Lecture_1__II__F24__Lipids___TAG_

Introduction to Biochemistry

Course: BIOC*2580

Focus: Second Half of the Course (Weeks 7-12)

Lipids

• Biological lipids are structurally diverse macromolecules that play crucial roles in various biological processes.

• They are defined by their common property: hydrophobicity, which means they repel water, making them insoluble in aqueous environments.

• To dissolve biological lipids, organic solvents such as a 2:1 mixture of chloroform and methanol are typically required.

Biological Functions of Lipids

• Energy Storage:

  • Triacylglycerols (fats and oils) serve as the primary forms of energy storage in organisms, providing insulation and structural support.

• Structural Elements of Membranes:

  • Phospholipids and sterols are integral components of cellular membranes, contributing to membrane fluidity and integrity.

• Signal Transduction:

  • Lipids such as steroid hormones and prostaglandins are involved in signal transduction pathways, influencing various physiological responses.

• Enzyme Cofactors:

  • Coenzyme Q (ubiquinone) plays a vital role in mitochondrial electron transport, assisting in ATP production.

• Vitamins:

  • Certain lipids function as vitamins, including fat-soluble vitamins A, D, E, and K, which are essential for numerous biological functions.

• Light-absorbing Pigments:

  • Carotene and other lipids are involved in light absorption for photosynthesis and other biochemical processes.

Lipid-Biomolecule Linkages

• Glycolipids:

  • These lipids contain both sugar and lipid portions, playing important roles in cell membranes and cell recognition (e.g., blood group antigens O, A, B).

• Lipoproteins:

  • Lipoproteins, such as VLDL, LDL, and HDL, are critical for transporting lipids in the bloodstream and are significant indicators of cardiovascular health.

Course Content Overview

• Fatty Acids:

  • Fatty acids serve as the building blocks of complex lipids and are central intermediates in metabolic pathways, influencing energy metabolism and membrane structure.

• Triacylglycerols (TAGs):

  • TAGs are the major form of stored fat, consisting of three fatty acids linked to a glycerol backbone.

• Phosphoglycerides:

  • These are key lipid constituents in biological membranes, including phosphatidylcholine and phosphatidylethanolamine.

Fatty Acids

• Definition:

  • Fatty acids are carboxylic acids with hydrocarbon chains typically containing 4-36 carbon atoms.

• Saturated Fatty Acids:

  • These fatty acids have no double bonds in their carbon chains, leading to a straight structure and higher melting points.

• Unsaturated Fatty Acids:

  • Unsaturated fatty acids contain one (monounsaturated) or more (polyunsaturated) double bonds, introducing kinks that reduce melting points.

• Example:

  • Lauric acid (12:0) is a saturated fatty acid commonly derived from coconut oil.

Naming Fatty Acids

• Nomenclature:

  • Fatty acids are named based on the number of carbon atoms followed by the number of double bonds (e.g., 12:0 for lauric acid, 18:2 for linoleic acid).

• Double Bonds Positioning:

  • The positions of double bonds are specified relative to the carboxyl carbon using a superscript notation (e.g., 18:2(Δ9,12)).

• PUFA Convention:

  • For polyunsaturated fatty acids (PUFAs), the position of double bonds is often indicated relative to the methyl carbon (ω), which is important for identifying ω-3 and ω-6 fatty acids.

Features of Common Fatty Acids

• Fatty acids typically consist of an even number of carbon atoms.

• They often have an unbranched structure and are commonly found in the cis configuration.

• In the case of polyunsaturated fats, the presence of methylene-bridged, non-conjugated double bonds causes kinks in their hydrocarbon chains, affecting their physical properties.

Common Saturated Fatty Acids

Mnemonic: "Let My Pal Stay Around" to remember these names.

Typical Unsaturated Fatty Acids

Trans Fatty Acids

• Trans fatty acids are formed through the partial hydrogenation of unsaturated fatty acids found in margarine, giving them a longer shelf life.

• Trans double bonds enable these fatty acids to adopt extended conformations, which are different from their cis counterparts.

• The consumption of trans fatty acids is linked to negative cardiovascular health effects, including increased risk of heart disease.

Melting Points of Fatty Acids

• Saturated Fatty Acids:

  • Generally have higher melting points due to the ability to pack closely together in an ordered manner.

• Unsaturated Fatty Acids:

  • Typically have lower melting points because the kinks introduced by cis double bonds hinder close packing; however, trans fats can pack more regularly, which increases their melting points.

Derivatives of Fatty Acids

• Formation of Esters:

  • Lipids can be formed by the reaction of carboxylic acids with alcohols, leading to the formation of esters.

• Formation of Acid Anhydrides:

  • Occurs when two carboxylic acids react, resulting in acid anhydrides.

Triacylglycerols (TAGs)

• The majority of fatty acids in biological systems exist as TAGs, which consist of three fatty acids esterified to glycerol.

• TAGs are the primary constituents of dietary fats and oils, serving as the main form of energy storage in the body.

• The melting points of TAGs depend on the chain length and degree of saturation of their fatty acids.

• Types of TAGs:

  • Simple TAGs: Contain the same fatty acid at all three positions of glycerol.

  • Mixed TAGs: Contain two or three different fatty acids, impacting their physical properties and nutritional value.

• Higher long-chain saturated fatty acid content in TAGs leads to higher melting temperatures, illustrating the differences between animal fats and plant oils.