Biological Chemistry I Unit 10 ReviewLipid Metabolism and Membranes

Lipid Structure and Classification

• Lipids are hydrophobic organic molecules insoluble in water due to their nonpolar nature
• Four main categories of lipids include fatty acids, triacylglycerols, phospholipids, and sterols
  • Fatty acids consist of a carboxylic acid head group attached to a hydrocarbon chain (palmitic acid)
  • Triacylglycerols (triglycerides) are composed of three fatty acids esterified to a glycerol backbone and serve as energy storage molecules
• Phospholipids contain a hydrophilic phosphate head group and two hydrophobic fatty acid tails
  • Phospholipids are the primary components of cell membranes (phosphatidylcholine)
• Sterols are characterized by a four-ring structure with a hydroxyl group and a hydrocarbon side chain (cholesterol)
• Unsaturated fatty acids contain one or more double bonds in their hydrocarbon chain, while saturated fatty acids lack double bonds
• The degree of unsaturation influences the fluidity and melting point of lipids
  • Increased unsaturation results in lower melting points and higher fluidity

Membrane Composition and Organization

• Cell membranes are composed primarily of phospholipids arranged in a bilayer with hydrophobic tails facing inward and hydrophilic heads facing outward
• Membrane proteins are embedded within the phospholipid bilayer and perform various functions (ion channels, receptors)
• The fluid mosaic model describes the dynamic nature of the membrane, with lipids and proteins able to diffuse laterally within the plane of the membrane
• Cholesterol is a key component of animal cell membranes that modulates membrane fluidity and permeability
  • Cholesterol intercalates between phospholipids, reducing fluidity at high temperatures and preventing solidification at low temperatures
• Glycolipids are lipids with attached carbohydrate groups that participate in cell recognition and signaling
• Membrane asymmetry refers to the different lipid and protein compositions of the inner and outer leaflets of the bilayer
• Lipid rafts are specialized membrane microdomains enriched in cholesterol and sphingolipids that facilitate protein interactions and signaling events

Lipid Biosynthesis Pathways

• Fatty acid synthesis occurs in the cytosol and involves the sequential addition of two-carbon units to a growing acyl chain
  • Acetyl-CoA carboxylase catalyzes the rate-limiting step, converting acetyl-CoA to malonyl-CoA
  • Fatty acid synthase is a multi-enzyme complex that catalyzes the elongation of the acyl chain using malonyl-CoA and NADPH
• Triacylglycerol synthesis takes place in the smooth endoplasmic reticulum and involves the sequential esterification of fatty acids to a glycerol backbone
• Phospholipid synthesis begins with the formation of phosphatidic acid, followed by the addition of a specific head group (choline, ethanolamine)
• Cholesterol biosynthesis is a multi-step process that starts with acetyl-CoA and involves the formation of isoprene units and cyclization reactions
  • HMG-CoA reductase is the rate-limiting enzyme in cholesterol synthesis and a target for cholesterol-lowering drugs (statins)
• Desaturation and elongation enzymes introduce double bonds and extend the length of fatty acid chains, respectively
• Regulation of lipid biosynthesis is mediated by transcription factors (SREBP) and hormones (insulin)

Lipid Breakdown and Energy Production

• Lipolysis is the hydrolysis of triacylglycerols into free fatty acids and glycerol, primarily occurring in adipose tissue
  • Hormone-sensitive lipase is activated by glucagon and epinephrine during fasting to mobilize stored fat
• Beta-oxidation is the primary pathway for fatty acid catabolism and takes place in the mitochondrial matrix
  • Fatty acids are activated by coenzyme A and transported into the mitochondria via the carnitine shuttle system
  • Each cycle of beta-oxidation removes a two-carbon unit (acetyl-CoA) from the fatty acid chain, generating NADH and FADH2
• Acetyl-CoA generated from beta-oxidation enters the citric acid cycle for further oxidation and ATP production
• Ketogenesis occurs in the liver during prolonged fasting or low-carbohydrate states, converting excess acetyl-CoA into ketone bodies (acetoacetate, beta-hydroxybutyrate)
  • Ketone bodies serve as an alternative fuel source for the brain and other tissues during glucose scarcity
• Peroxisomal beta-oxidation is responsible for the breakdown of very long-chain fatty acids and branched-chain fatty acids
• Disorders of fatty acid oxidation can lead to the accumulation of fatty acids and impaired energy production (medium-chain acyl-CoA dehydrogenase deficiency)

Membrane Transport Mechanisms

• Passive transport occurs down the concentration gradient without energy input and includes simple diffusion and facilitated diffusion
  • Simple diffusion allows small, nonpolar molecules (O2, CO2) to cross the membrane directly
  • Facilitated diffusion involves carrier proteins or channels that selectively transport specific molecules (glucose, ions)
• Active transport requires energy input (ATP) to move molecules against their concentration gradient
  • Primary active transport directly couples ATP hydrolysis to the movement of molecules (Na+/K+ ATPase)
  • Secondary active transport utilizes the electrochemical gradient established by primary active transport to drive the transport of another molecule (Na+/glucose cotransporter)
• Endocytosis is the process by which cells internalize extracellular materials and involves the invagination of the plasma membrane
  • Receptor-mediated endocytosis is a specific type of endocytosis that involves the binding of ligands to cell surface receptors (LDL receptor)
• Exocytosis is the fusion of intracellular vesicles with the plasma membrane to release their contents into the extracellular space
  • Synaptic vesicle exocytosis is a highly regulated process that mediates neurotransmitter release at synapses
• Aquaporins are specialized water channels that facilitate the rapid movement of water across membranes in response to osmotic gradients
• Ion channels are transmembrane proteins that allow the selective passage of specific ions (Na+, K+, Ca2+) across the membrane
  • Gated ion channels open or close in response to specific stimuli (voltage, ligands, mechanical stress)

Lipid Signaling and Regulation

• Lipids serve as important signaling molecules that regulate various cellular processes
• Eicosanoids are derived from 20-carbon polyunsaturated fatty acids (arachidonic acid) and include prostaglandins, thromboxanes, and leukotrienes
  • Prostaglandins are involved in inflammation, pain sensation, and smooth muscle contraction
  • Thromboxanes promote platelet aggregation and vasoconstriction
  • Leukotrienes are mediators of inflammation and allergic responses
• Steroid hormones (estrogen, testosterone) are derived from cholesterol and regulate gene expression by binding to nuclear receptors
• Phosphoinositides are membrane phospholipids that can be hydrolyzed to generate second messengers (IP3, DAG) in response to receptor activation
  • IP3 triggers the release of calcium from intracellular stores, while DAG activates protein kinase C
• Sphingolipids (ceramide, sphingosine) are involved in cell signaling pathways related to apoptosis and cell growth
• Lipid-derived second messengers (DAG, IP3) amplify and propagate signals received by cell surface receptors
• Lipid modifications (myristoylation, palmitoylation) of proteins can regulate their localization and function
  • Myristoylation involves the addition of a 14-carbon fatty acid to the N-terminus of a protein
  • Palmitoylation involves the addition of a 16-carbon fatty acid to cysteine residues

Disorders of Lipid Metabolism

• Obesity is characterized by excessive accumulation of body fat and is associated with increased risk of metabolic disorders (type 2 diabetes, cardiovascular disease)
• Atherosclerosis is the buildup of cholesterol-rich plaques in the arterial walls, leading to narrowing of the blood vessels and increased risk of heart attack and stroke
  • High levels of LDL cholesterol contribute to the development of atherosclerotic plaques
• Familial hypercholesterolemia is a genetic disorder characterized by defective LDL receptors, resulting in elevated blood cholesterol levels and premature cardiovascular disease
• Fatty liver disease is the accumulation of fat in the liver, which can lead to inflammation and liver damage (cirrhosis)
  • Non-alcoholic fatty liver disease (NAFLD) is associated with obesity and insulin resistance
• Lipid storage disorders (Gaucher disease, Niemann-Pick disease) are caused by defects in enzymes involved in the breakdown of specific lipids, leading to their accumulation in tissues
• Peroxisomal disorders (Zellweger syndrome) are characterized by defects in peroxisomal biogenesis or function, resulting in impaired lipid metabolism and neurological abnormalities
• Lipodystrophies are a group of disorders characterized by the selective loss of body fat, often accompanied by insulin resistance and metabolic complications

Lab Techniques and Applications

• Lipid extraction techniques (Folch method, Bligh-Dyer method) involve the use of organic solvents (chloroform, methanol) to isolate lipids from biological samples
• Thin-layer chromatography (TLC) is used to separate and identify different lipid classes based on their polarity and interaction with a stationary phase
  • Lipids are visualized on TLC plates using specific stains (iodine vapor, phosphomolybdic acid)
• Gas chromatography (GC) is used to separate and quantify fatty acid methyl esters (FAMEs) based on their volatility and interaction with a carrier gas
  • GC is often coupled with mass spectrometry (GC-MS) for the identification of fatty acid species
• High-performance liquid chromatography (HPLC) is used to separate and quantify lipid species based on their interaction with a stationary phase and a mobile phase
  • Reverse-phase HPLC is commonly used for the separation of neutral lipids (triacylglycerols, cholesterol esters)
• Mass spectrometry (MS) techniques (ESI-MS, MALDI-MS) are used for the identification and quantification of lipid molecular species
  • Lipidomics is the comprehensive analysis of lipid species in biological systems using MS-based approaches
• Fluorescence microscopy is used to visualize the distribution and dynamics of lipids in living cells using fluorescent lipid probes (NBD-labeled lipids)
• Lipid-protein interactions can be studied using techniques such as lipid overlay assays and surface plasmon resonance (SPR)
• Lipid nanoparticles (liposomes) are used as drug delivery vehicles and in the study of membrane properties and interactions