2.2.3. LDL particle structure
In blood circulation, TG and CE are packed into LDL particles forming a hydrophobic core surrounded by a surface monolayer of polar PL together with unesterified cholesterol (FC) and apoB. LDL normally also contains lipophilic antioxidants, mainly Vitamin E and ß-carotene. LDL is a large spherical particle, molecular weight of about 3 x 106 Da, with a diameter of 22-28 nm and density between 1.019-1.063 g/ml. The core is composed of some 1,600 molecules of CE (long chain fatty acid) and 170 molecules of TG. The CE is the main lipid of the lipoprotein core with the most fatty acyl chain in these esters being linoleate (Krieger et al. 1978). This core is shielded by a layer of PL (700 molecules), FC (600 molecules), and 1 molecule of apoB-100 (Steinberg 1997b, Stryer 1988, Yang et al. 1989). In the percent mass composition, each LDL particle consists of 35-45% CE, 7-10% FC, 7-10% TG, 15-20% PL, and 20-25% protein (Deckelbaum et al. 1987, Schultz and Liebman 1997).
Figure 2. Schematic model of LDL particle.
Overall structure of the LDL particle is shown in Fig. 2. The PLs are arrayed so that their hydrophilic heads are on the outside, allowing the LDL to be dissolved in the blood or intercellular fluid. Embedded in this hydrophilic coat is one apoB-100 molecule. Effective and efficient binding for apoB-100 to the LDL receptor is the prerequisite for cholesterol delivery into cells. In principle, LDL can also transport other lipophilic biosubstances into cells under the direction of apoB-100. On the other hand, apoB-100 gene mutation or conformational changes may affect LDL metabolism and plasma cholesterol levels (Innerarity et al. 1987, Aviram et al. 1988, Chen et al. 1994).
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