Phospholipid in eukaryotes

Phospholipids are hydrophobic molecules present in all living organisms. They are applied as building blocks of cellular membranes, but serve several other functions as well. The most important classes of phospholipids in eukaryotic cells are the sphingomyelins (SM, Figure 21.4) and glycerophospholipids (GPL, Table 21.2), while phosphoglycolipids are found in prokaryotic cells. Fast-atom bombardment ionization (FAE) first enabled the use of MS and MS-MS for the structural characterization of phospholipids. ESI-MS further facilitated this. ESI-MS characterization of phospholipids was reviewed by Pulfer and Murphy [3]. 

Figure 1. Structure of phosphatidylinositol. Phosphatidylinositol (Ptdins) constitutes about 10% of the total phospholipids in eukaryotic cells and is the precursor of the other phosphoinositides (polyphosphoinositides) through sequential phosphorylations by specific kinases. As indicated, its inositol head group can be phosphorylated at three positions (D-3, D-4 and D-5) by specific kinases in vivo. The cleavage by phosphoinositide-specific phospholipase C (PLC), which has as its preferred substrate PtdIns(4,5)P2, is also shown. PI3K, phosphoinositide 3-kinase. PI-K II and III, phosphatidylinositol kinase types II and III. PIP-K I, phosphatidylinositol monophosphate kinase type I. PIP-K II, phosphatidylinositol monophosphate kinase type II.

Cardiolipin or diphosphatidyl glycerol is one of the most ancient membrane phospholipids from phylogenic aspects. It is surprising for such a complex molecule as cardiolipin to have evolved as one of the major membrane lipids in prokaryotics, when steroids such as cholesterol and phytosterols did not. In eukaryotic cells, cardiolipin is exclusively localized within the mitochondria where it is particularly emiched in the outer leaflet of the inner membrane. Even though a molecular structure of cardiolipin has been conserved in entire organisms, its biological significance has escaped attention except in the case of anti-cardiolipin auto-antibodies which are clinically associated with the Wasserman reaction. 

Lipid transfer peptides and proteins occur in eukaryotic and prokaryotic cells. In vitro they possess the ability to transfer phospholipids between lipid membranes. Plant lipid transfer peptides are unspecific in their substrate selectivity. They bind phosphatidylcholine, phosphatidylethanolamine, phosphatidylinositol, and glycolipids. Some of these peptides have shown antifungal activity in vitro The sequences of lipid transfer proteins and peptides contain 91-95 amino acids, are basic, and have eight cysteine residues forming four disulfide bonds. They do not contain tryptophan residues. About 40% of the sequence adopts a helical structure with helices linked via disulfide bonds. The tertiary structure comprises four a-helices. The three-dimensional structure of a lipid transfer peptide from H. vulgare in complex with palmitate has been solved by NMR. In this structure the fatty acid is caged in a hydrophobic cavity formed by the helices. 

In eukaryotic cells, phospholipid synthesis occurs primarily on the surfaces of the smooth endoplasmic reticulum and the mitochondrial inner membrane. Some newly formed phospholipids remain at the site of synthesis, but most are destined for other cellular locations. 

In eukaryotes, phosphatidylglycerol, cardiolipin, and the phosphatidylinositols (all anionic phospholipids see Fig. 10-8) are synthesized by the same strategy used for phospholipid synthesis in bacteria. Phosphatidylglycerol is made exactly as in bacteria. Cardiolipin synthesis in eukaryotes differs slightly phosphatidylglycerol condenses with CDP-diacylglycerol (Fig. 21-26), not another molecule of phosphatidylglycerol as in E. coli (Fig. 21-25). 

PC and PE are the most abundant phospholipids in most eukaryotic cells. The primary route of their synthesis uses choline and ethanolamine obtained either from the diet or from the turnover of the body s phospholipids. [Note In the liver, PC also can be synthesized from phosphatidylserine (PS) and PE (see below).]... 

Figure 21-4 Biosynthesis of triacylglycerols, glycol ipids, and major phospholipids that are formed both in prokaryotes and eukaryotes. More complete schemes of phospholipid synthesis are shown in Figs. 21-3 and 21-5. Green arrow pathway occurring only in eukaryotes.

Cells continuously secrete materials via small cytoplasmic vesicles, which in eukaryotes arise largely from the Golgi apparatus (pp. 425-427 Fig. 20-8). The vesicles of this constitutive pathway may have diameters of 50 nm. They carry phospholipids, proteins, and other constituents for incorporation into the plasma membrane of the cell.618 619 In addition, there are... 

Synthesis of most phospholipids starts from glycerol-3-phosphate, which is formed in one step from the central metabolic pathways, and acyl-CoA, which arises in one step from activation of a fatty acid. In two acylation steps the key compound phosphatidic acid is formed. This can be converted to many other lipid compounds as well as CDP-diacylglycerol, which is a key branchpoint intermediate that can be converted to other lipids. Distinct routes to phosphatidylethanolamine and phosphatidylcholine are found in prokaryotes and eukaryotes. The pathway found in eukaryotes starts with transport across the plasma membrane of ethanolamine and/or choline. The modified derivatives of these compounds are directly condensed with diacylglycerol to form the corresponding membrane lipids. Modification of the head-groups or tail-groups on preformed lipids is a common reaction. For example, the ethanolamine of the head-group in phosphatidylethanolamine can be replaced in one step by serine or modified in 3 steps to choline. 

Phospholipid synthesis in eukaryotes is more complex than in E. coli. This relates to the other roles phospholipids play in membranes aside from their structural role. Eukaryotes... 

The first phase of phospholipid synthesis in E. coli and eukaryotes. Additional routes to and from phosphatidic acid, found predominantly in eukaryotes, are shown in brackets. 

The second phase of phospholipid synthesis in eukaryotes. Choline or ethanolamine enters the cell via active transport mechanisms and is immediately phosphorylated by the enzyme, choline (ethanolamine) kinase. The phosphorylated derivatives of choline and ethanolamine... 

A typical biological membrane is a complex structure composed primarily of lipids and proteins. The major structural components of the bilayer are various lipids. In eukaryotes, the most common type of lipids are phosphatidylcholines, whereas in prokaryotes (such as Escherichia coli), the main lipids are typically phosphatidylethanolamines (1). One example of a typical eukaryotic neutral (zwitterionic) phospholipid is palmitoyl-oleoy 1-phosphatidylcholine (POPC). The molecular structure of POPC is compared to those of dimyristoylphosphatidyl-choline (DMPC) and the negatively charged dimyristoylphosphatidylglycerol (DMPG), commonly used in membrane mimetics, in Fig. 1. 

Zachowski, A., 1993. Phospholipids in animal eukaryotic membranes transverse asymmetry and movement. Biochem. J., 294 1-14... 

Apart from purely artificial vesicles made from phospholipids and proteins of choice, it is also possible to make vesicles from the cytoplasmic membranes of cells, usually by a sonication procedure. Artificial vesicles and vesicles derived from natural membranes have proved very useful in studying transport phenomena across membranes. Vesicles also occur naturally, e.g.. by the budding of the Golgi apparatus in eukaryotic cells (Chap. 1). 

The bulk constituent of cells is water (H20). The cell membrane or plasma membrane (PM) that encloses the living cell is basically composed of a phospholipid bilayer, a 0.01 micrometre ( xm) (10 nm) thick bimolecular layer of hydrophobic (or water repelling) fatty molecules. In eukaryotes (organisms having a nucleus) there is a phospholipid bilayer PM enclosing the cell. Similar membranes bound specialized intracellular organelles, namely the endoplasmic reticulum (ER), ER-associated Golgi vesicles, lysosomes, vacuoles, peroxisomes, nucleus and mitochondria (and, additionally, the chloroplasts in plant cells). 

Phosphatidylserine also has a net negative charge. It is a widespread but minor lipid in eukaryotes, accounting usually for less than 10% of the total phospholipids. Its greatest concentration has been noted for myelin from brain tissue. Phosphatidylserine is concentrated in the inner monolayer of the plasma membrane and the other cellular membranes (Fig. 1). 

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