Apolipoproteins secondary structures

Rozek, A.. Sparrow, J.T., Weisgraber, K.H., and. Cushley, R. J. (1998) Sequence-specific H NMR resonance assignments and secondary structure of human apolipoprotein C-I in the presence of sodium dodecyl sulfate, Biochemistry and Cell Biology 76, 267-275. 

Larger molecules such as proteins usually do not fit these predictions, probably because the molecules adopt an ordered three-dimensional structure in which many of the hydrophobic residues are buried within the structure and unavailable for interaction with the reversed phase. As might be expected from the proposed mechanism of separation, the retention of proteins on reversed-phase columns is not related to molecular weight of the sample, but rather the surface polarity of the molecule. Table I shows that there is a correlation of hydrophobicity (measured by mole % of strongly hydrophobic residues) with retention order for seven different proteins. It is unlikely that the retention of all proteins on a reversed-phase column can be correlated in this manner, because many protein structures have few nonpolar residues exposed to the aqueous environment. For example, although the major A and C apolipoproteins are eluted from a ju-Bondapak alkylphenyl column in an order which can be related to the proposed secondary structures, there is little correlation with the content of hydrophobic residues in each protein and the degree of interaction with the stationary phase. A similar lack of correlation be-... 

Fig. 2. Predicted secondary structure of human apolipoprotein E3. One-letter amino acid designations as in Fig. 1. The predicted secondary structure was determined by applying the Chou-Fasman algorithm (Chou and Fasman, 1974) and is predicted to contain two ordered segments, one in the amino- and one in the carboxyl-terminal region of the protein (residues 1-164 and 200-290, respectively).

Fig. 19. Carboxyl-terminal truncations of apolipoprotein E3. The predicted secondary structure of the carboxyl terminus of each of the four carboxyl-terminal truncated proteins is compared with that of intact apoE (top) and the 22-kDa fragment (bottom). Shown on the right are the characteristics of each protein with respect to lipoprotein binding and the ability of the lipid-free form to tetramerize.

Given the widespread occurrence of sequences in apolipoproteins that evidently code for amphipathic helices, it is not surprising that many workers have attempted to identify possible secondary structural elements in apolipoproteins and to predict possible tertiary interactions and overall arrangements of secondary structure elements when these proteins are bound to lipids (Edelstein et al., 1979). Here we discuss models for apoA-1 and apoE-3 developed by Nolte and Atkinson (1992). These models resulted from an examination of the primary sequence of human plasma and apoA-1 and apoE-3 using a variety of approaches, and an integration of the resulting data into unihed predictions for the secondary structures of those molecules. 

The key structural features predicted for the amphipathic helix by the original model (Segrest et al., 1974) enabled three laboratories to study independently how amino acid variability determined the properties of the amphipathic helix (Kanellis et al., 1980 Fukushima et al., 1980 Sparrow et al., 1981). The strategy adapted by these investigators was based, not on the primary sequence of naturally occurring apolipoproteins, but on incorporating the periodicity of the secondary structural features of the amphipathic helix motif into the sequences of the peptide analogs. 

FIGURE 3 The domain structure of a plasma lipoprotein. The nonpolar lipids triglyceride and cholesterol ester are surrounded by the amphipathic lipids phospholipid and cholesterol. The latter are stabilized by apolipoproteins. These proteins have amphipathic a-helix and amphipathic y6-sheet secondary structures. 

The structure of the native protein can be important in influencing the rate, extent, and effects of denaturation. Proline-rich peptides and proteins capable of a secondary helical structure appear to favor partially folded structures under denaturing conditions these are simultaneously present with random-coil denatured molecules (32). The presence of these two slowly interconverting conformers then leads to increased reversed-phase band broadening as above. Structural factors which disfavor protein denaturation can also operate to reduce band width. For the HIC separation of various apolipoproteins (96), it was found that more hydrophobic proteins gave narrower bands. This was attributed to a more structure conformer in the retained state for the more hydrophobic protein. That is, a single (native ) conformer exists during the separation of more hydrophobic proteins in this system, but some denaturation of less hydrophobic proteins occurs. 

Jabs, H.U., Assmann, G, Greifendorf, D., Benninghoven, A. (1986) High performance hquid chromatography and time-of-flight secondary ion mass spectrometry a new dimension in structural analysis of apolipoproteins. J. Lipid Res.,27, 613-621. 

Apolipoproteins undergo changes in secondary protein structure when combined with phosphatidylcholine (Morrisett et al., 1977b). The circular dichroic spectrum and the blue-shifted tryptophan fluorescence spectrum are consistent with an amphipathic structure. Since lipoprotein lipase also can undergo hydrophobic association with phospholipids (Voyta et al., 1980), as indicated by blue-shifted tryptophan fluorescence, it seems probable that the interaction of the enzyme with the apoprotein activator at the lipid-water interface involves extensive lateral protein protein interactions (Smith and Scow, 1979). 

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