**4. Structure and function of CETP**

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

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

to perform its functions. Hence, the real mechanism of CETP-mediated lipid transfer still

Structural Basis and Functional Mechanism of Lipoprotein in Cholesterol Transport

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

11

In this chapter, we briefly summarized the functional mechanism and structural basis of lipoproteins (e.g., HDL, LDL and CETP) in cholesterol transport, as well as their structural dynamics during the transport process. Furthermore, the latest developments in the plasma lipoprotein (HDL and LDL) elevations were summarized, especially the conformational changes of lipoprotein particles. Due to the incapability of the current assays and highly heterogeneous of lipoprotein particles, the function of lipoprotein in cholesterol transport remains elusive with regard to many important questions, such as how the lipoprotein particle assembles and how the assembly modulates the neutral lipids dynamic exchanges at the molecular level. Cryo-EM coupled with MD simulations have revealed several important mechanisms of CETP-mediated lipid exchange and metabolism with all-atom detail [89, 95]. Further researches could pay more attention to simultaneously monitor the dynamic structural change of lipoproteins and the dynamic mechanism of lipid transfer, especially the internal motivation of physical mechanism during the process of lipid transport.

Project supported by the National Natural Science Foundation of China under Grant No. 11374237, 11504287, 11774279 and 11774280, Fundamental Research Funds for the Central Universities (xjj2017029), China Postdoctoral Science Foundation (2017 M613147) and Shaanxi

, Lei Zhang1

and Shengli Zhang1

\*

remains to be studied and verified.

**5. Conclusion**

**Acknowledgements**

**Conflict of interest**

**Author details**

Xi'an, Shaanxi, China

Xi'an, China

Zhiwei Yang1,2,3, Dongxiao Hao1

Province Postdoctoral Science Foundation (2017).

The authors have declared that no competing interests exist.

\*Address all correspondence to: zhangsl@xjtu.edu.cn

, Yizhuo Che1

1 Department of Applied Physics, School of Science, Xi'an Jiaotong University,

2 Department of Applied Chemistry, School of Science, Xi'an Jiaotong University,

3 School of Life Science and Technology, Xi'an Jiaotong University, Xi'an, China

**Figure 5.** The crystal structure of CETP (PDB: 2OBD) and three-dimensional density maps of CETP binging lipoproteins. (a) Atom figure of CETP. (b) Secondary structure of CETP. (c) Ternary complexes of HDL-CETP-LDL in cryo-EM micrographs. (d)~(f) the CETP insert into HDL, VLDL, LDL respectively in cryo-EM micrographs. (g) (color online) the tunnel model of CETP-mediated lipid transfer [89].

esters between HDL and LDL (or VLDL), the way of CETP interacts with lipoproteins is extremely essential. CETP shows a high binding affinity for nascent HDL and other lipoproteins to cover the lipoproteins surfaces owing to its proper curvature radius. They proposed a lipid transport mechanism, shuttle model. In this mechanism, the CETP in turn covers the surface of LDL (or VLDL) and HDL to swap LDL-cholesterol esters (or VLDL-cholesterol esters) with HDL-triglycerides. These steps are constantly recycled until the completion of the transport process, in which cholesterol esters move from LDL (or VLDL) to HDL [20]. This model based on the hydrophobic cavity of CETP and its feasibility of binding to lipoproteins, explains the mechanism of CETP-mediate lipid transfer reasonably, but there are not complex of CETP binding to lipoproteins in the cryo-EM micrographs intuitively to verify the authenticity of the model.

Zhang et al. [89] studied human recombinant CETP with cryo-EM by using an optimized negative-staining (OpNS) EM protocol [49, 90]. Applied the single-particle techniques, they obtained the 3D structure of CETP and the complexes of CETP binging to lipoproteins. In the 3D-map of complexes, they discovered the HDL-CETP binding structure which appears to be formed by N-terminal of CETP insert into HDL and the HDL-LDL (or HDL-VLDL) is formed by C-terminal of CETP insert into HDL (or LDL) (**Figure 5**c~f). This conclusion was later confirmed by Geraldine et al. by using large-scale atomistic molecular dynamics [91]. The measurement of the protrusion from the lipoproteins surface shows that ~48 Å of the tapered N-terminal end of CETP penetrates the HDL surface and ~25 Å of the C-terminal end of CETP penetrates the LDL surface (~20 Å of the C-terminal end of CETP penetrates the VLDL surface) reaching the lipid–rich, lipoproteins core. Furthermore, Zhang et al. proposed the tunnel model of lipid transfer mediated by CETP [89, 92, 93]. In this model, both CETP terminals finish penetrating surface sites on lipoproteins, N-terminal to HDL and C-terminal to LDL (or VLDL). Then neutral lipids, including cholesterol esters and triglycerides, transfer through the hydrophobic tunnel at the core of the CETP (**Figure 5**).

However, there are some discrepancies with the tunnel model mentioned above. Matthias et al. used the experiments which involve three monoclonal antibodies to demonstrate that the antibodies binding on both ends of CETP do not inhibit CETP's function of transshipment cholesterol esters, but the antibodies on the middle does [94]. In their research they supposed that the formation of the ternary tunnel complexes is not a mechanistic prerequisite by CETP to perform its functions. Hence, the real mechanism of CETP-mediated lipid transfer still remains to be studied and verified.
