Lipids Chemistry Lecture 3

Lipoproteins are spherical particles that transport lipids (triacylglycerols and cholesterol) in the body. They are composed of a core of hydrophobic lipids surrounded by a monolayer of phospholipids and free cholesterol, with apoproteins embedded in the outer layer.

Chylomicrons, very-low-density lipoproteins (VLDL), low-density lipoproteins (LDL), and high-density lipoproteins (HDL) are the four main types of lipoproteins.

The outer layer of a lipoprotein consists of a monolayer of phospholipids and free cholesterol, with their charged groups facing outwards towards the water molecules.

The core of a lipoprotein is composed of hydrophobic lipids, primarily esterified cholesterol and triacylglycerol.

Apoproteins are proteins embedded in the lipoprotein structure. They span the region between the core and the outer envelope, with some parts exposed at the surface.

Lipoproteins are classified based on density, which is determined by their ratio of protein to lipid. Chylomicrons are the least dense and largest, while HDL is the densest and smallest.

Chylomicrons primarily transport triacylglycerols, while LDL predominantly carries cholesterol. HDL is involved in reverse cholesterol transport, removing excess cholesterol from peripheral tissues and returning it to the liver.

VLDL particles lose triacylglycerols, transforming into intermediate-density lipoproteins (IDL). Further loss of triacylglycerols leads to the formation of LDL.

The primary function of LDL is to deliver cholesterol to cells throughout the body, especially those needing it for steroid hormone synthesis.

LDL particles are rich in cholesterol and cholesterol esters. About 60% is returned to the liver, while the remaining 40% goes to tissues like adrenal glands and gonads for steroid hormone production.

High levels of LDL in the bloodstream contribute to the formation of atherosclerotic plaques, which can lead to heart disease.

HDL particles can be created by the liver and intestines as small molecules with a shell composed of phospholipids, cholesterol, and apoproteins.

Nascent HDL particles accumulate phospholipids and cholesterol from blood vessel walls, growing larger and more globular. This transformation leads to mature HDL.

HDL particles remove excess cholesterol from cells and transport it back to the liver for breakdown or excretion. This process is crucial for maintaining healthy cholesterol levels.

HDL is known as 'good cholesterol' because it helps lower blood cholesterol levels by removing excess cholesterol from circulation.

HDL particles transport cholesterol from peripheral tissues back to the liver, where it can be utilized for bile salt synthesis or excretion.

High levels of HDL in the blood help reduce the risk of heart disease by facilitating the movement of cholesterol away from blood vessels and towards the liver for processing.

HDL cholesterol helps prevent cholesterol from leaving the HDL particle by attaching a fatty acid to it.

Cholesterol synthesized in the liver can be stored as cholesterol esters within lipid droplets, packaged into lipoproteins for transport to tissues, or converted into bile acids for fat absorption.

ACAT (Acyl-CoA-cholesterol acyl transferase) is an enzyme that attaches a fatty acid to cholesterol, converting it into a cholesterol ester.

Steroids are a class of lipids found in plants and animals. They share a common core structure called the steroid nucleus.

The steroid nucleus consists of 17 carbon atoms arranged in four fused rings, with two methyl groups.

Sterols, like cholesterol, have a hydroxyl group attached to carbon 3 and a side chain attached to carbon 17.

Bile acids are steroids that aid in fat digestion. They are formed from cholesterol in the liver.

Steroid hormones regulate various physiological processes.

Conjugation is a process where bile acids are modified by attaching either glycine or taurine to their structure. This process increases their water solubility, making them more effective at emulsifying fats in the digestive tract.

Vitamin D is a steroid involved in calcium absorption and bone health.

Bile salts are the conjugated forms of bile acids that are present in bile. They are more water-soluble than their unconjugated counterparts and are essential for the efficient digestion and absorption of fats.

The conjugation of bile acids with glycine or taurine increases their water solubility. This is essential for proper fat digestion and absorption because bile acids need to be dissolved in the aqueous environment of the digestive tract to effectively emulsify fats.

Bile acids are stored in the gallbladder and released into the small intestine. This release is triggered by the presence of fat in the diet. The bile acids emulsify fats, breaking them down into smaller droplets, which allows pancreatic lipase to effectively digest them.

Cholesterol is a sterol, an important component of animal cell membranes, crucial for maintaining membrane fluidity and structure. It is also a precursor to vital hormones like testosterone, estrogen, and cortisol.

Cholesterol's fused ring system makes it less flexible compared to other lipids.

High levels of cholesterol in the blood can lead to its accumulation in blood vessel walls, a condition called atherosclerosis. This buildup can restrict blood flow and increase the risk of heart disease.

Cholesterol is converted into cholesteryl esters by attaching a fatty acid to its C-3 hydroxyl group. This form is used for storage and transport in the blood.

7-dehydrocholesterol, a precursor to vitamin D3, is stored under the skin and converted to cholecalciferol (vitamin D3) by exposure to sunlight. Vitamin D3 plays a role in calcium absorption and bone health.

Low-density lipoproteins (LDL) transport cholesterol from the liver to the tissues, often referred to as 'bad cholesterol' due to its association with a higher risk of heart disease.

High-density lipoproteins (HDL) transport cholesterol from the blood back to the liver, where it can be metabolized. HDL is often called 'good cholesterol' due to its association with a lower risk of heart disease.

Bile acids, synthesized from cholesterol in the liver, are hydroxylated steroids essential for fat digestion. These amphipathic molecules aid in emulsifying fats, making them easier to digest and absorb.

Bile acids are conjugated with taurine or glycine residues to form bile salts, which contribute to their amphipathic nature and enhance their digestive function.

A diverse range of bile acids are synthesized and classified into primary, secondary, conjugated, and non-conjugated types based on their structural differences. Bile acids are essential for lipid metabolism and overall digestive health.