invariant
Asp residue in a highly conserved sequence SDKTGT[L/I/V/M][T/I/S]. We have already briefly discussed the
Na þ /K þ -ATPase in Chapter 9, and in Figure 11.4 a the architecture of the -subunit of the Ca 2 þ ATPase of rabbit a

FIGURE 11.4 Architecture of Ca 2 þ -ATPase and its ion pumping mechanism. (a) A ribbon representation of Ca 2 þ -ATPase in the E1 $ 2Ca 2 þ
state, viewed parallel to the membrane plane. Colours change gradually from the amino terminus (blue) to the carboxy terminus (red). Purple
spheres (numbered and circled) represent bound Ca 2 þ . Three cytoplasmic domains (A, N, and P), the a -helices in the A-domain (A1 e A3), and
those in the transmembrane domain (M1 e M10) are indicated. M1 0 is an amphipathic part of the M1 helix lying on the bilayer surface. Docked
ATP is shown in transparent space fill. Several key residues e E183 (A), F487 and R560 (N, ATP binding), D351 (phosphorylation site), D627
and D703 (P) e are shown in ball-and-stick. Axis of rotation (or tilt) of the A-domain is indicated with a thin orange line. PDB accession code is
1SU4 (E1 2Ca 2 þ ). (b) A cartoon illustrating the structural changes of the Ca 2 þ -ATPase during the reaction cycle, based on the crystal structures

in 7 different states.

$

(From Toyoshima, 2009. Copyright 2009, with permission from Elsevier.)

white skeletal muscle (SERCA1a) is presented. In addition to the ion transport domain, embedded in the
membrane, there are three cytoplasmic domains, the N-domain (nucleotide binding), the P-domain (phosphory-
lation) in which many highly conserved residues are clustered, and the A-domain (actuator 3 ) , which together
confer the ATP hydrolysing activity. The N-domain recognises and positions the

-phosphoryl of ATP for
nucleophilic attack, while the conserved Asp in the P-domain accepts the phosphoryl group and forms a high-
energy aspartyl-phosphate intermediate. A Glu residue in the signature sequence 181 TGES motif in the A-domain
positions a water molecule for subsequent hydrolysis, leading to release of the phosphoryl group. The cytoplasmic
domains are connected to the transmembrane (TM) segment by five linker regions ( Figure 11.4 ) that form the
crucial structural connection between the two-step release of energy on the cytoplasmic side and its conversion
into physical translocation of ions through the membrane.
In muscle contraction, Ca 2 þ is released from sarcoplasmic reticulum (SR) into muscle cells via a Ca 2 þ -release
channel. Ca 2 þ -ATPase then pumps back the released Ca 2 þ into the SR to cause relaxation. SERCA1a is both

g

3. Actuate e to put into action or motion.