biochem_Chapter_8

Chapter 8: Lipids

Key Concepts

  • Structure and function of various lipid types:

    • Fatty Acids

    • Triacylglycerols

    • Glycerophospholipids

    • Sphingolipids

    • Steroids

    • Waxes

    • Terpenes

  • Biological signaling roles of lipids and their metabolites.

Lipid Functions

  • Energy Storage

  • Cell Membranes

  • Endocrine Signaling

  • Types of lipids involved in functions:

    • Fatty Acids

    • Sphingolipids

    • Triacylglycerols

    • Glycerophospholipids

    • Eicosanoids

    • Steroids

Classes of Lipids

  • Fatty Acids

  • Triacylglycerols

  • Glycerophospholipids

  • Sphingolipids

  • Steroids and Hormones

  • Waxes

  • Isoprene-based Lipids

  • Note: Fatty acids have a pKa of ~4.8; at biological pH (7), all carboxylic acid groups are deprotonated.

  • Biological lipids are amphipathic, possessing both polar and nonpolar regions.

Common Fatty Acids

  • Saturated:

    • Lauric Acid (12 C)

    • Myristic Acid (14 C)

    • Palmitic Acid (16 C)

    • Stearic Acid (18 C)

    • Arachidic Acid (20 C)

  • Unsaturated:

    • Palmitoleic Acid (16:1)

    • Oleic Acid (18:1)

    • Linoleic Acid (18:2)

    • α-Linolenic Acid (18:3)

    • γ-Linolenic Acid (18:3)

    • Arachidonic Acid (20:4)

Chemistry of Fatty Acids

  • Saturated Fatty Acids:

    • Pack tightly, high intermolecular forces leading to high boiling points, solid at room temperature.

  • Unsaturated Fatty Acids:

    • Bend in structure, pack less orderly, lower intermolecular forces leading to lower boiling points, liquid at room temperature.

Naturally Occurring Fatty Acids

  • Typically in cis conformation.

  • Trans isomers result from commercial hydrogenation to increase shelf stability and melting points.

Trans Fatty Acids and Health

  • Increase in LDL cholesterol levels; primarily from partially hydrogenated fats in foods.

  • LDL (Low-Density Lipoprotein): Associated with plaque build-up in arteries (bad cholesterol).

  • HDL (High-Density Lipoprotein): Helps remove fat molecules from cells (good cholesterol).

Omega Nomenclature

  • Based on the methyl carbon, not the carboxylic acid.

  • ω-3 fatty acid derivatives regulate lipid metabolic enzymes through peroxisome proliferator-activated receptors.

Triacylglycerols (Triglycerides)

  • Predominant storage form of fatty acids in plants and animals.

  • Stored in adipocytes, transported as lipoprotein components in the bloodstream.

  • High energy yield relative to carbohydrates and proteins (38 kJ/g vs 17 kJ/g).

Stereospecific Numbering (sn) System for Triacylglycerols

  • sn-1: Top Carbon

  • sn-2: Middle Carbon

  • sn-3: Bottom Carbon

Glycerophospholipids: A Subset of Phospholipids

  • Essential components of cell membranes, important for signaling.

  • Triesters of glycerol-3-phosphate, featuring a polar head group linked through phosphate.

  • Phosphatidate acts as a precursor to other common glycerophospholipids.

Biological Signaling by Lipids

  • Glycerophospholipids and sphingolipids serve as signaling molecules, affecting local cellular processes.

  • Lifetimes are typically short, emphasizing the need for careful regulation of their production and degradation.

Sphingolipids

  • Present in biological membranes, important in signal transduction and cell recognition.

  • Ceramides: Fatty acid + sphingosine; key in skin barrier function.

  • Glycosphingolipids: Ceramides + sugars; includes cerebrosides (one sugar) and gangliosides (three or more sugars).

Cholesterol and Steroids

  • Cholesterol: Exists in membranes, gives structural integrity, precursor to other steroids.

  • Steroid structure: Three 6-membered and one 5-membered ring; hormones serve various functions including metabolic regulation.

  • Examples include cortisol (metabolism), testosterone (male hormone), and estradiol (female hormone).

Waxes

  • Esters of long-chain alcohols and long-chain fatty acids, water-insoluble, serve protective functions in nature.

Terpenes

  • Formed from isoprene units; classified into monoterpenes, sesquiterpenes, diterpenes, and triterpenes.

Eicosanoids

  • Signaling molecules derived from arachidonic acid; influence inflammation and metabolic responses.

Endocannabinoids

  • Operate as retrograde signaling molecules, influencing various physiological processes.