Lipid Study Guide

📖 Core Concepts Lipids – Hydrophobic or amphiphilic small molecules (fats, waxes, sterols, etc.) that self‑assemble in water into membranes, vesicles, or micelles. Amphiphilicity – Molecules have a polar “head” (e.g., carboxyl, phosphate) and a non‑polar “tail” (hydrocarbon chain), driving the hydrophobic effect. Primary lipid building blocks – ketoacyl units → fatty‑acyl lipids; isoprene units → sterol‑ and prenol‑derived lipids. Major biological roles – energy storage (triglycerides ≈ 38 kJ g⁻¹), membrane structure, and signaling (eicosanoids, sphingosine‑1‑phosphate, steroid hormones). Lipid classification (LIPID MAPS) – 8 categories: fatty acyls, glycerolipids, glycerophospholipids, sphingolipids, saccharolipids, polyketides, sterol lipids, prenol lipids. Membrane architecture – Phospholipid bilayer matrix + cholesterol (fluidity) + sphingolipids (order, rafts). Essential fatty acids – Linoleic acid (ω‑6) and α‑linolenic acid (ω‑3) cannot be synthesized and must be obtained from diet. 📌 Must Remember Triglyceride energy yield:  9 kcal g⁻¹ (38 kJ g⁻¹). β‑Oxidation of palmitate (C₁₆): produces 106 ATP total. Key enzymes: Fatty‑acid synthase (FAS) – repeats condensation‑reduction‑dehydration‑reduction. Acetyl‑CoA carboxylase – converts acetyl‑CoA → malonyl‑CoA (committed step). HMG‑CoA reductase – rate‑limiting step of the mevalonate pathway. Desaturation: Stearoyl‑CoA desaturase‑1 converts stearic (18:0) → oleic (18:1). Sphingolipid backbone: sphingosine + fatty‑acyl‑CoA → ceramide → sphingomyelin or glycosphingolipids. Sterol hallmark: 4‑ring core + OH at C‑3 (cholesterol). Vitamin‑D resistance clue: low serum Ca²⁺ & PO₄³⁻ despite normal/elevated 25‑OH‑D; VDR DNA‑binding mutations. 🔄 Key Processes Fatty‑acid synthesis (cytosolic): Acetyl‑CoA + ACC → malonyl‑CoA → FAS cycle (condensation, reduction, dehydration, reduction) → saturated fatty acyl‑CoA. Triglyceride assembly (ER): Glycerol‑3‑P + 3 fatty‑acyl‑CoA → phosphatidic acid → diacylglycerol → triacylglycerol (TAG). β‑Oxidation (mitochondria/peroxisome): Cycle: Acyl‑CoA → enoyl‑CoA (dehydrogenation) → hydroxyacyl‑CoA (hydration) → ketoacyl‑CoA (dehydrogenation) → acetyl‑CoA + shortened acyl‑CoA. Mevalonate → sterol pathway: 2 Acetyl‑CoA → HMG‑CoA → mevalonate → IPP/DMAPP → squalene → lanosterol → cholesterol (or ergosterol in fungi). Eicosanoid production: Arachidonic acid (20:4) → cyclooxygenase → prostaglandins / thromboxanes; → lipoxygenase → leukotrienes. S1P signaling: Ceramide → sphingosine → sphingosine‑kinase → S1P → GPCR activation → Ca²⁺ mobilization, vascular maturation. 🔍 Key Comparisons Saturated vs. Unsaturated fatty acids – No double bonds → straight chain → higher melting point; cis‑double bond → kink → increased membrane fluidity. Cis vs. Trans double bonds – Cis = kinked, fluid membranes; Trans = straight, behaves like saturated, raises LDL. Mevalonate vs. DXP (non‑mevalonate) pathway – Mevalonate: acetyl‑CoA origin, used by animals/archaea; DXP: pyruvate + GAP origin, used by many bacteria & plant plastids. Triglyceride vs. Phospholipid – TAG: three fatty acids, neutral energy storage; Phospholipid: two fatty acids + phosphate headgroup, membrane bilayer component. Ceramide vs. Sphingomyelin – Ceramide = backbone + amide‑linked FA; sphingomyelin = ceramide + phosphocholine headgroup (major phosphosphingolipid). ⚠️ Common Misunderstandings “All fats are bad.” – Only trans and excess saturated fats raise CVD risk; unsaturated (especially ω‑3) are cardioprotective. “Cholesterol is only harmful.” – Cholesterol is essential for membrane fluidity, precursor of steroid hormones, and vitamin D synthesis. “β‑Oxidation only occurs in mitochondria.” – Peroxisomes also β‑oxidize very‑long‑chain fatty acids, handing off shortened chains to mitochondria. “All essential fatty acids are the same.” – ω‑6 (linoleic) → arachidonic acid → pro‑inflammatory eicosanoids; ω‑3 (α‑linolenic) → EPA/DHA → anti‑inflammatory mediators. 🧠 Mental Models / Intuition “Lipid sandwich” – Picture a burger: polar head = bun, hydrophobic tails = patty. In water, buns face outward, patty hides inside → membrane bilayer. “FA synthesis = chain‑building LEGO” – Each malonyl‑CoA adds two carbons; the cycle repeats until the desired length is reached. “β‑Oxidation = molecular scissors” – Every round cuts off a two‑carbon “paper clip” (acetyl‑CoA) from the fatty‑acid tail. 🚩 Exceptions & Edge Cases Odd‑chain fatty acids – Yield propionyl‑CoA (→ succinyl‑CoA) after β‑oxidation; require methylmalonyl‑CoA mutase. Unsaturated fatty acid β‑oxidation – Needs enoyl‑CoA isomerase (cis‑Δ³) or 2‑methylacyl‑CoA racemase (odd‑chain methyl branch). Trans fats from overheating – Not only industrial hydrogenation; high heat can isomerize cis to trans bonds. Vitamin D resistance – High‑dose calcitriol can sometimes overcome partial VDR defects, but not all mutations respond. 📍 When to Use Which Identify a lipid class → Look at headgroup & backbone: Glycerol + 2 FA + phosphate → glycerophospholipid. Sphingosine + amide FA → sphingolipid. Steroid core → sterol lipid. Choose a metabolic pathway for synthesis → Need a sterol → Mevalonate pathway (animals, archaea). Need carotenoid or quinone in bacteria/plants → DXP pathway. Select a diagnostic marker → Elevated plasma triglycerides + low HDL → risk of CVD. High LDL variability → increased MI/stroke risk. 👀 Patterns to Recognize “C‑C‑C‑C‑OH” vs. “C‑C‑C‑C‑=O” – Presence of terminal OH often indicates sterol; terminal carbonyl suggests fatty‑acid derivative. Double‑bond location – “Δ9 cis” (oleic) → fluidity; “Δ9 trans” → rigid, atherosclerotic risk. Lipid‑linked signaling – Second messenger lipids (DAG, PA, PIP₂) appear after phospholipase C cleavage; watch for “hydrolyzed phospholipid → DAG + IP₃”. Membrane raft markers – Enrichment of sphingomyelin + cholesterol = ordered Lo phase. 🗂️ Exam Traps Confusing “essential” with “non‑essential” – Only linoleic (ω‑6) and α‑linolenic (ω‑3) are truly essential. Assuming all phospholipids have choline headgroup – Phosphatidylethanolamine, phosphatidylserine, and phosphatidylinositol are equally common. Mixing up β‑oxidation location – Remember peroxisomal β‑oxidation lacks ATP generation; it hands off shortened FA to mitochondria. Mistaking “cholesterol” for “cholesteryl esters” – Free cholesterol modulates membrane fluidity; cholesteryl esters are storage form in lipoproteins. Overlooking trans‑fat formation – Not only from industrial hydrogenation; high‑heat cooking can produce trans isomers. --- Use this guide to skim key facts, visualize mechanisms, and spot common pitfalls right before your exam.