particularly informative, and readers should look for more studies like this one
in the future literature.
6.4 PHOSPHORYL TRANSFER: P - TYPE ATPASES
6.4.1 Introduction,
P - type adenosine triphosphatases (ATPases) are proteins that maintain elec-
trochemical gradients and actively transport ions across biomembranes.^95 The
P - type ATPases, also known as cation pumps, use energy from ATP to facilitate
ion transport. During active transport, phosphorylation of an aspartic acid
residue takes place and a covalent aspartyl - phosphoanhydride intermediate
is formed with ATP. A magnesium ion appears to be necessary for the
phosphorylation to take place. P - type ATPases include Na + , K + - ATPases, H + ,
Na+ - ATPases, and Ca 2+ - ATPases. Na + , K + - ATPases were discussed in Section
5.4.1. Ca 2+ - ATPases, to be discussed in Section 6.4.2 , are found in the
sarco(endo)plasmic reticulum of skeletal muscle and are known as SERCA
ATPases. This large protein facilitates the re - uptake, into the sarcoplasmic
reticulum, of cytosolic Ca 2+ released during muscle contraction. SERCA is also
involved in signal pathways using calcium as a secondary messenger.
6.4.2 Calcium P - Type ATPases
Some sarco(endo)plasmic reticulum (SR) Ca 2+ - ATPases (SERCA) are P - type
ATPases that play a major role in muscle contraction - relaxation cycles and
are responsible for transporting calcium into the lumen of the sarcoplasmic
reticulum. Some defi nitions are useful in the discussion of Ca 2+ - ATPases:
- Every eukaryotic cell has three parts: a membrane with channels to let
materials into and out of the cell, cytoplasm containing organelles and
the cytosol, and a nucleus, containing the cell ’ s chromosomes, or genetic
material. The cytoplasm contains everything within the cell, except the
nucleus.
- The endoplasmic reticulum (ER) is responsible for the production of the
protein and lipid components of most of the cell ’ s organelles. The ER
contains a large number of folds, but the membrane forms a single sheet
enclosing a single closed sac. This internal space is called the ER lumen.
The smooth endoplasmic reticulum (ER) in muscle cells contains the
vesicles and tubules that serve as a store of calcium ions. These are
released as one step in the muscle contraction process. Calcium pumps,
Ca2+ - ATPases, serve to move the calcium from the cytoplasm to the ER
or SR lumen.
- The sarcoplasmic reticulum (SR) functions to uptake calcium from the
sarcoplasm (the cytoplasm of muscle fi ber). The sarcoplasmic reticulum
PHOSPHORYL TRANSFER: P-TYPE ATPASES 327
