Transport Proteins

What are the structures and functions of eukaryotic membranes?

• Separation of aqueous compartments by hydrophobic lipid bilayers permits regulation and specialization of biochemical processes. Selective exchange of nutrients and toxic waste products across cell membranes requires transmembrane proteins.

• The major components of cellular membranes are phospholipids, which are amphipathic molecules that contain both hydrophobic (water-fearing) and hydrophilic (water-loving) chemical groups. Phospholipids form lipid monolayers at the air–water interface or, upon vigorous mixing, generate lipid bilayers, micelles, and liposomes.

• Lipids can move laterally within cell membranes by simple diffusion; however, the fluidity of membranes can differ depending on temperature and lipid composition.

• Eukaryotic cells contain three major membrane types: (1) a plasma membrane that surrounds the entire cell; (2) an endomembrane system of cytoplasmic membrane structures; and (3) organelle membranes in mitochondria and chloroplasts that function in energy conversion processes.

• The endomembrane system is a network of lipid bilayers that includes the nuclear envelope, the smooth and rough endoplasmic reticulum, the Golgi apparatus, and vesicles carrying catabolic enzymes (lysosomes and peroxisomes).

• Mitochondria and chloroplasts are subcellular organelles in eukaryotic cells that contain membrane-embedded proteins, which carry out the energy-converting reactions that lead to the production of ATP.

What are the energetics of solute transport by membrane transport proteins?

• Membrane proteins must be able to associate with the hydrophobic environment of the lipid membrane by orienting nonpolar amino acid residues toward the outside of the protein. The three major types of membrane proteins are membrane receptor proteins, membrane-bound metabolic enzymes, and membrane transport proteins.

• Hydrophobic biomolecules are able to diffuse across cell membranes, whereas polar and charged molecules must be transported across the membrane by proteins. There are two classes of transporters: energy-independent passive transporters and energy-dependent active transporters.

• The value of ΔG for membrane transport can be calculated using the equation ΔG = RT ln(C₂/C₁) + ZFΔψ, in which R is the gas constant, T is temperature (in kelvin), C₁ is the starting-point solute concentration, C₂ is the destination solute concentration, Z is the electrical charge of the solute, F is the Faraday constant, and Δψ is the membrane potential (in volts).

How do passive transporters move specific ions and molecules across membranes without using energy?

• Porins are passive transporters consisting of β-barrel structures that provide a channel for ions and small molecules to pass through the outer membrane of Gram-negative bacteria. The Omp32 porin protein is a selective bacterial transport protein that uses positively charged Arg residues within the channel to permit anions, but not cations, to enter.

• The K⁺ channel protein is an α-helical passive transporter found in both prokaryotic and eukaryotic cells that displays a 10,000-fold selectivity for K⁺ ions over Na⁺ ions. The molecular basis for this exquisite selectivity is the specific placement of carbonyl oxygen atoms within the channel, which interact precisely with desolvated K⁺ ions but not Na⁺ ions, thereby allowing K⁺ ions, but not Na⁺ ions, to pass through the opening.

• Aquaporins are tetrameric passive transporters that provide a very efficient means for H₂O molecules to pass through biological membranes in response to an osmotic gradient. Selectivity within the channel is achieved by a physical restriction imposed by two short α helices containing Asn residues.

How do active transporters use energy to pump solutes against their concentration gradients?

• Primary active transporters require energy input, such as ATP hydrolysis, to drive protein conformational changes required for their “pumping” function. In contrast, secondary active transporters use the potential energy available in a downhill concentration gradient to transport other molecules across the membrane.

• Two primary active transporters, Na⁺–K⁺ ATPase and skeletal muscle SERCA, are both P-type active transporters that use the energy available in ATP hydrolysis to translocate ions across membranes.

• ABC transporters are another type of primary active transport protein and use ATP hydrolysis to drive the protein conformational changes required for pumping ions and small molecules across membranes. ABC transporters are homodimeric complexes containing two ATP binding half-sites that are activated by ligand-induced conformational changes; ATP hydrolysis restores the resting-state conformation.

• The E. coli lactose permease protein is a bacterial secondary active transporter that uses potential energy in a proton membrane gradient to import lactose into the cell via a symporter mechanism. Similarly, the human Na⁺–I⁻ symporter protein is also a secondary active membrane symporter; however, it uses the Na⁺ gradient maintained by the Na⁺–K⁺ ATPase primary active transporter to pump I⁻ ions into thyroid gland cells for use in thyroid hormone synthesis.

How do many common drugs work by targeting membrane transport proteins?

• Proton-pump inhibitors bind to the H⁺–K⁺ ATPase in the gastric tract. Blocking of this proton pump reduces the amount of H⁺ transported into the gastric tract, thereby increasing the pH of gastric juice.

• Low pH initiates the conversion of omeprazole to an active inhibitor (sulfenamide intermediate) that reacts with a sulfhydryl group on the H⁺–K⁺ ATPase, permanently blocking its function.

• Selective serotonin reuptake inhibitors block a serotonin transporter, which is a secondary active transporter. The serotonin transporter uses the energy from Na⁺ going down its concentration gradient to pump serotonin up its concentration gradient.

• The net effect of blocking the serotonin transporter is that serotonin remains in the synaptic cleft at a higher concentration for a longer period of time, thereby increasing signaling through the serotonin receptor.

• Thiazide diuretics are commonly prescribed to treat high blood pressure. These drugs bind to an Na⁺–Cl⁻ co-transporter in the distal convoluted tube of the kidney.

• The Na⁺–Cl⁻ co-transporter is a secondary active transporter that alternates inward- and outward-facing conformations to move Na⁺ and Cl⁻ across the membrane. Thiazide binding to the outward-facing conformation of the Na⁺–Cl⁻ co-transporter prevents the transition to the inward-facing conformation, thus blocking transport. This means Na⁺ remains in the kidney filtrate, where it will be excreted in urine.