الفسفوليبيدات وأيض الدهون

Lipids

• Lipids are compounds that are not soluble in water but are soluble in non-polar organic solvents like benzene, chloroform, ether, hot alcohol, and acetone.

Functions of Lipids

• Lipids are a significant source of high energy value.
  • 1 gram of fat = 9 calories
  • 1 gram of protein = 4 calories
  • 1 gram of carbohydrate = 4 calories
• Structural components of biomembranes (phospholipids and cholesterol).
• Protective coating for organs like the kidney.
• Source of fat-soluble vitamins (A, D, K, and E).
• Insulate against temperature changes (subcutaneous fat).
• Regulate metabolism (steroid hormones and prostaglandins).
• Transport forms of various metabolic fuels.
• Components of inner mitochondrial membranes involved in electron transport.

Clinical Applications of Lipids

• Excessive fat deposits lead to obesity.
• Abnormalities in cholesterol and lipoprotein metabolism contribute to atherosclerosis and cardiovascular diseases.
• In diabetes mellitus, the metabolisms of fatty acids and lipoproteins are deranged, leading to ketosis.

General Properties of Lipids

• Lipids are relatively soluble in organic solvents like chloroform and methanol.
• Insoluble in water.
• The hydrophobic nature of lipids is due to the prevalence of hydrocarbon chains (-CH2-) in their structure.

Classification of Lipids

• Lipids can be classified based on their chemical nature.

Simple Lipids

• Fatty acids are the major units of lipids.
• Straight-chain hydrocarbons with a terminal carboxyl group.
• Generally found in ester linkages in various lipid classes.
• Represented by the general formula R-COOH.
• Fatty acids can be saturated or unsaturated.
  • Unsaturated fatty acids have double bonds.
  • Polyunsaturated fatty acids have more than one double bond (but not more than four).
  • Saturated fatty acids have only single bonds (examples: palmitic and stearic acids).
• Odd-numbered fatty acids are rare.

Triglycerides (TG)

• Most body fatty acids are components of triglycerides, stored in adipose tissue as fat.
• Adipose cells convert fatty acids into triglycerides through esterification with glycerol-3-phosphate, a glucose derivative.
• Triglycerides are hydrolyzed during fasting, starvation, or uncontrolled diabetes.
• Excess carbohydrates can be temporarily stored as triglycerides.
• Hydrolysis of triglycerides releases free fatty acids + glycerol.
• Oxidation of lipids in the body produces peroxides and free radicals, which can damage tissues.
  • These radicals are linked to inflammatory diseases, aging, cancer, and atherosclerosis.

1,2-Diacylglycerol and Triacylglycerol

• 1,2-Diacylglycerol is a glycerol molecule with two fatty acyl groups.
• Triacylglycerol has three fatty acyl groups.

Compound Lipids

• Esters of fatty acids with alcohol that has additional groups, like phosphate, nitrogenous bases, carbohydrates, and proteins.
  • Phospholipids: Important components of cell membranes, found in all plant and animal cells, arranged in a lipid bilayer. Amphiphilic with a hydrophilic head and hydrophobic tail.
  • Glycolipids: Contain fatty acids, carbohydrates, and sphingosine. Examples include cerebrosides, which are crucial for brain and spinal cord cells.

Lipoproteins

• Macromolecular complexes of lipids and proteins, transporting lipids in the circulatory system.
  • Chylomicrons: Transport dietary lipids from the intestines to peripheral tissues.
  • VLDL (Very Low-Density Lipoproteins): Transport endogenously synthesized triglycerides from the liver to peripheral tissues.
  • LDL (Low-Density Lipoproteins): Transport cholesterol from the liver to peripheral tissues ("bad" cholesterol).
  • HDL (High-Density Lipoproteins): Transport cholesterol from peripheral tissues back to the liver ("good" cholesterol).
  • IDL (Intermediate-Density Lipoproteins): Derived from VLDL, denser than VLDL, has less triacylglycerol.

Cholesterol

• Cholesterol is a precursor for the synthesis of all other steroids in the body.
  • Includes steroid hormones, vitamin D, and bile acids.
• Structural component of cell membranes.
• Cholesterol synthesis occurs in hepatic cells from cytosolic acetyl-CoA, with glucose being a major carbon source.

Cholesterol Synthesis

• 5 Steps:
  1. Synthesis of HMG-CoA
  2. Formation of mevalonate (6C)
  3. Production of isoprenoid units (5C)
  4. Synthesis of squalene (30C)
  5. Conversion of squalene to cholesterol (27C)

Cholesterol Regulation

• Controlled by the rate-limiting enzyme HMG CoA reductase through:
  • Feedback control: Cholesterol concentration regulates HMG CoA reductase synthesis.
  • Hormonal regulation: Glucagon inhibits cholesterol synthesis, while insulin enhances it.

يتناول هذا الاختبار محور الفسفوليبيدات ودورها في الأيض، بما في ذلك تركيبها وخصائصها. كما يتطرق إلى العلاقة بين اختلال الأيض في مرض السكري وتأثيره على الكوليسترول والبروتينات الدهنية. هذا الاختبار مناسب للطلاب في مجالات البيولوجيا والكيمياء الحيوية.