What is Lipogenesis?

Fat metabolism is an important and complex biochemical reaction in the body. It refers to the process of digestion, absorption, synthesis and decomposition of fats in the body with the help of various related enzymes. It is processed into substances required by the body to ensure the normal physiological functions. Life activities are of great significance. Lipids are important substances for energy storage and energy supply in the body, and also important structural components of biofilms. Diseases caused by abnormal fat metabolism are common diseases in modern society.

Fat metabolism

Fat metabolism is an important and complex biochemical reaction in the body. It refers to the process of digestion, absorption, synthesis and decomposition of fats in the body with the help of various related enzymes. It is processed into substances required by the body to ensure the normal physiological functions. Life activities are of great significance. Lipids are important substances for energy storage and energy supply in the body, and also important structural components of biofilms. Diseases caused by abnormal fat metabolism are common diseases in modern society.

The digestion of fat is mainly in the upper part of the small intestine, which is hydrolyzed into glycerol and fatty acids by various enzymes and bile salts. There are two types of lipid absorption: triglycerides composed of medium-chain and short-chain fatty acids can be absorbed after emulsification and enter the blood through the portal vein; triglycerides composed of long-chain fatty acids are combined with apolipoprotein and cholesterol to form chyle particles Finally, blood was introduced through the lymph.

The biochemical process of metabolism in the body after fat absorption is mainly divided into four types of lipids: triglycerides, phospholipids, cholesterol, and plasma lipoproteins. It is complicated and precise by insulin, glucagon, diet nutrition, and biochemical enzyme activity in the body. Regulation, into the body's various fine biochemical reactions required by the material components. Liver, adipose tissue, and small intestine are important places to synthesize fat, and the liver has the strongest synthesis ability. After synthesis, it is combined with apolipoprotein, cholesterol, etc. to form very low density lipoprotein (VLDL), which is transported into the blood to be stored or used in extrahepatic tissues. If the triglycerides synthesized by the liver cannot be transported in time, fatty liver will be formed. When chronic hunger and insufficient sugar supply, a large amount of fatty acids are used to generate acetyl CoA for oxidative energy supply and a large amount of ketone bodies. The liver is an organ that produces ketone bodies, but cannot use ketone bodies. Brain tissue cannot utilize fatty acids, while ketone bodies are soluble in water and have small molecules that can cross the blood-brain barrier. In severe diabetic patients, glucose is not effectively used, fatty acids are converted to generate a large number of ketone bodies, which exceeds the ability of extrahepatic tissues to use, which leads to an increase in blood ketone bodies and can cause ketoacidosis.

Chinese name

Fat metabolism

the way

Anabolic and Catabolic

Nature

science

Category

biological

The lipids mainly include the following 4 types:

Fat: It is synthesized from glycerin and fatty acids. There are two sources of fatty acids in the body: one is the body itself

Phospholipids containing phosphoric acid can be divided into two categories:

Classification of Lipid Metabolism in Lipid Metabolism

1. Electrophoresis method: Lipoprotein can be divided into pre-beta, beta lipoprotein and chylomicrons (CM).

2. Ultracentrifugation: Divided into chylomicron, very low density lipoprotein (VLDL), low density lipoprotein (LDL) and high density lipoprotein (HDL) equivalent to CM, pre-, , -lipoprotein separated by electrophoresis .

Lipid metabolism plasma lipoprotein composition

Plasma lipoproteins are mainly composed of proteins, triglycerides, phospholipids, cholesterol and their esters. Free fatty acids are combined with albumin for transport and are not included in plasma lipoproteins. CM is the largest, contains the most triglycerides, and has the least protein, so it has the smallest density. VLDL also contains many triglycerides, but its protein content is higher than CM. LDL contains the most cholesterol and cholesterol esters. HDL contains the most protein.

Structure of fat metabolism lipoprotein

Plasma lipoproteins have roughly similar basic structures. Hydrophobic triglycerides and cholesterol esters are located in the core of lipoproteins, while amphoteric molecules such as apolipoproteins, phospholipids, and free cholesterol are covered with a monolayer on the surface of lipoproteins, which are non-polar inward and internal. The hydrophobic core is connected with its polar groups facing outward and the lipoprotein molecules are spherical. CM and VLDL are mainly based on triglycerides, while LDL and HDL are mainly based on cholesterol esters. Because the polar groups of the lipoprotein molecules facing the surface are hydrophilic, the hydrophilicity of the lipoprotein particles is increased so that they can be evenly dispersed in the blood. From CM to HDL, the diameter is getting smaller and smaller, so the proportion of the outer layer increases, so HDL contains apolipoprotein and phospholipids are the highest.

Apolipoprotein

The protein part of lipoprotein is called apolipoprotein, and there are five types of apoA, B, C, D, and E. Different lipoproteins contain different apolipoproteins. Apolipoproteins are amphoteric molecules. Hydrophobic amino acids form a non-polar surface. Hydrophilic amino acids are polar surfaces. The non-polar surface is connected to a hydrophobic lipid core to make the structure of lipoproteins more stable.

Fat metabolism

1. Chylomicrons

The main function is to transport exogenous triglycerides and cholesterol. There is no CM in fasting blood. After digestion and absorption of exogenous triglycerides, triglycerides and cholesterol are resynthesized in the small intestinal mucosal cells, forming CM with apolipoprotein, and transported to the extrahepatic tissues via lymphatic blood. Under the action of lipoprotein lipase, glycerol The triester is hydrolyzed, the product is utilized by extrahepatic tissues, and CM residues are taken up and utilized by the liver.

2. Very low density lipoprotein

VLDL is the main form of transport of endogenous triglycerides. Hepatocytes and small intestinal mucosal cells synthesize triglycerides and apolipoproteins, cholesterol, etc. to form VLDL, which is secreted into the blood, and is hydrolyzed and utilized under the action of lipase in extrahepatic tissues. During the hydrolysis, VLDL and HDL exchange with each other, and VLDL becomes IDL It is ingested and metabolized by the liver, and IDL that is not ingested continues to become LDL.

3 Low-density lipoprotein

LDL in human plasma is converted from VLDL, which is the main form of endogenous cholesterol transported by the liver. The liver is the main organ that degrades LDL. There are LDL receptors on the surface of the cell membrane of the liver and other tissues. LDL can be taken up. The cholesterol lipids are hydrolyzed into free cholesterol and fatty acids. Further uptake, and promote the storage of free cholesterol esterification in the cytosol. This reaction is catalyzed by the endoplasmic reticulum fatty acid CoA cholesterol lipid acyltransferase (ACAT). In addition to the LDL receptor pathway, LDL in plasma can also be cleared by the monocyte phagocytic system.

4 High density lipoprotein

The main role is to reversely transport cholesterol, transporting cholesterol from extrahepatic tissues to liver metabolism. After the newborn HDL is released into the blood, it will undergo a series of transformations, continuously transferring cholesterol and its esters from the CM and VLDL to HDL. The main role of this is the plasma lecithin cholesterol lipid acyltransferase (LCAT). Finally, the newborn HDL becomes mature. HDL, mature HDL is taken up in combination with HDL receptors of liver cell membranes, and the cholesterol in them synthesizes bile acids or is excreted through the bile, so that cholesterol in senescent cell membranes in peripheral tissues can be transported to the liver for metabolism and excreted.

Hyperlipidemia

Hyperlipidemia when the blood lipid is higher than the upper limit of the normal person is manifested by an increase in triglyceride and cholesterol levels, which is manifested in lipoproteins. CM, VLDL, and LDL can all be increased, but HDL generally does not increase.