Apoprotein properties

R.J. Carrico and H.F. Deutsch, Presence of zinc in human cytocuprein and some properties of the apoprotein. J. Biol. Chem. 245, 723-727 (1970). 

Moncrief MBC, Hausinger RR 1996. Purification and activation properties of UreD-UreF-Urease apoprotein complexes. J Bacteriol 178 5417-21. 

The information available in this polypeptide or these polyeptides is far from definitive. Controversy exists on size and molecular weight of the apo LDL monomers, their number, and chemical properties (Table 8). Also, the question of polypeptide heterogeneity has not been resolved this is not surprising in view of the difficulty of protein solubilization discussed earlier. At the time of this writing, apo LDL is the mostly poorly characterized protein of all serum lipoproteins. This apoprotein is... 

One approach for identifying the type of iron center in a protein has been to remove the center, intact, by ligand exchange between the protein and an exogenous acceptor ligand (reviewed by Berg and Holm, 1982). The removed, or extruded, center is identified by comparison of its spectral properties (usually absorption spectrum) to known model compounds. Alternatively, the extruded center can be inserted into a second apoprotein which has been previously determined to accept only one type of iron center. The reconstructed standard protein is then analyzed by EPR. The latter method, interprotein cluster transfer, requires that acceptor apoproteins for all known classes of centers are included in the reaction mixture and that the reconstituted reporter ... 

The properties of the semiquinone from of the ETF isolated from the methylotrophic bacterium resemble those of the bacterial flavodoxins with the exception that flavodoxins form neutral semiquinones whereas this ETF forms an anionic semiquinone. Nearly quantitative anionic semiquinone formation is observed either in the presence of excess dithionite or when excess trimethylamine and a catalytic amount of trimethylamine dehydrogenase are added. Of interest is the apparent stability of the anionic semiquinone towards oxidation by O2 but not to oxidizing agents such as ferricyanide. This appears to be the first example of an air-stable protein-bound anionic flavin semiquinone. Future studies on the factors involved in imparting this resistance to O2 oxidation by the apoprotein are looked forward to with great interest. 

Properties of the Apoproteins of the Major Human Lipoprotein Classes... 

The structure and function of these apoproteins has been intensely studied in the past decade, and some of the properties of these apoproteins are summarized in table 20.3. All of the apoproteins from human plasma have been sequenced and contain regions that are rich in hydrophobic amino acids, which facilitate binding of lipid. 

Investigations of the kinetics of the reaction of these new siderophores with iron-saturated transferrin showed a rapid formation of a ternary complex with transferrin, followed by a slow step in which the ferric siderophore was released from the apoprotein. Weitl et al.257 have evaluated the ferric-chelating properties of several of these siderophores and found the following order of effectiveness for removing iron from transferrin enterobactin > MECAMS > MECAM > LICAMS > DFOA > TRIM-CAMS. 

Lipoproteins are globular, micelle-like particles consisting of a hydrophobic core of triacylglycerols and cholesterol esters surrounded by an amphipathic coat of protein, phospholipid and cholesterol. The apolipoproteins (apoproteins) on the surface of the lipoproteins help to solubilize the lipids and target the lipoproteins to the correct tissues. There are five different types of lipoprotein, classified according to their functional and physical properties chylomicrons, very low density lipoproteins (VLDLs), intermediate density lipoproteins (IDLs), low density lipoproteins (LDLs), and high density lipoproteins (HDLs). The major function of lipoproteins is to transport triacylglycerols, cholesterol and phospholipids around the body. 

Know lipid transport systems throughout the organism the different types of lipoproteins and their properties, their metabolism, their component apoproteins, and the functions of each individual lipoprotein and apoprotein. 

Several types of proteins are associated with lipoproteins. These are termed apolipoproteins, or simply apoproteins. Table 19.2 shows the various apolipo-proteins (Apos), their chemical properties and occurrence, and their function, which is discussed later. Note that the A apoproteins are found largely in HDL, the B-100 is found largely in LDL, VLDL, and IDL, and C apoproteins are largely seen in chylomicrons. Nevertheless, there is a large degree of apoprotein overlap among the various lipoprotein classes. 

The dwell time of a transferrin molecule with the reticulocyte may be only a minute or two (52, 80) or possibly as long as 10 min (86), after which the protein is released for another cycle of iron transport. The affinity of reticulocyte receptors for apotransferrin appears less than for iron-loaded molecules (80). This property may facilitate the release of the protein from the receptor after it has donated its iron and ensures that the apoprotein will not impede the delivery of iron to reticulocytes by competing with iron-bearing molecules for available receptors on the cell surface. 

See Fig. 6-4. The polar surface of the spherical particle renders the assembly soluble in water. This structure can be considered to be a tentative one only. The amount of polar material in chylomicrons and VLDL is astonishingly small. Moreover, when lipoproteins come into contact with the membranes of the cells of target tissue, the proteins remain soluble and do not become incorporated into the membrane. This suggests that the proteins of lipoproteins have unusual properties. It is known that several species of proteins (apoproteins AI, All. B4K, B1(K), Cl, CII, CIII, D, and E) occur. The amino acid sequences of some of them have been determined, and they possess hydrophobic regions i.e., they have properties suggesting that parts of their structure are compatible with hydrocarbons (e.g., TAGs and the tails of phospholipids). 

The above rationalizations rest on the assumption that iron is displaced toward Lg, putting, perhaps, undue weight on the premise that enzyme properties are solely governed by the relative positions of Lg, L , and iron. At best, it is but one facet of the possible constraints imposed by the apoprotein upon the environment of the prosthetic group. 

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