Sarcoplasmic membrane enzyme properties

The number of different proteins in a membrane varies from less than a dozen in the sarcoplasmic reticulum to over 100 in the plasma membrane. Most membrane proteins can be separated from one another using sodium dodecyl sulfate polyacrylamide gel electrophoresis (SDS-PAGE), a technique that has revolutionized their study. In the absence of SDS, few membrane proteins would remain soluble during electrophoresis. Proteins are the major functional molecules of membranes and consist of enzymes, pumps and channels, structural components, antigens (eg, for histocompatibility), and receptors for various molecules. Because every membrane possesses a different complement of proteins, there is no such thing as a typical membrane structure. The enzymatic properties of several different membranes are shown in Table 41-2. 

A biologic reason for the abundance of nonlamellar lipids in membranes is that they possess the ability to modulate the activities of membrane proteins (15, 16). It has been recognized that membranes exist in a state of curvature frustration, which may be sufficiently large to have significant effect on certain protein conformations (17). Many examples show that the lipid bilayer elastic curvature stress indeed couples to conformational changes of membrane proteins (15, 18, 19). Protein kinase C is one such example of an enzyme activated by lipids that exhibit a propensity for nonlamellar phase formation (20). The activity of Ca " -ATPase from sarcoplasmic reticulum membranes also strongly correlates with the occurrence of nonbilayer lipids in the membrane and increases with the increase of their amount. It is noteworthy that the protein activity does not depend on the chemical structure of the lipids but only on their phase propensity thus specific binding interactions are ruled out. The list of proteins with activities that depend on the phase properties... 

Ghosh et al. [48] have isolated two isoforms of nSMase from rabbit skeletal muscle (92 and 53 kDa). Peptide mapping revealed important structural similarities, and the catalytic activities were also similar, except that the 53 kDa protein was Mg" -independent. These nSMases are located in the transverse tubules of the muscle cells, which may be related to the observation that sphingosine modulates calcium release from sarcoplasmic reticulum membranes [49]. Two Mn -and Mg" -dependent nSMases located in the microsomal membranes of seminiferous tubes of immature Wistar rats have been characterized [50] whose properties do not appear to differ significantly from other mammalian nSMases. Finally, we mention two other nSMases purified from eukaryotic natural sources, namely the Mg -dcpcridcril nSMase isolated from Saccharomyces cerevisiae [51], and that obtained from membrane fractions of intraeryfhrocytic Plasmodium falciparum, the malaria parasite [52]. The latter enzyme was activated by phosphatidylserine and other anionic phosphohpids, and was sensitive to scyphostatin, an inhibitor of mammalian nSMase (see below). 

We will consider the structural and mechanistic features of P-type ATPases by examining the Ca ATPase found in the sarcoplasmic reticulum (SR Ca ATPase, or SERCA) of muscle cells. The properties of this member have been established in great detail, by relying on crystal structures of the pump in five different states. This enzyme, which constitutes 80% of the protein in the sarcoplasmic reticulum membrane, plays an important role in muscle contraction, a process triggered by an abrupt rise in the cytoplasmic calcium ion level. Muscle relaxation depends on the rapid removal of from the cytoplasm into the sarcoplasmic reticulum, a specialized compartment for Ca storage, by SERCA, This pump maintains a Ca" concentration of approximately 0.1 pM in the cytoplasm compared with 1.5 mM in the sarcoplasmic reticulum. 

Otherwise, the observed plateau could suggest an association of these aromatic amines with membrane components. In fact, it was shown that azido acridines do interact with proteins of submitochondrial particles (9) and that an acridine derivative covalently bounded to impermeable molecules did retain the uncouling property (10). Moreover, quinacrine is an inhibitor of the FI ATPase solubilized from bovine heart mitochondria (11) and this property can be extended to other kind of ATPase, like the calcium dependent enzyme purified from sarcoplasmic reticulum (12). 

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