Chapter 13: Membrane Channels & Pumps

Membrane Channels & Pumps from Biochemistry (Eighth Edition) presents a rigorous examination of membrane channels and pumps, the essential protein machinery that regulates the transport of ions and molecules across the impermeable lipid bilayer of the cell. The text distinguishes between passive transport, or facilitated diffusion, which occurs spontaneously down a concentration gradient through channels, and active transport, which requires energy input to drive molecules uphill against gradients via pumps. Thermodynamics are applied to quantify this energy, defining the electrochemical potential as the sum of concentration and electrical terms. A major focus is placed on ATP-driven pumps, specifically the P-type ATPases like the sarcoplasmic reticulum Ca2+ ATPase (SERCA) and the Na+-K+ ATPase, which maintain critical ionic gradients by forming a phosphorylated aspartate intermediate and undergoing conformational changes between E1 and E2 states. The clinical relevance of these pumps is highlighted by the action of cardiotonic steroids like digitalis, which inhibit the dephosphorylation of the Na+-K+ pump to treat heart failure. The chapter also details ABC transporters, such as the multidrug-resistance (MDR) protein, which utilize ATP-binding cassettes to pump substrates without phosphorylation, a mechanism often implicated in chemotherapy resistance. Secondary active transport is explored through carriers like the lactose permease symporter, which couples the thermodynamically unfavorable flow of one species to the favorable flow of another, utilizing an eversion mechanism. Moving beyond pumps, the text analyzes ion channels that enable rapid, precise transport for physiological processes like nerve impulses. The generation of action potentials is described through the coordinated opening of voltage-gated Na+ and K+ channels, utilizing the patch-clamp technique to observe single-channel conductance. Structural biology reveals how the potassium channel achieves high selectivity through a specific filter sequence (TVGYG) that strips water from K+ ions while rejecting smaller Na+ ions due to the high energetic cost of desolvation. The mechanics of voltage gating are explained via charged S4 paddles that move through the membrane, while rapid inactivation is described using the ball-and-chain model where an N-terminal domain occludes the pore. Ligand-gated channels are exemplified by the acetylcholine receptor, a pentameric channel that undergoes rotation of membrane-spanning helices to open upon neurotransmitter binding. Finally, the chapter covers gap junctions formed by connexins that allow direct cytoplasmic communication between cells, and aquaporins, specialized channels that facilitate rapid water transport while strictly excluding protons to maintain membrane potential.