Signal Transduction, Membrane Transport, and Lipids: Study Notes for Biochemistry

Lipids and Fatty Acids

Fatty Acids: Nomenclature and Omega Fatty Acids

Fatty acids are carboxylic acids with hydrocarbon chains, essential for membrane structure and energy storage. Their nomenclature is based on chain length, degree of saturation, and position of double bonds.

• Saturated fatty acids: No double bonds (e.g., palmitic acid, stearic acid).

• Unsaturated fatty acids: One or more double bonds; cis configuration is most common in nature.

• Omega fatty acids: Named by the position of the first double bond from the methyl (omega) end (e.g., omega-3, omega-6).

• Polyunsaturated fatty acids (PUFAs): Multiple double bonds; important for cell membrane fluidity and signaling.

Example: Linoleic acid (18:2, omega-6) and alpha-linolenic acid (18:3, omega-3) are essential fatty acids in the human diet.

Phospholipids in Membranes

Phospholipids are amphipathic molecules forming the bilayer of biological membranes. They consist of a glycerol backbone, two fatty acid tails, and a phosphate-containing head group.

• Phosphatidylcholine, phosphatidylethanolamine, phosphatidylserine, phosphatidylinositol: Major classes of phospholipids, differing in their head groups.

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

Example: Phosphatidylcholine is abundant in the outer leaflet of the plasma membrane, contributing to membrane fluidity and signaling.

Steroids and Cholesterol

Steroids are lipids with four fused rings. Cholesterol is a major animal steroid, modulating membrane fluidity and serving as a precursor for steroid hormones and bile acids.

• Cholesterol: Intercalates between phospholipids, reducing membrane permeability and increasing stability.

• Clinical relevance: LDL (low-density lipoprotein) delivers cholesterol to tissues; high LDL is associated with atherosclerosis.

Example: Statins inhibit HMG-CoA reductase, lowering cholesterol synthesis and reducing cardiovascular risk.

Eicosanoids

Eicosanoids are signaling molecules derived from arachidonic acid (20:4, omega-6). They include prostaglandins, thromboxanes, leukotrienes, and lipoxins, which mediate inflammation, immunity, and other physiological functions.

• Clinical relevance: NSAIDs inhibit cyclooxygenase, reducing prostaglandin synthesis and inflammation.

Biological Membranes and Transport

Membrane Composition and Structure

Biological membranes are lipid bilayers with embedded proteins, providing compartmentalization and selective permeability. Membrane fluidity depends on lipid composition, temperature, and cholesterol content.

• Lipid asymmetry: Different lipids are distributed unequally between the inner and outer leaflets.

• Cholesterol: Modulates membrane fluidity and stability.

Membrane Proteins

Membrane proteins are classified as integral (embedded in the bilayer) or peripheral (associated with the membrane surface). They perform functions such as transport, signaling, and cell recognition.

• Integral proteins: Span the membrane, often as alpha-helices or beta-barrels.

• Peripheral proteins: Attach to membrane surfaces via non-covalent interactions.

Membrane Transport

Transport across membranes can be passive (down a concentration gradient) or active (against a gradient, requiring energy).

• Passive transport: Includes simple diffusion and facilitated diffusion via channels or carriers.

• Active transport: Requires ATP hydrolysis (primary) or uses gradients established by primary transporters (secondary).

• Na+/K+ ATPase: Pumps 3 Na+ out and 2 K+ in, maintaining membrane potential.

• Glucose transporters (GLUT, SGLT): Facilitate glucose uptake; SGLT uses Na+ gradient (secondary active transport).

Example: SGLT2 inhibitors are used to treat diabetes by blocking renal glucose reabsorption.

Ion Channels

Ion channels are proteins that allow selective passage of ions across membranes, crucial for electrical signaling in nerves and muscles.

• Voltage-gated channels: Open in response to changes in membrane potential (e.g., Na+, K+, Ca2+ channels).

• Ligand-gated channels: Open in response to binding of a specific molecule (e.g., acetylcholine receptor).

Signal Transduction

General Features

Signal transduction is the process by which cells convert extracellular signals into intracellular responses, often involving cascades of protein modifications and second messengers.

• Receptors: Detect signals (ligands) and initiate cellular responses.

• Second messengers: Small molecules (e.g., cAMP, Ca2+, IP3) that amplify and propagate signals.

G Protein-Coupled Receptors (GPCRs)

GPCRs are a large family of membrane receptors with seven transmembrane helices. They activate heterotrimeric G proteins upon ligand binding, leading to diverse cellular responses.

• Mechanism: Ligand binding → G protein activation (GDP-GTP exchange) → effector enzyme activation (e.g., adenylyl cyclase, phospholipase C) → second messenger production.

• Clinical relevance: Many drugs target GPCRs (e.g., beta-blockers, antihistamines).

Receptor Tyrosine Kinases (RTKs)

RTKs are membrane receptors with intrinsic kinase activity. Ligand binding induces dimerization and autophosphorylation, triggering downstream signaling pathways (e.g., MAPK, PI3K/AKT).

• Example: Insulin receptor is an RTK that regulates glucose uptake and metabolism.

Receptor Guanylyl Cyclases

These receptors convert GTP to cGMP upon ligand binding, mediating responses such as vasodilation.

Phosphoinositide Pathway (Gq-coupled GPCRs)

Activation of Gq-coupled GPCRs stimulates phospholipase C, which cleaves PIP2 into IP3 and DAG. IP3 releases Ca2+ from the endoplasmic reticulum, while DAG activates protein kinase C (PKC).

Sensory Transduction

Sensory transduction involves the conversion of physical or chemical stimuli into electrical signals, as seen in vision, olfaction, and taste.

Steroid Hormone Regulation of Transcription

Steroid hormones diffuse across membranes, bind nuclear receptors, and regulate gene expression by acting as transcription factors.

Clinical Examples and Test Strategies

Clinical Examples

• Cholera toxin: ADP-ribosylates Gsα, causing persistent cAMP elevation and watery diarrhea.

• Statins: Inhibit HMG-CoA reductase, lowering cholesterol synthesis.

• NSAIDs: Inhibit cyclooxygenase, reducing prostaglandin synthesis and inflammation.

• Warfarin: Inhibits vitamin K epoxide reductase, affecting clotting factor synthesis.

• SGLT2 inhibitors: Block renal glucose reabsorption, used in diabetes treatment.

Example Questions and Thought Process

• Which describes a typical Gs-coupled transduction pathway resulting in hyperpolarization?

• How do SGLT2 inhibitors affect glucose reabsorption in the kidney?

• What is the effect of statins on cholesterol biosynthesis?

• How does the Na+/K+ ATPase maintain membrane potential?

Summary Table of Key Molecules and Functions

Molecule/Protein	Function	Clinical Relevance
Na+/K+ ATPase	Pumps 3 Na+ out, 2 K+ in; maintains membrane potential	Target of ouabain/digoxin
SGLT2	Na+-glucose symporter	SGLT2 inhibitors treat diabetes
GLUT	Passive glucose transporter	Deficiency causes hypoglycemia
GPCR	Signal transduction via G proteins	Targets ~50% FDA drugs
Adenylyl cyclase	Converts ATP to cAMP	Cholera toxin modifies Gsα
PKA	Phosphorylates proteins in response to cAMP	Regulates metabolism
Phospholipase C	Cleaves PIP2 to IP3 and DAG	Activates PKC, DAG activates PKC
PKC	Serine/threonine kinase	Regulates cell growth, differentiation
PI3K	Phosphorylates PIP3	Activates AKT pathway
AKT	Serine/threonine kinase	Promotes cell survival, metabolism
Glycogen synthase kinase 3 (GSK3)	Modulates glycogen metabolism	Regulates GPCR signaling
Cholesterol	Membrane structure, steroid precursor	Statins inhibit synthesis
Leukotrienes	Eicosanoids, bronchoconstriction	Asthma treatment target