Structure of apolipoprotein B100 in low density lipoproteins (original) (raw)
Related papers
Low density lipoprotein: structure, dynamics, and interactions of apoB-100 with lipids
2011
Low-density lipoprotein (LDL) transports cholesterol in the bloodstream and plays an important role in the development of cardiovascular diseases, in particular atherosclerosis. Despite its importance to health, the structure of LDL is not known in detail. This is worrying since the lack of LDL's structural information makes it more difficult to understand its function. In this work, we have combined experimental and theoretical data to construct LDL models comprised of the apoB-100 protein wrapped around a lipid droplet of about 20 nm in size. The models are considered by near-atomistic multi-microsecond simulations to unravel structural as well as dynamical properties of LDL, with particular attention paid to lipids and their interactions with the protein. We find that the distribution and the ordering of the lipids in the LDL particle are rather complex. The previously proposed 2-and 3layer models turn out to be inadequate to describe the properties of the lipid droplet. At the surface of LDL, apoB-100 is found to interact favorably with cholesterol and its esters. The interactions of apoB-100 with core molecules, in particular cholesteryl esters, are rather frequent and arise from hydrophobic amino acids interacting with the ring of cholesteryl esters, and also in part from the rather loose packing of lipids at the surface of the lipoparticle. The loose packing may foster the function of transfer proteins, which transport lipids between lipoproteins. Finally, the comparison of the several apoB-100 models in our study suggests that the properties of lipids in LDL are rather insensitive to the conformation of apoB-100. Altogether, the findings pave the way for further studies of LDL to better understand the central steps in the emergence of atherosclerosis.
Biochimica et Biophysica Acta (BBA) - Molecular and Cell Biology of Lipids, 2000
Low density lipoprotein (LDL) particles are the major cholesterol carriers in circulation and their physiological function is to carry cholesterol to the cells. In the process of atherogenesis these particles are modified and they accumulate in the arterial wall. Although the composition and overall structure of the LDL particles is well known, the fundamental molecular interactions and their impact on the structure of LDL particles are not well understood. Here, the existing pieces of structural information on LDL particles are combined with computer models of the individual molecular components to give a detailed structural model and visualization of the particles. Strong evidence is presented in favor of interactions between LDL lipid constituents that lead to specific domain formation in the particles. A new three-layer model, which divides the LDL particle into outer surface, interfacial layer, and core, and which is capable of explaining some seemingly contradictory interpretations of molecular interactions in LDL particles, is also presented. A new molecular interaction model for the Lsheet structure and phosphatidylcholine headgroups is introduced and an overall view of the tertiary structure of apolipoprotein B-100 in the LDL particles is presented. This structural information is also utilized to understand and explain the molecular characteristics and interactions of modified, atherogenic LDL particles. ß 2000 Elsevier Science B.V. All rights reserved.
Molecular structure of low density lipoprotein: current status and future challenges
European Biophysics Journal, 2009
This review highlights recent advances in structural studies on low density lipoprotein (LDL) with particular emphasis on the apolipoprotein moiety of LDL, apolipoprotein B100 (apoB100). Various molecular aspects of LDL are outlined and obstacles to structure determination are addressed. In this context, the prevailing conceptions of the molecular assembly of LDL and how the synergy of complementary biochemical, biophysical and molecular simulation approaches has lead to the current structural model of LDL are discussed. Evidence is presented that structural heterogeneity and the intrinsic dynamics of LDL are key determinants of the functionality of LDL in both health and disease. Some key research directions, remaining open questions and rapidly emerging new concepts for medical applications of LDL, are furthermore outlined. The article concludes by providing an outlook concerning promising future strategies for the clariWcation of the molecular details of LDL, in particular of apoB100, combining recent advances in molecular modeling with developments of novel experimental techniques. Although new insights into the molecular organization of LDL are forthcoming, many open questions remain unanswered. The major challenge of the next decade will certainly be the elucidation of the molecular structural and dynamic features of apoB100.
Structure of apolipoprotein B-100 of human low density lipoproteins
Arteriosclerosis, Thrombosis, and Vascular Biology, 1989
We have analyzed low density lipoproteins (LDL) apolipoprotein (apo) B structure by direct sequence analysis of LDL apo B-100 tryptlc peptides. Native LDL were digested with trypsin, and the products were fractionated on a Sephadex G-50 column. The partially digested apo B-100 still associated with liplds was recovered In the void volume (designated trypsln-nonreleasable, TN, peptides). The released peptides (designated trypsln-releasable, TR, peptides) in subsequent peaks were repurifled on two successive high-performance liquid chromatogaphy (HPLC) columns. The TN peak was dellpidated and redigested with trypsin, and the resulting peptides were purified on two successive HPLC columns. Using this approach, we sequenced over 88% of LDL apo B-100, extending and refining our previous study (Nature 1986;323:738-742) which covered 52% of the protein. TN peptides made up 31%, and the TR peptides, 34% of the apo B-100 sequence; 23.7% were found under both TN and TR categories. Based on its differential trypsin releasabllity, apo B-100 can be divided into five domains: 1) residues 1-1000, largely TR; 2) residues 1001-1700, alternating TR and TN; 3) residues 1701-3070, largely TN; 4) residues 3071-4100, mainly TR and mixed; and 5) residues 4101-4536, almost exclusively TN. Domain 1 contained 14 of the 25 Cys residues In apo B. Domain 4 encompassed seven N-glycosylatIon sites, and contained the putative receptor binding domains. All 19 potential N-glycosylatlon sites were directly sequenced: 16 were found to be glycosylated and three were not Three pairs of dlsurfide bridges were also mapped. Finally, a combination of cDNA sequencing, direct mRNA sequencing, and comparison of published apo B-100 sequences allowed us to Identify specific amino acid residues within apo B-100 that seem to represent bona fide allellc variations. Our study provides information on LDL apo B-100 structure that will be important to our understanding of its conformation and metabolism.
Arteriosclerosis, Thrombosis, and Vascular Biology, 2000
Apolipoprotein B (apoB)-100-containing lipoproteins are secreted from the liver as large triglyceride-rich very low density lipoproteins (VLDLs) into the circulation, where they are transformed, through the action of lipases and plasma lipid transfer proteins, into smaller, less buoyant, cholesteryl ester-rich low density lipoproteins (LDLs). As a consequence of this intravascular metabolism, apoB-containing lipoproteins are heterogeneous in size, in hydrated density, in surface charge, and in lipid and apolipoprotein composition. To identify specific regions of apoB that may undergo conformational changes during the intravascular transformation of VLDLs into LDLs, we have used a panel of 29 well-characterized anti-apoB monoclonal antibodies to determine whether individual apoB epitopes are differentially expressed in VLDL, intermediate density lipoprotein (IDL), and LDL subfractions isolated from 6 normolipidemic subjects. When analyzed in a solid-phase radioimmunoassay, the expression of most epitopes was remarkably similar in VLDLs, IDLs, and LDLs. Two epitopes that are close to the apoB LDL receptor-binding site show an increased expression in large (1.019 to 1.028 g/mL), medium (1.028 to 1.041 g/mL), and small (1.041 to 1.063 g/mL) LDLs compared with VLDLs and IDLs, and 2 epitopes situated between apoB residues 4342 and 4536 are significantly more immunoreactive in small and medium-sized LDLs compared with VLDLs, IDLs, and large LDLs. Therefore, as VLDL is converted to LDL, conformational changes identified by monoclonal antibodies occur at precise points in the metabolic cascade and are limited to well-defined regions of apoB structure. These conformational changes may correspond to alterations in apoB functional activities.
Structure of apolipoprotein A-I in spherical high density lipoproteins of different sizes
Proceedings of the National Academy of Sciences, 2008
Spherical high density lipoproteins (HDL) † predominate in human plasma. However, little information exists on the structure of the most common HDL protein, apolipoprotein (apo) A-I, in spheres vs. better studied discoidal forms. We produced spherical HDL by incubating reconstituted discoidal HDL with physiological plasmaremodeling enzymes and compared apoA-I structure in discs and spheres of comparable diameter (79 -80 and 93-96 Å). Using cross-linking chemistry and mass spectrometry, we determined that the general structural organization of apoA-I was overall similar between discs and spheres, regardless of diameter. This was the case despite the fact that the 93 Å spheres contained three molecules of apoA-I per particle compared with only two in the discs. Thus, apoA-I adopts a consistent general structural framework in HDL particles-irrespective of shape, size and the number of apoA-Is present. Furthermore, a similar cross-linking pattern was demonstrated in HDL particles isolated from human serum. We propose the first experiment-based molecular model of apoA-I in spherical HDL particles. This model provides a new foundation for understanding how apoA-I structure modulates HDL function and metabolism.
Journal of lipid research, 1995
Naturally occurring mutant forms of apolipoprotein B (apoB)-100 may be able to provide valuable information on the structure-function relationships of apoB with the low density lipoprotein (LDL) receptor. ApoB-67, recently identified in a kindred displaying apoB levels 25% of normal (Welty et al. J. Clin. Invest. 1991. 87: 1748-1754), is predicted to contain 3040 amino acids and therefore, contains part of the epitope for antibody 4G3, which blocks binding of LDL to the LDL-receptor. To determine whether the amino terminal 67% of apoB-100 is important for binding to the LDL receptor, the apoB-67-containing lipoprotein particle was purified from plasma by gradient ultracentrifugation. The fractions containing apoB-67 were in the density range 1.049-1.070 g/ml. These fractions were pooled and adsorbed onto an affinity chromatography column containing the monoclonal antibody, MB-47. The epitope for MB-47 is two nonlinear domains between amino acids 3429 to 3453 and 3507 to 3523; theref...
Biochemistry, 1993
Recombinant apolipoprotein(a) has been studied by hydrodynamic techniques and electron microscopy. Recombinant apo(a) was primarily a monomer in solution with an ~~~0 ,~ of 9.3 S, a D z o ,~ of 2.29 ficks, and a molecular weight of 325 000 from sedimentation equilibrium and 3 18 000 from combining the sedimentation and diffusion coefficients. A small amount, approximately lo%, of the recombinant apo(a) was present as a high molecular weight aggregate. The Stokes radius of the monomer, determined either from the diffusion coefficient or by combining the sedimentation equilibrium data with the sedimentation velocity data, was 94 A. The frictional ratio was 2.2, suggesting a highly asymmetric or random coil structure. In the electron microscope, recombinant apolipoprotein(a) was visualized as a long, highly flexible chain of domains forming large, open coiled structures on the EM grid with contour lengths of about 800 A. Addition of 6-aminohexanoic acid at 50 mM, a concentration which should saturate the weak lysine binding sites, did not alter the sedimentation behavior. In vivo, apolipoprotein(a) is associated tightly with LDL to form a highly atherogeniclipoprotein, Lp(a). A single molecule of recombinant apo(a) also associated tightly with LDL to yield a 13.34 Lp(a)-like complex. This complex dissociated upon the addition of 50 mM 6-aminohexanoic acid. A novel sucrose gradient centrifugation technique was employed to determine a dissociation constant for the reversible equilibrium between recombinant apo(a) and LDL; at physiological ionic strength the dissociation constant was 0.3 nM. Raising the salt concentration to 5 M NaBr caused the dissociation constant to increase to 500 nM. Hydrodynamic modeling suggests recombinant apo(a) made contact with the LDL through, at most, a few kringles, with the remainder of the molecule extending into solution. Our results suggest that, in addition to the apoB-apo(a) disulfide bond, strong noncovalent forces hold the Lp(a) molecule together. Furthermore, the bulk of apo(a) is extended away from the lipoprotein surface, where it may readily interact with other ligands.