Đề 2 – Bài tập, đề thi trắc nghiệm online Hóa sinh lipid

Adipose tissue (fat tissue) is specialized for the storage of triglycerides (fat) as a long-term energy reserve. It also releases fatty acids and glycerol through lipolysis when energy is needed by the body. Adipose tissue also plays a role in endocrine function.

Acyl-CoA synthetase (also known as thiokinase) activates fatty acids by attaching them to Coenzyme A (CoA), forming fatty acyl-CoA. This is a necessary first step for both fatty acid beta-oxidation and fatty acid synthesis (in some contexts). The reaction is driven by ATP hydrolysis.

HDL facilitates the removal of excess cholesterol from tissues and transports it back to the liver for processing or excretion, earning it the nickname `good cholesterol`.

Bile salts, synthesized in the liver and secreted into the small intestine, act as detergents. They emulsify large fat globules into smaller droplets, increasing the surface area for pancreatic lipases to efficiently digest triglycerides.

While lipids serve crucial roles in energy storage, membrane structure, and signaling, catalysis of biochemical reactions is primarily the function of enzymes, which are proteins (or sometimes RNA).

Statin drugs competitively inhibit HMG-CoA reductase, a key enzyme in the mevalonate pathway, which is the major pathway for cholesterol biosynthesis in the liver. By inhibiting this enzyme, statins reduce cholesterol production.

Sphingolipids are important structural components of cell membranes, especially enriched in nerve tissue and the myelin sheath surrounding nerve fibers. They play roles in cell signaling and recognition as well.

Long-chain fatty acids cannot directly cross the inner mitochondrial membrane. The carnitine shuttle system facilitates their transport into the mitochondrial matrix, where beta-oxidation occurs. Carnitine palmitoyltransferase I and II are key enzymes in this shuttle.

Leptin is an adipokine (hormone produced by adipose tissue) that acts on the hypothalamus in the brain to regulate energy balance. It signals satiety, reducing appetite and increasing energy expenditure. Resistance to leptin can contribute to obesity.

Triglycerides are formed by esterification of three fatty acid molecules with a single glycerol molecule. This structure makes them efficient for energy storage.

Lipid rafts are specialized microdomains within cell membranes, enriched in cholesterol and sphingolipids. They are thought to organize membrane proteins, concentrating them for signaling processes and other membrane functions.

Beta-oxidation is the primary pathway for fatty acid catabolism. In this process, fatty acids are sequentially broken down two carbon atoms at a time, generating acetyl-CoA, which can then enter the citric acid cycle for ATP production.

Both glycerophospholipids and sphingolipids are types of complex lipids that contain fatty acid chains. Glycerophospholipids have a glycerol backbone, while sphingolipids have a sphingosine or dihydrosphingosine backbone, but both classes are characterized by fatty acid components.

Lipoprotein lipase (LPL) is an enzyme that hydrolyzes triglycerides in lipoproteins (like chylomicrons and VLDL) in the bloodstream, allowing tissues to take up fatty acids. A deficiency in LPL leads to impaired triglyceride clearance, resulting in hypertriglyceridemia.

Chylomicrons are assembled in the endoplasmic reticulum of intestinal epithelial cells (enterocytes). They are responsible for transporting dietary lipids (triglycerides, cholesterol, and fat-soluble vitamins) from the intestine to the rest of the body via the lymphatic system and bloodstream.

Lipids are characterized by their hydrophobic nature, meaning they are poorly soluble in water and readily soluble in nonpolar solvents like chloroform or ether.

`De novo` synthesis, in general biochemistry, means `from new` or `from scratch`. In lipid metabolism, it refers to the synthesis of lipids, particularly fatty acids, from non-lipid precursors such as acetyl-CoA, which can be derived from carbohydrates and amino acids.

Omega-3 and omega-6 fatty acids are both polyunsaturated fatty acids, but they are named based on the position of the first double bond from the omega (methyl) end of the fatty acid chain. Omega-3s have the first double bond at the 3rd carbon, and omega-6s at the 6th carbon.

LDL carries cholesterol from the liver to tissues. High levels of LDL cholesterol are associated with increased risk of atherosclerosis, hence the term `bad cholesterol`.

Fatty acid synthesis (lipogenesis) takes place in the cytoplasm, with key enzymes often associated with the endoplasmic reticulum. It starts with acetyl-CoA and uses NADPH as a reducing agent.

Pancreatic lipase specifically hydrolyzes triglycerides at the sn-1 and sn-3 positions, releasing two fatty acids and leaving a 2-monoglyceride. Further hydrolysis of monoglycerides can occur, but the primary products are monoglycerides and free fatty acids.

Ketone bodies (acetone, acetoacetate, and beta-hydroxybutyrate) are produced in the liver from acetyl-CoA derived from fatty acid breakdown, especially during prolonged fasting or in uncontrolled diabetes when glucose availability is limited. They serve as an alternative energy source for the brain and other tissues.

Cholesterol acts as a membrane buffer, increasing fluidity at low temperatures by preventing fatty acid tails from packing tightly, and decreasing fluidity at high temperatures by restricting phospholipid movement.

Saturated fatty acids are `saturated` with hydrogen atoms and contain only single bonds between carbon atoms. Unsaturated fatty acids contain one or more double bonds.

Lipids, being hydrophobic, are not soluble in the aqueous environment of blood. Lipoproteins, with their outer shell of proteins and phospholipids, encapsulate lipids, allowing them to be transported in the bloodstream.

The amphipathic nature of phospholipids arises from their polar (hydrophilic) phosphate head and nonpolar (hydrophobic) fatty acid tails. This property is crucial for forming lipid bilayers in cell membranes.

Eicosanoids are potent signaling molecules involved in inflammation, pain, and blood clotting. They are synthesized from arachidonic acid, a polyunsaturated fatty acid, via pathways involving enzymes like cyclooxygenases and lipoxygenases.

Cholesterol is the precursor for all steroid hormones, including glucocorticoids (like cortisol), mineralocorticoids (like aldosterone), and sex hormones (like testosterone and estrogen). Steroid hormone synthesis involves modifications of the cholesterol structure.

Phospholipases are enzymes that hydrolyze phospholipids at various ester bonds. For example, phospholipase A2 (PLA2) releases arachidonic acid from membrane phospholipids, which is a precursor for eicosanoid synthesis. Phospholipases play roles in signal transduction, membrane remodeling, and digestion.

Peroxisomes are organelles involved in various metabolic pathways, including the beta-oxidation of very long-chain fatty acids (VLCFAs), which are too long to be efficiently processed in mitochondria initially. They shorten VLCFAs so they can then be fully oxidized in mitochondria.