Comprehensive Study Notes: Lipids, Membranes, and Carbohydrate Metabolism

Introduction to Lipids and Fatty Acids

Functions of Lipids

Lipids are a diverse group of hydrophobic biomolecules with essential roles in biological systems.

• Energy Storage: Triacylglycerols store energy efficiently due to their high caloric content.

• Structural Components: Phospholipids and cholesterol are key components of biological membranes.

• Signaling Molecules: Steroid hormones and eicosanoids are derived from lipids.

• Insulation and Protection: Adipose tissue cushions organs and insulates against temperature changes.

Structure and Classification of Fatty Acids

• Fatty Acids: Carboxylic acids with long hydrocarbon chains (typically 12–24 carbons).

• Saturated Fatty Acids: No double bonds; straight chains allow tight packing, leading to higher melting points.

• Unsaturated Fatty Acids: Contain one (monounsaturated) or more (polyunsaturated) double bonds, introducing kinks that lower melting points.

• Cis vs. Trans: Cis double bonds cause bends in the chain; trans double bonds result in straighter chains, similar to saturated fats.

Example: Oleic acid (18:1 cis-Δ9) is a common monounsaturated fatty acid.

Naming Conventions

• IUPAC System: Numbering starts from the carboxyl carbon; double bond positions indicated by Δ (delta) notation.

• Omega (ω) System: Numbering starts from the methyl (omega) end; e.g., ω-3 fatty acids have a double bond three carbons from the methyl end.

Melting Temperature (Tm) of Fatty Acids

• Longer chains and fewer double bonds increase Tm.

• Cis double bonds lower Tm more than trans double bonds.

Triacylglycerols and Phospholipids

Triacylglycerols

• Structure: Glycerol backbone esterified to three fatty acids.

• Function: Main energy storage form in animals.

• Melting Temperature: Influenced by fatty acid composition; more unsaturation lowers Tm.

Glycerophospholipids

• Structure: Glycerol backbone, two fatty acids, and a phosphate group with a polar head group (e.g., choline, ethanolamine).

• Function: Major component of cell membranes.

Phospholipases

• Enzymes that hydrolyze specific bonds in phospholipids.

• Different types (A1, A2, C, D) cleave at distinct sites.

Other Lipid Classes

• Plasmalogens: Ether-linked phospholipids found in heart and brain tissue.

• Sphingolipids: Built on a sphingosine backbone; important in neural tissue.

• Ceramides: Sphingosine + fatty acid; precursors to more complex sphingolipids.

Cholesterol and Lipoproteins

Cholesterol

• Structure: Four fused hydrocarbon rings with a hydroxyl group (steroid nucleus).

• Function: Modulates membrane fluidity; precursor for steroid hormones and bile acids.

Lipoprotein Particles

• Function: Transport cholesterol and other lipids in the bloodstream.

• Composition: Core of triglycerides and cholesterol esters, surrounded by phospholipids, free cholesterol, and apolipoproteins.

Particle	Main Function
Chylomicrons	Transport dietary lipids from intestine to tissues
LDL (Low-Density Lipoprotein)	Delivers cholesterol to peripheral tissues
HDL (High-Density Lipoprotein)	Removes excess cholesterol from tissues to liver

Biological Membranes

Functions and Structure

• Define cell boundaries, compartmentalize functions, and mediate transport and signaling.

• Composed of lipid bilayers with embedded proteins.

Membrane Assembly and Properties

• Forces: Hydrophobic effect drives bilayer formation; van der Waals and electrostatic interactions stabilize structure.

• Monolayers vs. Bilayers: Monolayers form at air-water interfaces; bilayers form in aqueous environments.

• Self-Annealing: Membranes can reseal after disruption due to hydrophobic interactions.

• Diffusion: Lateral diffusion is rapid; transverse (flip-flop) diffusion is slow.

• Permeability: Membranes are selectively permeable; small nonpolar molecules cross easily, ions and polar molecules do not.

• Asymmetry: Inner and outer leaflets have different lipid compositions.

• Fluidity: Increased by shorter chains, more unsaturation, and cholesterol (modulates fluidity and stability).

Concentration and Charge Gradients

Membrane Gradients

• Membranes maintain concentration and electrical gradients essential for cellular function.

• Free energy change for transport ($\Delta G$) depends on concentration and charge differences.

Equation:

$\Delta G = RT \ln \left( \frac{[C]_2}{[C]_1} \right) + zF\Delta \Psi$

Where $[C]_1$ and $[C]_2$ are concentrations on each side, $z$ is ion charge, $F$ is Faraday's constant, and $\Delta \Psi$ is membrane potential.

Membrane Transport

Types of Transport

• Simple Diffusion: Movement of small, nonpolar molecules down their concentration gradient.

• Facilitated Diffusion: Protein-assisted movement down a gradient (channels, carriers).

• Active Transport: Movement against a gradient, requiring energy (primary: ATP-driven; secondary: driven by another gradient).

Transport Proteins

• Channels: Form pores for specific ions (e.g., potassium channel).

• Carriers: Bind and transport specific molecules.

• Selectivity: Determined by size, charge, and binding sites (e.g., K+ channel selectivity filter).

• Gating: Channels can open or close in response to stimuli.

Transport Mechanisms

• Uniport: Single substance moves in one direction.

• Symport: Two substances move in the same direction.

• Antiport: Two substances move in opposite directions.

Signal Transduction

Hormones and Signaling

• Hormones act as first messengers to initiate cellular responses.

• Signal transduction involves three steps: reception, transduction, and response.

• Key Terms: Receptor (binds signal), transducer (relays signal), effector enzyme (generates second messenger), second messenger (intracellular signal).

G-Protein Coupled Receptors (GPCRs)

• GPCRs activate G-proteins, which act as molecular switches (GTP-bound = active, GDP-bound = inactive).

• Two major pathways:

  • GPCR/cAMP (cyclic AMP as second messenger)

  • Tyrosine kinase receptor/PIP3 (phosphatidylinositol-3,4,5-trisphosphate as second messenger)

• Signaling must be terminated to reset the system.

Concepts in Metabolism

Terminology and Energy Flow

• Intermediate Metabolism: All chemical reactions in cells.

• Catabolism: Breakdown of molecules to release energy (exergonic, oxidative).

• Anabolism: Synthesis of molecules using energy (endergonic, reductive).

• Metabolite: Intermediate or product of metabolism.

• Autotrophs: Synthesize organic molecules from CO2.

• Heterotrophs: Obtain organic molecules from other organisms.

Regulation of Metabolic Pathways

• Feedback Inhibition: End product inhibits an early step.

• Feedforward Activation: Early metabolite activates a later step.

• Compartmentalization: Pathways occur in specific organelles or regions.

Oxidation-Reduction Reactions in Carbohydrate Metabolism

Redox Reactions

• Oxidation: Loss of electrons; reduction: gain of electrons.

• Electron carriers: NAD+/NADH, NADP+/NADPH, FAD/FADH2.

• Oxidation state of carbon can be determined by counting bonds to more electronegative atoms.

High Energy Bonds and ATP

ATP as Energy Currency

• ATP contains two high-energy phosphoanhydride bonds.

• Hydrolysis of ATP releases energy to drive cellular processes.

• Adenylate Energy Charge: Reflects the energy status of the cell.

Equation:

$\text{Adenylate Energy Charge} = \frac{[ATP] + 0.5[ADP]}{[ATP] + [ADP] + [AMP]}$

• High-energy thioesters (e.g., acetyl-CoA) also store and transfer energy.

• Standard free energy change ($\Delta G^\circ'$) differs from actual free energy change ($\Delta G$) in the cell.

Basis of Metabolic Regulation

• NADH and NADPH cycles are maintained by separate pathways.

• Regulation focuses on reactions far from equilibrium (large negative $\Delta G$).

• Opposing pathways (e.g., glycolysis and gluconeogenesis) are reciprocally regulated to prevent futile cycles.

Glycolysis and Related Pathways

Glycolysis

• Two phases: energy investment (uses ATP) and energy payoff (produces ATP and NADH).

• Key enzymes: kinases, isomerases, dehydrogenases, mutases, enolases.

• Each step is catalyzed by a specific enzyme, often requiring cofactors (e.g., Mg2+).

• Overall reaction:

$\text{Glucose} + 2\,\text{NAD}^+ + 2\,\text{ADP} + 2\,\text{P}_i \rightarrow 2\,\text{Pyruvate} + 2\,\text{NADH} + 2\,\text{ATP} + 2\,\text{H}_2\text{O} + 2\,\text{H}^+$

Anaerobic Glycolysis

• In absence of oxygen, pyruvate is reduced to lactate to regenerate NAD+.

• Lactic acid fermentation is catalyzed by lactate dehydrogenase.

• The Cori Cycle recycles lactate from muscle to liver for gluconeogenesis.

Glycogen Metabolism

• Glucose-6-phosphate can enter glycolysis, pentose phosphate pathway, or be stored as glycogen.

• Glycogen Synthesis: Involves glycogen synthase and branching enzyme.

• Glycogen Breakdown: Involves glycogen phosphorylase and debranching enzyme.

• Regulated reciprocally by insulin (stimulates synthesis) and glucagon (stimulates breakdown).

Pentose Phosphate Pathway

• Alternative fate of glucose-6-phosphate.

• Oxidative Phase: Generates NADPH and ribose-5-phosphate.

• Products depend on cellular needs for NADPH and nucleotides.

Gluconeogenesis

• Pathway for synthesizing glucose from non-carbohydrate precursors.

• Four unique steps require specific enzymes (e.g., pyruvate carboxylase, PEP carboxykinase, fructose-1,6-bisphosphatase, glucose-6-phosphatase).

• Other steps are reversals of glycolysis.

Regulation of Glucose Metabolism

Reciprocal Regulation

• Anabolic and catabolic pathways are regulated in opposite directions to prevent futile cycles.

• Key regulatory enzymes: glucokinase, phosphofructokinase-1 (PFK-1), pyruvate kinase, PEP carboxykinase, pyruvate carboxylase.

• Regulation by energy charge, allosteric effectors (e.g., fructose-2,6-bisphosphate), and hormones.

Fructose-2,6-Bisphosphate (F2,6BP) Regulation

• F2,6BP is a potent allosteric activator of PFK-1 (glycolysis) and inhibitor of fructose-1,6-bisphosphatase (gluconeogenesis).

• Synthesized and degraded by a bifunctional enzyme (PFK-2/FBPase-2), regulated by phosphorylation in response to insulin and glucagon.

Hormonal Regulation: Insulin and Glucagon

• Insulin promotes glucose uptake, glycogen synthesis, and glycolysis.

• Glucagon stimulates glycogen breakdown and gluconeogenesis, especially in liver and kidney.

• Signal transduction involves receptor activation, second messengers, and enzyme phosphorylation/dephosphorylation.

• Epinephrine also stimulates glycogen breakdown in muscle.

Additional info: These notes synthesize and expand upon the study guide outline, providing definitions, context, and key equations for each topic. For detailed reaction mechanisms and structures, refer to standard biochemistry textbooks or lecture materials.