Binding of Lipids to Proteins

The PI anchor maintains adhesion of acetylcholinesterase (of the red blood cell) and of some proteoglycans (su I fated proteinsof the extracellular matrix) to the cell membrane Palmitic acid is bound via thiol-ester bonds toCys 322 and Cys 323 of rhodopsln (see the section on vitamin A), a 327-amino-add protein. The polypeptide chain of rhodopsin loops in and out of the membrane several times, leaving the possible function of the lipid as an anchor in question. Myristic add is bound to the catalytic subunit of the cAMP-dependent protein kinase, though this protein is cytosolic and soluble. 

G Protein is a class of proteins, occurring at the plasma membrane, that participates in transmitting signals from the extracellular fluid to various enzymes within the cell. Myristic acid is bound to G protein. Specifically, myristic acid is attached to the a subunit of G protein. This lipid is bound via an amide linkage to the N-terminal glycine. A geranylgeranyl group is attached via a thiol-ester bond to a cysteine residue on the y subunit of G protein (Casey and Seabra, 1996). 

I he average daily intake of total dietary cholesterol is 400-500 mg. Cholesterol also enters the gastrointestinal tract via the bile. Between fiOO and 1200 mg of free cholesterol is secreted in the bile per day. By weight, bile consists of 92% water, 6% bile salts, 0,3% cholesterol, and small amounts of bilirubin, fatty acids, phosphatidylcholine, and sails. The cholesteryl esters of the diet are hydmlyzed to free cholesterol and a fatty add by pancreatic cholesterol esterase. After entry into the enterocyte, the free cholesterol is nmverted back to cholesteryl esters by acyl CoA cholesterol acyl transferase. Some evidence suggests that the absorption of dietary cholesterol (from the bile salt micelles) is mediated by a membrane-bound transport protein of the brush border (1 humhofer and Hauser, 1990), 

Plasma cholesterol levels are not changed very much by changes in the quantity of cholesterol in the diet for a number of reasons. First, reduction of dietary intake below typical le els results in a change in available cholesterol that is small compared with the rate of endogenous synthesis 9-12 mg/kg body weight per day. Hence, a 70-kg man synthesizes about 7(X) mg per day. Second, a reduction in the dietary intake may result in an increase in the rate of cholesterol biosynthesis in the body iTiis increase is due to an increase in the activity of one of the enzymes 

In the laboratory, synthetic membranes can be made from phospholipids alone. They cannot be made from cholesterol alone, but can be made from combinations of phospholipid and cholesterol. Cholesterol slabiliTcs the structure of a membrane. A minor form of cholesterol, cholesterol sulfate, may prevent the membrane from fusing with other membranes. Cholesterol sulfate is especially prevalent in the PM of sperm Cheetbam cl ai., 1990). 

---

The association of lipids with proteins in dilute aqueous solutions was studied by Tanford (15). He identified different types of interaction that depend on the number of associated lipid molecules. He also analyzed the relation between lipid association into micelles and the competing binding of lipids to proteins. 

Influence of Dietary Fats on Phospholipids in the Body Sphingosine-Based Lipids Covalent Binding of Lipids to Proteins Cholesterol Lipoproteins... 

Hantke K and Braun V (1973) Covalent binding of lipid to protein. Eur J Biochem 34 284-296 Heller B A, Holten D and Kirmaier C (1995a) Control of electron transfer between the L- and M-sides of photosynthetic reaction centers. Science 269 940-945... 

Covalent binding of carbohydrate to protein or lipid brings about large changes in the physical properties of these substances that allow them to serve specialized biochemical functions. Sulfated polysaccharides in glycoproteins, for example, are effective biological lubricants and linking carbohydrates to lipids allows them to be inserted into membranes. 

Experimental data offered as evidence for hydrophobic bonding of lipids to protein by Green and Tzagaloff (1966) can be readily given interpretations which exclude hydrophobic bond involvement of the protein with lipid chains. For example, (1) the stronger binding to mitochondrion structural protein of lipids with longer acyl chains... 

Protein kinase A (PKA) is a cyclic AMP-dependent protein kinase, a member of a family of protein kinases that are activated by binding of cAMP to their two regulatory subunits, which results in the release of two active catalytic subunits. Targets of PKA include L-type calcium channels (the relevant subunit and site of phosphorylation is still uncertain), phospholam-ban (the regulator of the sarcoplasmic calcium ATPase, SERCA) and key enzymes of glucose and lipid metabolism. 

Next we want to use the microresonator to detect the binding of a membrane protein to the lipid bilayer formed on the tube wall. The protein we choose is Annexin V, since its binding to the lipid membrane has been well studied in the... 

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. 

Recoverin is a Ca receptor with four EF structures and two Ca binding sites it can exist in the cytosol or associated with the membrane and has an N-terminal myristoyl residue as a lipid anchor. The distribution between free and membrane-associated forms is regulated by Ca. Binding of Ca to recoverin leads to its translocation from the cytosol to the membrane of the rod cells. Structural determination of recoverin in the Ca bound and Ca free forms (Ames et al., 1997) indicates that membrane association of recoverin is regulated by a Ca -myristoyl switch. The myristoyl residue can adopt two alternative positions in recoverin. In the absence of Ca, recoverin exists in a conformation in which the myristoyl residue is hidden in the iimer of the protein and is not available for membrane association. On Ca binding, a conformation change of recoverin takes place the myristoyl residue moves to the outside and can now associate with the membrane. 

---