BIOCHEM WEEK 6

Tomas Claudio Colleges MORONG, RIZAL COLLEGE OF NURSING PROF. F. JR. M. GABUT, RN | 1 BIOCHEMISTRY WEEK 6 Lipids and the Organization of Their Supramolecular Assemblies Introduction to Lipids • Definition: Lipids are a broad class of organic molecules that are either hydrophobic (water-insoluble) or amphipathic (containing both hydrophobic and hydrophilic parts). • Key feature: Unlike proteins, nucleic acids, or polysaccharides, lipids are defined more by their physical properties (solubility, hydrophobicity) than by a common structural motif. • Major classes include: o Fatty acids and triacylglycerols – storage lipids. o Phospholipids and sphingolipids – structural components of membranes. o Steroids and isoprenoids – signaling molecules and membrane stabilizers. o Glycolipids – components of membranes involved in recognition and signaling. Amphipathic vs. Hydrophobic Lipids 1. Hydrophobic lipids • Nature: Entirely nonpolar, consisting of hydrocarbon chains or rings. • Examples: o Triacylglycerols (TAGs) – three fatty acids esterified to glycerol. o Cholesterol esters – cholesterol bound to a fatty acid, eliminating its polar hydroxyl group. • Biological behavior: o Insoluble in water; aggregate into oily droplets. o Serve primarily as energy storage molecules because they can be densely packed without water. • Key point: Hydrophobic lipids do not form organized structures in water on their own, except droplets. •.

Tomas Claudio Colleges MORONG, RIZAL COLLEGE OF NURSING PROF. F. JR. M. GABUT, RN | 2 2. Amphipathic lipids • Nature: Contain polar (hydrophilic) head groups and nonpolar (hydrophobic) tails. • Examples: o Phospholipids (e.g., phosphatidylcholine, phosphatidylserine) – major membrane lipids. o Glycolipids – lipids with carbohydrate head groups involved in cell recognition. o Cholesterol (unesterified) – weakly amphipathic because of its hydroxyl group. • Biological behavior: o Self-assemble in water to minimize unfavorable interactions between hydrophobic tails and polar solvent. o Form micelles, bilayers, and vesicles, depending on their geometry. o Provide the structural framework of biological membranes. Physical basis for amphipathic behavior • The hydrophobic effect: Water molecules form ordered cages (clathrate structures) around nonpolar groups → this is entropically unfavorable. When hydrophobic groups cluster together, fewer water molecules are ordered, increasing entropy and stabilizing the assembly. • Result: Amphipathic molecules spontaneously organize into supramolecular structures with minimal free energy. Biological Roles of Lipids 1. Energy storage • Triacylglycerols (TAGs) are the main long-term energy reservoir in animals. o Highly reduced carbon atoms → yield more ATP upon oxidation than carbohydrates. o Anhydrous nature: Lipid droplets do not bind water → compact energy storage (about 2× energy density of glycogen). • Metabolic importance: During fasting or extended exercise, fatty acids from TAGs are mobilized via lipolysis and oxidized to produce ATP. • Clinical note: Excess TAG storage leads to obesity; insufficient storage occurs in lipodystrophy..

Tomas Claudio Colleges MORONG, RIZAL COLLEGE OF NURSING PROF. F. JR. M. GABUT, RN | 3 2. Membrane structure • Phospholipids and sphingolipids form the bilayer framework of cell membranes and organelles. • Membrane properties provided by lipids: o Barrier function: Selectively permeable to ions and molecules. o Fluidity and flexibility: Controlled by lipid composition (e.g., unsaturated fatty acids increase fluidity; cholesterol modulates rigidity). o Asymmetry: Inner and outer leaflets have different lipid compositions important for signaling (e.g., exposure of phosphatidylserine in apoptosis). • Dynamic nature: Membranes are not static—lipids diffuse laterally and rearrange, supporting cell signaling and transport..

Tomas Claudio Colleges MORONG, RIZAL COLLEGE OF NURSING PROF. F. JR. M. GABUT, RN | 4 3. Signaling roles • Steroid hormones (derived from cholesterol): regulate development, metabolism, reproduction (e.g., cortisol, estrogen, testosterone). • Eicosanoids (derived from arachidonic acid): local mediators in inflammation, immunity, and blood clotting (e.g., prostaglandins, leukotrienes, thromboxanes)..

Tomas Claudio Colleges MORONG, RIZAL COLLEGE OF NURSING PROF. F. JR. M. GABUT, RN | 5 • Intracellular second messengers: o Diacylglycerol (DAG) and phosphoinositides activate protein kinase C and regulate calcium signaling. o Sphingosine-1-phosphate (S1P) regulates cell proliferation and migration. • Clinical relevance: o Nonsteroidal anti-inflammatory drugs (NSAIDs, e.g., aspirin) block prostaglandin synthesis by inhibiting cyclooxygenase (COX). o Statins lower cholesterol synthesis, reducing substrate for steroid hormone and bile acid production but improving cardiovascular health. Self-Assembly of Lipids in Aqueous Media.

Tomas Claudio Colleges MORONG, RIZAL COLLEGE OF NURSING PROF. F. JR. M. GABUT, RN | 6 1. Introduction • Lipids do not float randomly in water; their chemical nature determines how they organize. • Hydrophobic vs. amphipathic behavior: o Purely hydrophobic lipids (like triacylglycerols) aggregate into oily droplets. o Amphipathic lipids (like phospholipids, glycolipids, and unesterified cholesterol) self-organize to minimize contact between hydrophobic regions and water. • This self-organization is spontaneous and thermodynamically driven, producing stable supramolecular structures critical for cell membranes, transport, and drug delivery. 2. Hydrophobic Effect as the Driving Force Basic thermodynamics • Water is highly ordered due to extensive hydrogen bonding. • When nonpolar molecules are exposed to water: o Water molecules arrange into ordered cages (clathrates) around them, decreasing entropy (ΔS < 0). o This is energetically unfavorable because the system loses randomness. • When hydrophobic regions cluster together: o Fewer water molecules are forced into an ordered structure. o Entropy increases (ΔS > 0), making the process spontaneous (ΔG < 0). 3. Supramolecular Structures Formed by Amphipathic Lipids A. Micelles.

Tomas Claudio Colleges MORONG, RIZAL COLLEGE OF NURSING PROF. F. JR. M. GABUT, RN | 7 • Shape: Spherical aggregates with hydrophobic tails inward and polar head groups outward, facing water. • Lipid requirement: Single-tailed amphipathic lipids with a conical shape (e.g., detergents, bile salts, fatty acids). • Critical micelle concentration (CMC): The threshold concentration at which micelles spontaneously form. • Function: o Solubilize hydrophobic molecules in an aqueous environment. o Facilitate lipid digestion and absorption (bile salts form micelles to emulsify dietary fats). B. Bilayers • Shape: Planar double-layered sheets with hydrophobic tails facing inward and hydrophilic heads outward. • Lipid requirement: Double-tailed cylindrical amphipathic lipids (e.g., phosphatidylcholine, sphingomyelin). • Function: o Structural basis of all biological membranes. o Provide a semi-permeable barrier to ions and polar molecules. C. Vesicles (Liposomes) • Shape: Closed, spherical bilayers surrounding an internal aqueous compartment. • Formation: When lipid bilayers curve and seal to avoid exposing hydrophobic edges. • Function: o Compartmentalization of biochemical reactions. o Experimental and therapeutic use as drug delivery systems (liposomal formulations improve solubility, reduce toxicity, and allow targeted delivery). 5. Biological and Practical Examples Micelle Formation by Detergents • Bile salts in the small intestine emulsify dietary fats → micelles make TAGs accessible to pancreatic lipases..

Tomas Claudio Colleges MORONG, RIZAL COLLEGE OF NURSING PROF. F. JR. M. GABUT, RN | 8 • Synthetic detergents (e.g., SDS) are used in laboratories to solubilize hydrophobic proteins or lipids. Liposome Drug Delivery Systems • Liposomal drugs (e.g., liposomal doxorubicin) encapsulate hydrophilic drugs in aqueous cores or hydrophobic drugs in the bilayer. • Advantages: o Improved solubility for poorly soluble drugs. o Reduced systemic toxicity by controlled release. o Targeted delivery by attaching antibodies or ligands to the liposome surface. • Clinical impact: Used in cancer therapy, antifungal therapy (liposomal amphotericin B), and vaccine delivery. The Structure of Biological Membranes 1. Introduction to Biological Membranes • All cells are bounded by a plasma membrane, and eukaryotic cells also contain internal membranes that form organelles (nucleus, ER, mitochondria, etc.). • Functions of membranes: o Barrier function – separate cell contents from the external environment. o Compartmentalization – allow organelles to maintain specialized environments. o Communication – receptors in membranes receive and transmit signals. o Transport – regulate movement of ions and molecules. o Energy conversion – mitochondrial and chloroplast membranes perform oxidative phosphorylation and photosynthesis. 2. The Fluid Mosaic Model (Singer and Nicolson, 1972) Key principles • Membranes are dynamic, not rigid: lipids and proteins are free to move laterally within the plane of the membrane, giving it “fluid” character. • Mosaic nature: composed of a heterogeneous mixture of lipids, proteins, and carbohydrates arranged in a discontinuous pattern..

Tomas Claudio Colleges MORONG, RIZAL COLLEGE OF NURSING PROF. F. JR. M. GABUT, RN | 9 • Asymmetry: the lipid and protein composition of the inner leaflet differs from the outer leaflet, giving functional polarity. Structural organization • Lipid bilayer as the basic structural scaffold. • Proteins interspersed like “tiles in a mosaic,” performing transport, signaling, and enzymatic roles. • Carbohydrates attached to lipids (glycolipids) or proteins (glycoproteins) on the extracellular surface for cell recognition. 3. Lateral Mobility of Membrane Components Lipid dynamics • Lateral diffusion: phospholipids move side-to-side within the same leaflet very rapidly (microseconds to milliseconds). • Rotation and flexion: individual lipid molecules rotate around their long axis and flex hydrocarbon tails. • Flip-flop: movement from one leaflet to the other is very slow without enzymes (flipases, flopases, scramblases)..

Tomas Claudio Colleges MORONG, RIZAL COLLEGE OF NURSING PROF. F. JR. M. GABUT, RN | 10 Protein dynamics • Integral membrane proteins also diffuse laterally, but more slowly than lipids due to their size and interactions with cytoskeleton or extracellular matrix. • Experimental evidence: Fluorescence Recovery After Photobleaching (FRAP) shows that when a membrane patch is photobleached, fluorescence recovers as unbleached molecules diffuse in. Physiological importance • Membrane fluidity is crucial for: o Transport protein function. o Signal transduction. o Endocytosis, exocytosis, and vesicle fusion. • Regulation of fluidity: o Fatty acid saturation: unsaturated fatty acids increase fluidity; saturated fatty acids make membranes more rigid. o Cholesterol content: acts as a “fluidity buffer,” preventing membranes from becoming too rigid at low temperatures or too fluid at high temperatures. 4. Integral vs. Peripheral Membrane Proteins Integral (intrinsic) proteins • Embedded deeply in the lipid bilayer. • Often have hydrophobic transmembrane α-helices or β-barrels. • Functions: o Ion channels, transporters, pumps. o Signal receptors (e.g., GPCRs). o Cell adhesion molecules. Peripheral (extrinsic) proteins • Loosely associated with the membrane surface through electrostatic interactions or binding to integral proteins. • Functions: o Cytoskeletal anchoring. o Signal transduction. o Enzymatic regulation. Carbohydrate components.

Tomas Claudio Colleges MORONG, RIZAL COLLEGE OF NURSING PROF. F. JR. M. GABUT, RN | 11 • Glycolipids and glycoproteins are exclusively on the outer leaflet, forming the glycocalyx, which mediates cell-cell recognition and protection. 5. Asymmetry of Lipid Distribution (Inner vs. Outer Leaflet) Outer leaflet lipids • Rich in phosphatidylcholine and sphingomyelin. • Display glycolipids and glycoproteins → carbohydrate chains project outward for cell recognition. Inner leaflet lipids • Rich in phosphatidylserine (PS), phosphatidylethanolamine (PE), and phosphatidylinositol (PI). • These play roles in intracellular signaling and anchoring proteins to membranes. Physiological and pathological roles • Phosphatidylserine externalization: normally restricted to the inner leaflet; its exposure on the cell surface marks cells for apoptosis, allowing macrophages to recognize and remove dying cells. • Signal transduction: PI is phosphorylated to PIP2 and PIP3, key messengers for intracellular signaling pathways..

Tomas Claudio Colleges MORONG, RIZAL COLLEGE OF NURSING PROF. F. JR. M. GABUT, RN | 12.

Tomas Claudio Colleges MORONG, RIZAL COLLEGE OF NURSING PROF. F. JR. M. GABUT, RN | 13 6. Clinical Link: Membrane Defects in Hereditary Spherocytosis Overview • Hereditary spherocytosis (HS): a genetic defect affecting red blood cell (RBC) membrane proteins (e.g., spectrin, ankyrin, band 3). • Leads to loss of membrane structural integrity, making RBCs spherical instead of biconcave. Consequences • Spherical RBCs are less deformable, causing premature destruction (hemolysis) in the spleen. • Symptoms: anemia, jaundice, splenomegaly. • Laboratory findings: increased osmotic fragility test, spherocytes on peripheral smear. Membrane biology relevance • Shows how defects in lipid–protein interactions can compromise cell survival. • Demonstrates that membranes are supported by an underlying cytoskeleton, not just a lipid bilayer. Treatment.

Tomas Claudio Colleges MORONG, RIZAL COLLEGE OF NURSING PROF. F. JR. M. GABUT, RN | 14 • Splenectomy is often performed to prevent destruction of RBCs, though the membrane defect remains..

Tomas Claudio Colleges MORONG, RIZAL COLLEGE OF NURSING PROF. F. JR. M. GABUT, RN | 15 The Structure of Lipoproteins 1) Lipoprotein architecture: how to float fat through water Problem: Neutral lipids (triacylglycerols, cholesteryl esters) are hydrophobic. Solution: Pack them into spherical nanoparticles with a detergent-like coat. • Hydrophobic core: o Triacylglycerols (TAG) o Cholesteryl esters (CE) • Amphipathic surface monolayer: o Phospholipids (polar heads face plasma; acyl tails face core) o Unesterified (free) cholesterol o Apolipoproteins (apos) — the “IDs and control panels”: ▪ ApoB-48 (chylomicrons, intestine origin): assembly & structural scaffold. ▪ ApoB-100 (VLDL/IDL/LDL, liver origin): structural; ligand for LDL receptor..

Tomas Claudio Colleges MORONG, RIZAL COLLEGE OF NURSING PROF. F. JR. M. GABUT, RN | 16 ▪ ApoA-I (HDL): structural; activates LCAT; ligand for SR-BI. ▪ ApoC-II: activates lipoprotein lipase (LPL)—key for TAG clearance. ▪ ApoC-III: inhibits LPL and hepatic uptake (↑TG when high). ▪ ApoE: ligand for remnant receptors (LRP1/LDLR) → hepatic clearance..

Tomas Claudio Colleges MORONG, RIZAL COLLEGE OF NURSING PROF. F. JR. M. GABUT, RN | 17.

Tomas Claudio Colleges MORONG, RIZAL COLLEGE OF NURSING PROF. F. JR. M. GABUT, RN | 18 2) Classes & functions: size ↔ density ↔ function Rule of thumb: Bigger → TAG-rich → lower density. Smaller → cholesterol-rich → higher density. Class Size (largest → smallest) Core cargo (major) Signature apos Primary function / route Chylomicron Very large Dietary TAG B-48, A-I, C- II, E Deliver intestinal TAG to tissues (LPL) → liver takes remnants (ApoE) VLDL Large Hepatic TAG B-100, C-II, E Deliver liver TAG to tissues (LPL) → becomes IDL IDL Medium TAG + CE (mixed) B-100, E Half cleared by liver (ApoE), half remodeled to LDL LDL Smaller Cholesteryl ester B-100 Deliver cholesterol to tissues via LDL receptor (LDLR) HDL Smallest (dense) CE (after LCAT) A-I (±A-II) Reverse cholesterol transport → liver via SR-BI; exchange via CETP.

Tomas Claudio Colleges MORONG, RIZAL COLLEGE OF NURSING PROF. F. JR. M. GABUT, RN | 19 3) Metabolic pathways (the “traffic map”) A) Exogenous pathway (dietary fat → body) 1. Intestine packages dietary TAG + cholesterol into nascent chylomicrons (ApoB-48). 2. In plasma, they pick up ApoC-II (for LPL) and ApoE from HDL. 3. LPL (anchored on capillary endothelium; adipose, heart, muscle) hydrolyzes TAG → fatty acids (stored or oxidized) + glycerol. 4. Chylomicron remnants (cholesterol-rich, ApoE) are taken up by the liver via LRP1/LDLR. B) Endogenous pathway (liver fat → body) 1. Liver secretes VLDL (ApoB-100) carrying hepatic TAG. 2. LPL trims VLDL → IDL (TAG↓, CE↑). 3. IDL: ~50% is cleared by liver (ApoE-dependent); remainder loses more TAG (hepatic lipase) → LDL. 4. LDL delivers CE to cells via LDL receptor (binds ApoB-100); most LDL is taken up by the liver..

Tomas Claudio Colleges MORONG, RIZAL COLLEGE OF NURSING PROF. F. JR. M. GABUT, RN | 20 C) Reverse cholesterol transport (periphery → liver) 1. Nascent HDL (discoidal; ApoA-I) is secreted by liver & intestine. 2. Cells export free cholesterol to HDL via ABCA1/ABCG1 → LCAT (activated by ApoA-I) esterifies cholesterol → moves into the HDL core (matures HDL). 3. HDL delivers CE to liver directly via SR-BI and/or exchanges CE for TAG with apoB particles via CETP (then apoB particles bring CE to liver). Enzymes & players to remember: • LPL: clears TAG from chylomicrons/VLDL (needs ApoC-II). • LCAT: esterifies cholesterol on HDL (needs ApoA-I). • Hepatic lipase: remodels IDL→LDL and HDL size. • CETP: swaps HDL-CE for VLDL/LDL-TAG. • LDLR/LRP1/SR-BI: key hepatic receptors for clearance. 4) “Bad” LDL vs “Good” HDL — what that really means • LDL (“bad”): Main courier of cholesterol to tissues; when excessive, it can deposit cholesterol in the arterial wall—especially when retained and oxidatively modified. • HDL (“good”): Returns cholesterol from tissues to liver and supports anti-inflammatory, antioxidant functions (via ApoA-I, enzymes). • Nuance: High HDL-C isn’t always protective if HDL is dysfunctional; ApoB (particle number) can be a better risk marker than LDL-C alone. 5) Pathophysiology: how oxidized LDL (oxLDL) drives atherosclerosis 1. Subendothelial retention: ApoB-containing particles (LDL, remnants) bind arterial proteoglycans and linger. 2. Oxidative modification: Reactive oxygen species, myeloperoxidase, lipoxygenases oxidize LDL lipids & ApoB-100 → oxLDL. 3. Scavenger receptor uptake: Macrophages take up oxLDL via SR-A, CD36 (not down-regulated by intracellular cholesterol) → foam cells. 4. Fatty streak → plaque: Foam cells release cytokines, recruit smooth muscle, lay extracellular matrix → fibrous cap over lipid core. 5. Instability & events: Thin caps can rupture → thrombosis → MI/stroke. Risk amplifiers: diabetes/insulin resistance (small dense LDL, high remnants), hypertension, smoking, chronic inflammation, high Lp(a). 6) Therapeutic targets & what they change on the map • Statins (↓HMG-CoA reductase) → ↑LDLR expression → ↓LDL-C/apoB. • Ezetimibe (↓NPC1L1 intestinal cholesterol absorption) → additional LDL-C lowering. • PCSK9 inhibitors (monoclonal antibodies or siRNA) → preserve LDLR → major LDL-C/apoB lowering. • Bempedoic acid (↓ATP-citrate lyase) → upstream statin-like effect, LDL-C↓. • Bile acid sequestrants → ↑bile acid excretion → LDL-C↓ (watch TG). • Fibrates (PPAR-α) → ↑LPL, ↓ApoC-III → TG↓, ↑HDL-C (variable). • Omega-3 fatty acids (EPA/DHA) → TG↓ in severe hypertriglyceridemia. • Niacin (less used) → ↓VLDL synthesis, ↑HDL-C; limited by side effects. • Lifestyle (dietary pattern, weight loss, physical activity) → improves TG, HDL-C, LDL particle quality..

Tomas Claudio Colleges MORONG, RIZAL COLLEGE OF NURSING PROF. F. JR. M. GABUT, RN | 21 7) Clinical pearls & inherited disorders • Non-HDL-C (= TC – HDL-C) tracks all apoB particles; ApoB directly counts atherogenic particle number. • Triglycerides ≥500 mg/dL → pancreatitis risk; first goal is TG lowering. • Familial hypercholesterolemia (FH) (LDLR/ApoB/PCSK9 variants): very high LDL-C, tendon xanthomas, early ASCVD. • Familial dysbetalipoproteinemia (ApoE2/E2): remnant accumulation, palmar xanthomas, high TC & TG. • LPL or ApoC-II deficiency: severe chylomicronemia, eruptive xanthomas, pancreatitis. • Abetalipoproteinemia (MTTP deficiency): no ApoB lipoproteins → fat malabsorption, neurologic issues. • Tangier disease (ABCA1 defect): very low HDL, orange tonsils, neuropathy..