Membrane enzymes lipid specificity

Several enzymatic activities of the membrane are not specific to plasma membranes but instead are widely distributed throughout intracellular membranes. Activities of several enzymes normally associated with lysosomes have been reported in lipid globule membranes,... 

As illustrated by the examples above, the key to understanding the PI pathway is to comprehend the nature of the regulation of discrete subcellular microdomains of the lipids. Any changes in a specific plasma membrane signaling pool, whether a transient oscillation in PtdIns(4,5)P2 or a more sustained change in rate of flux through the pathway in response to a stimulus, will result in changes in cytoskeletal structure, membrane enzyme activity, and pump or channel activity within the microdomains where the lipid resides. 

Membrane enzymes often show a transition in the Arrhenius plot of the enzyme activity, which is usually attributed to a change in the lipid configuration. (K" " -h H" ")-ATPase preparations show such a transition at 27-28°C [71,86]. However, a transition in the polarisation signal of a lipid viscosity probe (diphenylhexatriene), was not observed in this region [71], indicating that the transition in the ATPase activity is either due to a change in the rate-limiting step of the reaction or to the presence of an annulus of specific lipids with physical properties different from those of the bulk lipids. 

It was originally thought that there was a strict lipid specificity for restoration of activity of purified enzymes. It is now clear that membrane lipids activate more efficiently than single lipids (see Sandermann, 1978). It is also important to consider changes which might occur in the kinetic properties... 

The NADH dehydrogenase isolated from the archaebacterium Sulfolobus acidocaldarius is specific for NADH and reduces certain quinones, ferricyanide and 2,6-dichlorophenolindophenol [148]. The actual pH optimum of the enzyme was dependent upon the electron acceptor used, being considerably lower for ferricyanide (4.5) than for 2,6-dichlorophenolindophenol. The membrane enzyme from Bacillus caldotenax had similar properties when solubilized and partially purified, activity being stimulated by membrane lipids and non-ionic detergent [149]. The Bacillus caldovelox" enzyme was more stable in the membrane bound form than when solubilized and the addition of membrane lipids to the soluble enzyme preparation resulted in restoration of thermostability to levels comparable... 

In 1987 Brozek et al., characterized an enzyme found in membranes of E. coli that catalyzes the transfer of a palmitate moiety from the sm-1 position of glycer-ophospholipids, such as phosphatidylethanolamine or phosphatidylglycerol, to the hydroxyl of the iV-linked acyl chain of lipid X [50], This enzyme is specific for glycerophospholipids as acyl donors. Acyl-ACP and acyl-CoA are not substrates [50]. The reaction is very specific for acyl chain length. No other fatty acyl chain can substitute for palmitate [50]. 

There are other ways in which the lateral organization (and asymmetry) of lipids in biological membranes can be altered. Eor example, cholesterol can intercalate between the phospholipid fatty acid chains, its polar hydroxyl group associated with the polar head groups. In this manner, patches of cholesterol and phospholipids can form in an otherwise homogeneous sea of pure phospholipid. This lateral asymmetry can in turn affect the function of membrane proteins and enzymes. The lateral distribution of lipids in a membrane can also be affected by proteins in the membrane. Certain integral membrane proteins prefer associations with specific lipids. Proteins may select unsaturated lipid chains over saturated chains or may prefer a specific head group over others. 

The mechanisms involved in the establishment of lipid asymmetry are not well understood. The enzymes involved in the synthesis of phospholipids are located on the cytoplasmic side of microsomal membrane vesicles. Translocases (flippases) exist that transfer certain phospholipids (eg, phosphatidylcholine) from the inner to the outer leaflet. Specific proteins that preferentially bind individual phospholipids also appear to be... 

Purified membrane proteins or enzymes can be incorporated into these vesicles in order to assess what factors (eg, specific lipids or ancillary proteins) the proteins require to reconstitute their function. Investigations of purified proteins, eg, the Ca " ATPase of the sarcoplasmic reticulum, have in certain cases suggested that only a single protein and a single lipid are required to reconstitute an ion pump. 

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