lipid A

Examples of lipid A in the following topics:

• Lipid Biosynthesis

  • Many of the immune activating abilities of lipopolysaccharide can be attributed to the lipid A unit.
  • Lipids constitute a broad group of naturally occurring molecules that include fats, waxes, sterols, fat-soluble vitamins (such as vitamins A, D, E, and K), monoglycerides, diglycerides, triglycerides, phospholipids, and others .
  • Lipids may be broadly defined as hydrophobic or amphiphilic small molecules.
  • Using this approach, lipids may be divided into eight categories: fatty acids, glycerolipids, glycerophospholipids, sphingolipids, saccharolipids, and polyketides (derived from condensation of ketoacyl subunits); and sterol lipids and prenol lipids (derived from condensation of isoprene subunits ).
  • The synthesis of unsaturated fatty acids involves a desaturation reaction, whereby a double bond is introduced into the fatty acyl chain.

• Lipid Soluble Vitamins

  • The lipid soluble vitamins, shown in the diagram below, are not as easily eliminated and may accumulate to toxic levels if consumed in large quantity.
  • From this data it is clear that vitamins A and D, while essential to good health in proper amounts, can be very toxic.
  • Vitamin D, for example, is used as a rat poison, and in equal weight is more than 100 times as poisonous as sodium cyanide.
  • From the structures shown here, it should be clear that these compounds have more than a solubility connection with lipids.
  • Vitamins A is a terpene, and vitamins E and K have long terpene chains attached to an aromatic moiety.

• Chemistry of Hormones

  • There are three classes of hormones: peptide hormones, lipid hormones, and monoamine hormones.
  • A hormone is a chemical released by a cell or a gland in one part of the body that sends out messages that affect cells in other parts of the organism.
  • Lipid and phospholipid-derived hormones are produced from lipids such as linoleic acid and arachidonic acid.
  • Steroid hormones, which form the majority of lipid hormones, are derived from carbohydrates; for example, testosterone is produced primarily in the testes and plays a key role in development of the male reproductive system.
  • Eicosanoids are also lipid hormones that are derived from fatty acids in the plasma membrane.

• Lipid Metabolism

  • Biological lipids, which are broken down and utilized though β-oxidation, represent a potent energy source.
  • Lipids are universal biological molecules.
  • Among these, lipids can be metabolized by microbes for use as a primary energy source.
  • It is commonly used as a building block of more structurally complex lipids (such as phospholipids and triglycerides) .
  • Lipids are an energy and carbon source.

• Fatty Acids

  • The lipids are a large and diverse group of naturally occurring organic compounds that are related by their solubility in nonpolar organic solvents (e.g. ether, chloroform, acetone & benzene) and general insolubility in water.
  • There is great structural variety among the lipids, as will be demonstrated in the following sections.
  • The common feature of these lipids is that they are all esters of moderate to long chain fatty acids.
  • The trans-double bond isomer of oleic acid, known as elaidic acid, has a linear shape and a melting point of 45 ºC (32 ºC higher than its cis isomer).
  • These acids are also precursors to the prostaglandins, a family of physiologically potent lipids present in minute amounts in most body tissues.

• Mechanisms of Hormone Action

  • A hormone is a chemical messenger that enables communication between cells.
  • Hormones activate target cells by diffusing through the plasma membrane of the target cells (lipid-soluble hormones) to bind a receptor protein within the cytoplasm of the cell, or by binding a specific receptor protein in the cell membrane of the target cell (water-soluble proteins).
  • Lipid: Steroid hormones that contain lipids synthesized from cholesterol and eicosanoids that contain lipids synthesized from the fatty acid chains of phospholipids found in the plasma membrane.
  • Nuclear hormone receptors are activated by a lipid-soluble hormone such as estrogen, binding to them inside the cell.
  • Lipid-soluble hormones can cross the plasma membrane.

• Digestion and Absorption

  • Intestinal bacteria also play a role in synthesizing vitamin B and vitamin K as well as metabolizing bile acids, sterols and xenobiotics.
  • The salivary enzyme amylase begins the breakdown of food starches into maltose, a disaccharide.
  • A large part of protein digestion takes place in the stomach.
  • Emulsification is a process in which large lipid globules are broken down into several small lipid globules.
  • Protein digestion is a multistep process that begins in the stomach and continues through the intestines.

• Connecting Lipids to Glucose Metabolism

  • Lipids can be both made and broken down through parts of the glucose catabolism pathways.
  • Like sugars and amino acids, the catabolic pathways of lipids are also connected to the glucose catabolism pathways.
  • The lipids that are connected to the glucose pathways are cholesterol and triglycerides.
  • Cholesterol contributes to cell membrane flexibility and is a precursor to steroid hormones.
  • The acetyl groups are picked up by CoA to form acetyl CoA that proceeds into the citric acid cycle as it combines with oxaloacetate.

• Lipid-Derived, Amino Acid-Derived, and Peptide Hormones

  • All hormones in the human body can be divided into lipid-derived, amino acid-derived, and peptide hormones.
  • Most lipid hormones are derived from cholesterol, so they are structurally similar to it .
  • The primary class of lipid hormones in humans is the steroid hormones.
  • For example, cortisol has a half-life of 60 to 90 minutes, whereas epinephrine, an amino acid derived-hormone, has a half-life of approximately one minute.
  • Amino acid-derived and polypeptide hormones are water-soluble and insoluble in lipids.

• Chemical Digestion of Carbohydrates, Proteins, Lipids, and Nucleic Acids

  • Digestion of certain fats begins in the mouth, where lingual lipase breaks down short chain lipids into diglycerides.
  • Complete digestion of one molecule of fat (a triglyceride) results in three fatty acid molecules and one glycerol molecule.
  • Lipid digestion involves the formation of micelles in the presence of bile salts, and the passage of micelles and fatty acids through the unstirred layer.