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Module 5 Overview

  • Module 5 discusses the structure and function of hemoglobin, focusing on allosteric control of oxygen transport.

    • Hemoglobin's differential oxygen affinity results from effector molecules binding to it.

    • These effector molecules induce conformational changes in hemoglobin, shifting the equilibrium between the R (relaxed) state and the T (tense) state.

  • The module further covers the transport of polar molecules across cell membranes, detailing the role of membrane proteins that shield transported molecules from the hydrophobic interior of membranes.

  • The actin-myosin motor mechanism is explained as it mediates muscle contraction in response to calcium release.

    • Muscle contraction is examined at both the cellular and molecular levels within a biochemical context.

Module 5 Lecture Topics and Textbook Readings

  • Topic 1: Hemoglobin: Allostery and Evolution

    • Refer to Chapter 6.2

  • Topic 2: Membrane Transport Proteins

    • Refer to Chapter 6.3

  • Topic 3: Passive Transport

    • Refer to Chapter 6.3

  • Topic 4: Active Transport

    • Refer to Chapter 6.3

  • Topic 5: Muscle Contraction

    • Refer to Chapter 6.4

Mechanisms of Active Transport

  • Active Membrane Transporters

    • Active transporters can be categorized as:

    • Primary Active Transporters: Directly use energy from ATP.

    • Secondary Active Transporters: Utilize stored potential energy from gradients created by primary active transport.

  • Types of Active Transporters:

    • Antiporter: Moves two or more molecules in opposite directions.

    • Symporter: Co-transports two or more molecules in the same direction.

Primary Active Transporters: P-type Proteins

  • Na+/K+ ATPase

    • A P-type primary active transporter.

    • Mechanism:

    • Exports 3 Na⁺ ions out of the cell for every 2 K⁺ ions imported into the cell.

    • Utilizes energy derived from ATP hydrolysis.

    • Process: Binding of ATP occurs at the N domain, leading to phosphorylation of the P domain. This triggers conformational changes in the A domain, facilitating transport.

  • Skeletal Muscle SERCA (Sarco/Endoplasmic Reticulum Ca²⁺-ATPase)

    • Responsible for pumping Ca²⁺ ions from cytoplasm into the sarcoplasmic reticulum (SR), essential for muscle relaxation.

    • Catalytic phosphorylation of the Asp351 residue in the P domain of SERCA by ATP is necessary for Ca²⁺ pumping.

  • Ca²⁺ Transport Model

    • A four-step transport model is illustrated based on distinct structures of SERCA under varying binding conditions.

    • Analogy: Similar to a Japanese puzzle box.

Primary Active Transporters: ABC Transporters

  • ABC Transporters Overview

    • Most bacterial ABC transporters are import proteins composed of two identical subunits.

    • A substrate carrier protein delivers the transported molecule to the periplasmic side of the ABC transporter.

  • Transport Process Model

    • A three-step model describes how substrate molecules are imported into the cytosol via bacterial ABC transport proteins.

  • Mechanism Analogy

    • ABC transport proteins operate like an airlock, ensuring only one door is open at any time to avert equilibration across an impermeable barrier.

Secondary Active Transporters

  • Lactose Permease

    • A secondary active membrane transport protein that uses the potential energy from a concentration gradient, often derived from ATP hydrolysis or redox energy.

    • Function: Utilizes a symporter mechanism that harnesses energy from a steep proton gradient to move lactose across the inner bacterial membrane.

  • Na⁺/I⁻ Symporter

    • A type of secondary active transporter which capitalizes on the Na⁺ gradient to import iodide into thyroid cells.

    • Iodine uptake in thyroid cells has clinical applications:

    • Used for diagnostics and treatment, such as visualizing the thyroid gland following radioactive iodine (131I) ingestion.