Đề 11 – Bài tập, đề thi trắc nghiệm online Hóa sinh lipid

Waxes are esters of long-chain fatty acids and long-chain alcohols. Due to their highly hydrophobic nature and solid consistency, their primary function in biological systems is waterproofing and protection, such as in plant leaves, animal skin, and insect exoskeletons.

Sphingolipids are characterized by a sphingosine or dihydrosphingosine backbone, in contrast to glycerophospholipids which have a glycerol backbone. This structural difference defines the sphingolipid class.

Adipocytes lack glycerol kinase, the enzyme needed to phosphorylate glycerol and use it in glycolysis or glycerolipid synthesis. Therefore, glycerol released from triglyceride hydrolysis is transported to the liver, where it can be phosphorylated and used in gluconeogenesis or glycolysis.

Saturated fatty acids are characterized by the absence of carbon-carbon double bonds in their hydrocarbon chain, meaning they are `saturated` with hydrogen atoms.

Both omega-3 and omega-6 fatty acids are classified as essential fatty acids because humans lack the enzymes to introduce double bonds beyond the delta-9 carbon in fatty acid chains. Therefore, they must be obtained from dietary sources. The `omega` designation refers to the position of the first double bond from the methyl end of the fatty acid chain, not the total number of double bonds.

LDL (Low-Density Lipoprotein) is often referred to as `bad cholesterol` because high levels of LDL cholesterol in the blood are associated with the development of atherosclerosis, due to its role in delivering cholesterol to peripheral tissues, potentially leading to plaque formation in arteries.

Unsaturation, with its cis double bonds, introduces bends or kinks in the fatty acid chain. These kinks prevent efficient packing of fatty acid chains, reducing the intermolecular forces and thus lowering the melting point.

Lipoprotein density is inversely related to lipid content and directly related to protein content. Chylomicrons are the least dense due to their high triglyceride content, followed by VLDL, LDL, and finally HDL, which is the most dense due to its high protein-to-lipid ratio.

Chylomicrons are the largest and least dense lipoproteins, specifically designed to transport dietary (exogenous) triglycerides and other lipids from the intestine to the rest of the body via the lymph and bloodstream.

The three ketone bodies produced during ketogenesis are acetoacetate, β-hydroxybutyrate, and acetone. Oxaloacetate is a dicarboxylic acid involved in the citric acid cycle and gluconeogenesis, but it is not a ketone body.

Flippases are ATP-dependent enzymes that move phospholipids from the exoplasmic (outer) leaflet to the cytoplasmic (inner) leaflet of a cell membrane. Floppases, also ATP-dependent, move phospholipids in the opposite direction, from the cytoplasmic leaflet to the exoplasmic leaflet. These enzymes are crucial for maintaining lipid asymmetry in cell membranes.

Beta-oxidation, the process of breaking down fatty acids into acetyl-CoA, takes place in the mitochondrial matrix in eukaryotic cells, allowing the acetyl-CoA to enter the citric acid cycle for energy production.

Hormone-sensitive lipase (HSL) is the primary enzyme responsible for hydrolyzing stored triglycerides in adipocytes. Upon activation by hormones like epinephrine and glucagon (and inhibition by insulin), HSL breaks down triglycerides into free fatty acids and glycerol, which are then released into the bloodstream to provide energy to other tissues.

PPARs are ligand-activated transcription factors that regulate the expression of genes involved in various metabolic pathways, including fatty acid oxidation, lipid storage, glucose homeostasis, and inflammation. They play a broad role in controlling lipid and energy metabolism at the gene expression level.

Triacylglycerols, also known as triglycerides, are composed of a glycerol backbone esterified to three fatty acid molecules.

Glycerophospholipids, as the name suggests, are all based on a glycerol-3-phosphate backbone. This glycerol molecule is esterified with two fatty acids at positions 1 and 2 and has a phosphate group at position 3, which is further esterified to a head group alcohol. This glycerol-3-phosphate structure is the defining feature of glycerophospholipids.

Lipases are enzymes that specifically hydrolyze ester bonds, and in the context of lipid digestion, they break down triacylglycerols into fatty acids and glycerol.

Arachidonic acid, a 20-carbon polyunsaturated fatty acid, is the major precursor for the synthesis of most eicosanoids, including prostaglandins, thromboxanes, leukotrienes, and lipoxins, which are involved in inflammation, pain, and other signaling processes.

Amphipathic molecules, like phospholipids, possess both a polar (hydrophilic) head and a nonpolar (hydrophobic) tail, allowing them to form bilayers in aqueous environments.

Acetyl-CoA carboxylase (ACC) catalyzes the committed step in fatty acid synthesis, converting acetyl-CoA to malonyl-CoA. Its activity is regulated by various factors including citrate (activator) and palmitoyl-CoA (inhibitor), making it a key control point.

Acyl-CoA synthetases, also known as thiokinases, are enzymes that activate fatty acids by catalyzing the formation of a thioester bond between the carboxyl group of a fatty acid and the thiol group of Coenzyme A. This activation step is necessary for both fatty acid synthesis and beta-oxidation, as fatty acyl-CoA is the substrate for these pathways.

Insulin is an anabolic hormone that promotes energy storage. In terms of lipid metabolism, insulin stimulates fatty acid synthesis by activating acetyl-CoA carboxylase and inhibits fatty acid breakdown (beta-oxidation) by reducing the activity of hormone-sensitive lipase and decreasing the transport of fatty acids into mitochondria.

Carnitine acyltransferase I, located on the outer mitochondrial membrane, catalyzes the transfer of acyl groups from fatty acyl-CoA to carnitine, forming acyl-carnitine. This is a crucial step in the carnitine shuttle system for transporting long-chain fatty acids into the mitochondrial matrix for beta-oxidation.

Lipids primarily function in energy storage, membrane structure, and signaling. Catalysis of biochemical reactions is mainly the function of proteins, specifically enzymes.

Ketogenesis is upregulated during periods of low glucose availability, such as prolonged fasting or in uncontrolled diabetes, when the body shifts to using fatty acids as a primary energy source and produces ketone bodies as an alternative fuel.

Bile acids act as detergents to emulsify dietary fats in the small intestine. This emulsification process breaks down large fat globules into smaller droplets, significantly increasing the surface area available for pancreatic lipases to digest triglycerides.

Fatty acid synthesis starts with acetyl-CoA, which is carboxylated to malonyl-CoA. Acetyl-CoA is derived from carbohydrate and amino acid metabolism but directly serves as the building block for fatty acid chains.

Lipid rafts are characterized by their enrichment in saturated fatty acids, cholesterol, and sphingolipids. These lipids pack more tightly together, creating more ordered and less fluid membrane microdomains compared to the surrounding membrane.

Cholesterol acts as a membrane buffer. At high temperatures, it interacts with phospholipid fatty acid chains, reducing their movement and decreasing fluidity. At low temperatures, it disrupts the packing of fatty acids, preventing them from solidifying and thus increasing fluidity. This bidirectional effect helps maintain membrane fluidity over a range of temperatures.

Cholesterol is a precursor for steroid hormones and bile acids, and modulates cell membrane fluidity. However, it is not a primary energy source for muscle contraction; carbohydrates and lipids (fatty acids) are the main energy sources.