**3. Structure and function of LDL**

In normal human body, there are about 70% plasma cholesterol contained in LDLs, and the endocytosis of cholesterol-rich LDLs is mediated by LDL Receptor (LDL-R) on the surface of body cell. Hence, LDLs work as the vehicle for cholesterol transportation between liver and cells to maintain a constant cholesterol supply in human body [58, 59]. In some abnormal conditions, LDL might induce over-accumulation of cholesterol to form foam cells, resulting in the development of atherosclerosis [60]. The apo-B48 (apoprotein B48) and apo-B100 (apoprotein B100) located in surface of LDL particles tend to interact with extracellular material, which make LDL particles easy to bind with blood vessel intima [61]. The oxidation-LDL can promote lipoproteins aggregation [62, 63] and provoke inflammation by recruiting the circulating monocytes to the site followed invade the vessel wall and differentiate into macrophages, to finally produce atherosclerotic plaque [62, 64–66]. Cryo-EM combined with single particle technology and small angle scattering model reconstruction technology have been effectively applied to analyze the LDL structures, and molecular components [67]. LDLs include difference in density (~1.019–1.063), shape, size (diameter ~18–25 nm), surface charge and chemical composition [68]. A general consensus is that LDLs particles all have two compartments, an amphipathic surface phospholipid monolayer which surrounded by one single copy of apoB-100, and a hydrophobic lipid-cholesteryl esters core [69]. The structure and physical function of LDLs predominantly depend on the core-lipid composition and the conformation of the apoB-100 [70, 71].

**3.2. apoB-100 in LDL**

**4. Structure and function of CETP**

ApoB-100 (4536 residues, ~20% of overall LDL) is the only protein component of LDL, and wrapped around the phospholipid monolayer on the surface of LDL particle, with an irregular ring shape. N- and C-terminus of apoB-100 touch each other, with the formation of a protruding globular structure at N-terminal [81]. A more generally accepted structural model of apoB-100 is "pentapartite" structure, which generated by molecular simulations. In this model, apoB-100 has five consecutive functional domains, NH2-βα1-β1-α2-β2-α3-COOH [79]. As shown in **Figure 4**, a new LDL reconstruction in which lipid core is revealed an organized three-layer structure by using the single particle approach, including a pair of "paddles" configurations with several long "fingers" extensions which have similar length and interval [82].

Structural Basis and Functional Mechanism of Lipoprotein in Cholesterol Transport

http://dx.doi.org/10.5772/intechopen.76015

9

**Figure 4.** Overall structure and core structure of LDL above (a) or below (b) the critical temperature.

CETP acts as a medium between lipoproteins for elevating plasma LDL-C (or VLDL-C) level and lowering HDL-C level [19]. A series of CETP inhibitors have been investigated in clinical, such as torcetrapib, dalcetrapib, evacetrapib, and anacetrapib [83–85]. However, current inhibitors represent the turbulent beginning of CETP inhibition and an increased mortality rate related to off-target effects and lack of efficacy [86–88]. Accompanying adverse effects call

CETP is a hydrophobic transfer protein composed of 476 amino acids and reveals a so-called banana-shape (the size is 135 × 30 × 35 Å, see **Figure 5**) [20]. Its crystal structure includes two different β-barrel structures in N- and C- terminal respectively, and a central β-sheet with an ~60 Å-long hydrophobic central cavity, which can hold two phospholipids and two cholesterol molecules. Moreover, the two phospholipid molecules that located in two pores near the central domain expose the hydrophilic terminal to the aqueous environment and hydrophobic terminal to the hydrophobic cavity. Because of its special function to transfer cholesterol

for a deeper exploration of the mechanism for CETP-mediated lipid transfer.

#### **3.1. Lipid core of LDL**

Lipid core of LDL mainly consists of cholesteryl esters, some triglycerides, and some free-cholesterol. Structural changes of LDL are strikingly related to physiological temperature [72]. Lipids located in core show order arranged to a liquid-crystalline phase below the critical temperature, indicated by the results of X-ray and neutron small angle scattering technology, with the transition temperature of 15~35°C [73, 74]. Besides, the overall structure of LDL is a classical spherical particle when core structure is composed of radial cholesteryl esters arranged into a concentric spherical shell [75, 76]. However, the core-located lipids present in the liquid-crystalline state within an ellipsoidal shape particle revealed by the cryo-EM data [76, 77]. It seems reasonable to speculate that the change of temperature might indirectly change the shape of LDL particles from roughly spherical to ellipsoid [67]. Many efforts have been made to explore the structure of LDL at different temperatures, such as 4, 6 [77–79] and 37°C [80].

Structural Basis and Functional Mechanism of Lipoprotein in Cholesterol Transport http://dx.doi.org/10.5772/intechopen.76015 9

**Figure 4.** Overall structure and core structure of LDL above (a) or below (b) the critical temperature.

#### **3.2. apoB-100 in LDL**

sHDL contains similar amount of core lipid in reconstituted sHDL and has obviously less surface lipid monolayers, indicating that the apoA-I package on native spheres is much closer than the typical recombinant particles [46]. When a HDL disc alters to a sphere (LCAT converts free cholesterol to cholesteryl ester), global apoA-I conformation does not change significantly between particles of different shapes or origins, with similar protein–protein

In normal human body, there are about 70% plasma cholesterol contained in LDLs, and the endocytosis of cholesterol-rich LDLs is mediated by LDL Receptor (LDL-R) on the surface of body cell. Hence, LDLs work as the vehicle for cholesterol transportation between liver and cells to maintain a constant cholesterol supply in human body [58, 59]. In some abnormal conditions, LDL might induce over-accumulation of cholesterol to form foam cells, resulting in the development of atherosclerosis [60]. The apo-B48 (apoprotein B48) and apo-B100 (apoprotein B100) located in surface of LDL particles tend to interact with extracellular material, which make LDL particles easy to bind with blood vessel intima [61]. The oxidation-LDL can promote lipoproteins aggregation [62, 63] and provoke inflammation by recruiting the circulating monocytes to the site followed invade the vessel wall and differentiate into macrophages, to finally produce atherosclerotic plaque [62, 64–66]. Cryo-EM combined with single particle technology and small angle scattering model reconstruction technology have been effectively applied to analyze the LDL structures, and molecular components [67]. LDLs include difference in density (~1.019–1.063), shape, size (diameter ~18–25 nm), surface charge and chemical composition [68]. A general consensus is that LDLs particles all have two compartments, an amphipathic surface phospholipid monolayer which surrounded by one single copy of apoB-100, and a hydrophobic lipid-cholesteryl esters core [69]. The structure and physical function of LDLs predominantly depend on the core-lipid composition and the con-

Lipid core of LDL mainly consists of cholesteryl esters, some triglycerides, and some free-cholesterol. Structural changes of LDL are strikingly related to physiological temperature [72]. Lipids located in core show order arranged to a liquid-crystalline phase below the critical temperature, indicated by the results of X-ray and neutron small angle scattering technology, with the transition temperature of 15~35°C [73, 74]. Besides, the overall structure of LDL is a classical spherical particle when core structure is composed of radial cholesteryl esters arranged into a concentric spherical shell [75, 76]. However, the core-located lipids present in the liquid-crystalline state within an ellipsoidal shape particle revealed by the cryo-EM data [76, 77]. It seems reasonable to speculate that the change of temperature might indirectly change the shape of LDL particles from roughly spherical to ellipsoid [67]. Many efforts have been made to explore the structure of LDL at different temperatures, such as 4, 6 [77–79] and

contacts.

8 Cholesterol - Good, Bad and the Heart

**3. Structure and function of LDL**

formation of the apoB-100 [70, 71].

**3.1. Lipid core of LDL**

37°C [80].

ApoB-100 (4536 residues, ~20% of overall LDL) is the only protein component of LDL, and wrapped around the phospholipid monolayer on the surface of LDL particle, with an irregular ring shape. N- and C-terminus of apoB-100 touch each other, with the formation of a protruding globular structure at N-terminal [81]. A more generally accepted structural model of apoB-100 is "pentapartite" structure, which generated by molecular simulations. In this model, apoB-100 has five consecutive functional domains, NH2-βα1-β1-α2-β2-α3-COOH [79]. As shown in **Figure 4**, a new LDL reconstruction in which lipid core is revealed an organized three-layer structure by using the single particle approach, including a pair of "paddles" configurations with several long "fingers" extensions which have similar length and interval [82].
