Biochemistry Study Guide: Lipids, Membranes, and Metabolic Pathways (Chapters 11-12, 15-17)

Lipids: Structure, Classification, and Nomenclature

Systematic, Shorthand, and Omega Naming of Lipids

Lipids are a diverse group of biomolecules characterized by their hydrophobicity. Fatty acids are named systematically (IUPAC), by shorthand notation, and by omega (ω) nomenclature.

• Systematic Naming: Based on the number of carbons and double bonds (e.g., octadecanoic acid for an 18-carbon saturated fatty acid).

• Shorthand Notation: Indicates carbon count and double bonds (e.g., 18:1 Δ9 for oleic acid).

• Omega Naming: Counts from the methyl (ω) end; e.g., ω-3 fatty acids have a double bond at the third carbon from the end.

• Example: Linoleic acid: 18:2 Δ9,12, ω-6 fatty acid.

Major Types of Lipids: Structure and Function

Lipids are classified based on their structure and biological roles.

• Free Fatty Acids: Long hydrocarbon chains with a carboxyl group; energy storage and signaling.

• Triacylglycerols: Glycerol backbone esterified with three fatty acids; main energy storage in animals.

• Phospholipids: Glycerol backbone, two fatty acids, and a phosphate group; major membrane component.

• Glycolipids: Lipids with carbohydrate groups; important in cell recognition.

• Steroids: Four fused rings; includes cholesterol, hormones.

• Example: Phosphatidylcholine is a common phospholipid in membranes.

Membrane Fluidity and Composition

Factors Affecting Membrane Fluidity

Membrane fluidity is influenced by fatty acid tail length, saturation, and cholesterol content.

• Longer Fatty Acid Tails: Increase van der Waals interactions, decrease fluidity, raise melting temperature.

• Cis-Double Bonds: Introduce kinks, prevent tight packing, increase fluidity, lower melting temperature.

• Cholesterol: Modulates fluidity; stabilizes membrane at high temperatures, prevents solidification at low temperatures.

• Example: Membranes rich in unsaturated fatty acids are more fluid.

Membrane Components

Biological membranes are composed of lipids and proteins.

• Phospholipids: Form bilayer structure.

• Cholesterol: Intercalates between phospholipids.

• Proteins: Integral (span membrane) and peripheral (associate with surface).

• Glycolipids: Contribute to cell surface properties.

• Example: Aquaporins are integral membrane proteins facilitating water transport.

Membrane Permeability and Transport

Passive Transport: Permeability Order

Different molecules cross membranes at varying rates based on size and polarity.

• Order of Increasing Permeability: Small nonpolar molecules (O2, CO2) > Small uncharged polar molecules (H2O, urea) > Large uncharged polar molecules (glucose) > Ions (Na+, K+).

• Example: Oxygen diffuses rapidly, ions require channels.

Active Transporters

Active transport moves molecules against concentration gradients using energy.

• Symporters: Transport two substances in the same direction.

• Antiporters: Transport two substances in opposite directions.

• ATPases: Use ATP hydrolysis to drive transport (e.g., Na+/K+ pump).

• Example: Sodium-glucose symporter uses Na+ gradient to import glucose.

Activated Carriers in Metabolism

Identification and Function

Activated carriers are molecules that store and transfer energy or functional groups.

• NAD+: Carries electrons (hydride ions) in catabolic reactions.

• NADP+: Carries electrons in anabolic reactions.

• FAD: Carries electrons (two hydrogens).

• CoA: Carries acyl groups (e.g., acetyl).

• ATP: Carries phosphoryl groups; energy currency.

• Example: NADH is produced in glycolysis and used in oxidative phosphorylation.

Metabolic Pathways: Anabolism vs. Catabolism

Pathway Differences and Carrier Usage

Metabolism is divided into anabolic (building) and catabolic (breaking down) pathways.

• Anabolic Pathways: Synthesize complex molecules; use NADPH as electron donor.

• Catabolic Pathways: Degrade molecules; generate NADH.

• Example: Fatty acid synthesis (anabolic) uses NADPH; glycolysis (catabolic) produces NADH.

ATP: Properties and Role in Metabolism

Phosphoryl-Transfer Potential

ATP is a high-energy molecule due to its triphosphate structure.

• Resonance Stabilization: Hydrolysis products are stabilized by resonance.

• Electrostatic Repulsion: Multiple negative charges repel each other.

• Hydration: Hydrolysis products are well hydrated.

• Example: ATP hydrolysis releases energy to drive cellular reactions.

ATP Hydrolysis and Coupling

ATP hydrolysis can drive unfavorable reactions by coupling.

• Coupling Principle: Combine an unfavorable reaction with ATP hydrolysis to make the net reaction favorable.

• Example: Glucose phosphorylation: $\text{Glucose} + \text{ATP} \rightarrow \text{Glucose-6-phosphate} + \text{ADP}$

Thermodynamics of Metabolic Pathways

ΔG Calculations

Free energy change (ΔG) determines reaction spontaneity.

• Equation: $\Delta G = \Delta G^\circ + RT \ln Q$

• Q: Reaction quotient; R is gas constant; T is temperature.

• Example: Negative ΔG indicates a spontaneous reaction.

Enzyme Classification in Metabolism

Enzyme Classes

Enzymes are classified by the type of reaction they catalyze.

• Oxidoreductases: Catalyze oxidation-reduction reactions.

• Transferases: Transfer functional groups.

• Hydrolases: Catalyze hydrolysis reactions.

• Lyases: Add or remove groups to form double bonds.

• Isomerases: Catalyze isomerization.

• Ligases: Join two molecules.

• Example: Hexokinase (transferase) catalyzes phosphorylation of glucose.

Glycolysis and Gluconeogenesis

Net Reactions

Glycolysis and gluconeogenesis are central metabolic pathways.

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

• Gluconeogenesis Net Reaction: $2\,\text{Pyruvate} + 4\,\text{ATP} + 2\,\text{GTP} + 2\,\text{NADH} + 6\,\text{H}_2\text{O} \rightarrow \text{Glucose} + 4\,\text{ADP} + 2\,\text{GDP} + 6\,\text{P}_i + 2\,\text{NAD}^+ + 2\,\text{H}^+$

Regulated Steps and Enzyme Modulation

Key steps in glycolysis and gluconeogenesis are regulated by specific enzymes and modulators.

• Glycolysis:

  • Hexokinase: Inhibited by glucose-6-phosphate.

  • Phosphofructokinase-1 (PFK-1): Activated by AMP, inhibited by ATP and citrate.

  • Pyruvate kinase: Activated by fructose-1,6-bisphosphate, inhibited by ATP and alanine.

• Gluconeogenesis:

  • Fructose-1,6-bisphosphatase: Inhibited by AMP, activated by citrate.

  • Pyruvate carboxylase: Activated by acetyl-CoA.

• Example: High ATP inhibits glycolysis, favoring gluconeogenesis.

Enzymes, Intermediates, and Activated Carriers in Glycolysis

Glycolysis involves specific enzymes, intermediates, and carriers.

• Enzymes: Hexokinase, PFK-1, pyruvate kinase.

• Intermediates: Glucose-6-phosphate, fructose-1,6-bisphosphate, pyruvate.

• Activated Carriers: NADH, ATP.

• Example: Glyceraldehyde-3-phosphate dehydrogenase produces NADH.

Differences Between Glycolysis and Gluconeogenesis

Some steps are unique to each pathway due to irreversibility.

• Glycolysis: Irreversible steps catalyzed by hexokinase, PFK-1, pyruvate kinase.

• Gluconeogenesis: Uses glucose-6-phosphatase, fructose-1,6-bisphosphatase, pyruvate carboxylase, and PEP carboxykinase.

• Example: Pyruvate kinase step in glycolysis is bypassed by pyruvate carboxylase and PEP carboxykinase in gluconeogenesis.

Pathway Context: Precursors and Redox Balance

Understanding what happens before and after glycolysis and gluconeogenesis is essential.

• Precursors: Glycolysis starts with glucose; gluconeogenesis starts with pyruvate or lactate.

• Redox Balance: NADH produced in glycolysis must be reoxidized (e.g., via lactate dehydrogenase in anaerobic conditions).

• Example: In muscle, pyruvate is converted to lactate to regenerate NAD+.

Summary Table: Major Lipid Types

Lipid Type	Structure	Function
Free Fatty Acids	Long hydrocarbon chain with carboxyl group	Energy storage, signaling
Triacylglycerols	Glycerol + 3 fatty acids	Energy storage
Phospholipids	Glycerol + 2 fatty acids + phosphate	Membrane structure
Glycolipids	Lipid + carbohydrate	Cell recognition
Steroids	Four fused rings	Hormones, membrane fluidity

Summary Table: Activated Carriers

Carrier	What is Carried	Pathway
NAD+	Electrons (hydride)	Catabolic
NADP+	Electrons (hydride)	Anabolic
FAD	Electrons (2H)	Catabolic
CoA	Acyl groups	Both
ATP	Phosphoryl groups	Both

Additional info: Academic context and examples were added to clarify and expand upon brief study guide points.