MODULE 5: Chapter 6.3 Flashcards
What is the primary function of membrane proteins?
They regulate the molecules passing through a membrane.
What are the three major types of membrane proteins?
- Membrane receptor proteins
- Membrane-bound metabolic enzymes
- Membrane transport proteins
How do passive transporters facilitate biomolecule movement?
They move molecules across a membrane in the same direction as the concentration gradient without requiring an external energy source.
What is the difference between active and passive transporters?
Active transporters require energy to move biomolecules against a concentration gradient, while passive transporters do not.
What determines if a molecule can be transported passively or must be actively transported?
The change in free energy (ΔG) required to traverse the membrane.
In simple diffusion, what type of molecules can cross the lipid bilayer?
Hydrophobic molecules.
What is the role of membrane transport proteins for polar molecules?
They shield polar molecules from the hydrophobic interior of the membrane.
What is facilitated diffusion?
A process where passive transporters facilitate biomolecule movement across a membrane.
What happens to the rate of transport for passive transporters at high substrate concentrations?
It reaches a maximum when transporter function becomes rate-limiting.
What is the structure of gramicidin A?
A linear polypeptide consisting of 15 amino acid residues that forms a helix with a channel.
What type of structure do porin proteins have?
A β barrel structure.
How do porin proteins facilitate the diffusion of molecules?
By having hydrophobic residues facing outward and hydrophilic residues facing the interior of the channel.
What is an example of a selective porin protein?
The Omp32 protein of Delftia acidovorans.
What is the function of the Ca2⁺ transporter in muscle cells?
To transport Ca2⁺ from the cytosol into the sarcoplasmic reticulum.
What is the primary energy source for active transporters?
ATP hydrolysis.
What is the typical concentration of Ca2⁺ in skeletal muscle cells?
0.1 µM in the cytosol and 1.5 mM inside the sarcoplasmic reticulum.
What is the diameter of the channel in gramicidin A?
4 Å.
What type of ions can gramicidin A allow to leak out from bacterial cells?
Na⁺ and K⁺ ions.
What is the primary role of the E. coli maltoporin protein?
To facilitate diffusion of sugar molecules across the bacterial outer membrane.
What is the significance of the electrical charge in free energy calculations for transport?
It affects the overall change in free energy (ΔG) for charged solutes.
Fill in the blank: The equation for calculating ΔG for membrane transport includes R, T, C1, C2, Z, F, and _______.
Δψ
True or False: Passive transporters can become saturated at high substrate concentrations.
False.
What is the common characteristic of the β strands in porin proteins?
They have alternating polar and nonpolar residues.
What is the function of the K⁺ channel protein?
Facilitates diffusion of K⁺ ions across cell membranes in a passive manner
It operates by allowing K⁺ ions to pass through a selectivity channel.
Who elucidated the molecular structure of a K⁺ channel protein?
Rod MacKinnon and his colleagues in 1998
They used X-ray crystallography to characterize the K⁺ channel protein of Streptomyces lividans.
What is the selectivity ratio of K⁺ over Na⁺ ions in the K⁺ channel protein?
10,000 to 1
This selectivity is due to the narrow opening called the selectivity channel.
How do amino acids within the selectivity channel of the K⁺ channel protein facilitate ion selectivity?
They provide favorable desolvation energy for K⁺ ions
Main-chain carbonyl oxygen atoms interact with K⁺ ions, allowing them to shed their hydration layers.
Why do Na⁺ ions not pass through the selectivity channel of the K⁺ channel protein?
Smaller ionic radius makes desolvation energy unfavorable
Na⁺ cannot effectively replace its hydration layers with carbonyl oxygen interactions.
What is the main structural feature of aquaporins that allows for water transport?
Six transmembrane α helices forming a water channel
Aquaporins are responsible for transporting H2O across hydrophobic cell membranes.
How many different aquaporin genes do humans have?
11 different aquaporin genes
Each gene encodes a protein with six transmembrane α helices.
What is the diameter at the constriction point of aquaporins?
2.8 Å
This diameter allows only a single H2O molecule to pass through.
What is the conserved tripeptide sequence in aquaporins that contributes to selectivity?
Asn-Pro-Ala (NPA)
The Asn residue forms hydrogen bonds with H2O molecules to aid in selectivity.
What is the role of the gastric proton pump (H⁺–K⁺ ATPase)?
Transports H⁺ into the stomach to lower pH
It is inhibited by the drug Prilosec (omeprazole).
What type of transporter is the Na⁺–K⁺ ATPase?
P-type primary active transporter
It maintains an electrochemical gradient across animal cell membranes.
What is the ratio of Na⁺ exported to K⁺ imported by the Na⁺–K⁺ ATPase?
3 Na⁺ out for every 2 K⁺ in
This unequal transport establishes a membrane potential critical for neuronal function.
What type of transporters move molecules against an electrochemical gradient?
Active transporters
They require energy input to function.
What are the two basic classes of active transporters?
Primary and secondary active transporters
Primary transporters use ATP hydrolysis, while secondary transporters use energy from an electrochemical gradient.
What is the function of secondary active transporters?
Co-transport a second molecule against an uphill electrochemical gradient
This mechanism is usually coupled with a primary active transporter.
What is the effect of inhibiting the serotonin transporter protein?
Increases serotonin levels in neuronal synapses
This is the mechanism by which Zoloft (sertraline) treats depression.
What happens when cocaine inhibits the dopamine transporter?
Increases levels of synaptic dopamine
This leads to overstimulation of dopamine receptors and can result in addictive behavior.
What is the primary function of the Na⁺–K⁺ ATPase?
Exports Na⁺ ions and imports K⁺ ions to maintain an electrochemical gradient
This gradient is essential for axon firing and secondary active transport.
What is the result of ATP hydrolysis in the Na⁺–K⁺ ATPase?
Leads to conformational changes in the protein that enable transport against concentration gradients.
How many Na⁺ ions are exported and K⁺ ions imported by the Na⁺–K⁺ ATPase?
Three Na⁺ ions are exported and two K⁺ ions are imported.
What are the three distinct functional domains of the Na⁺–K⁺ ATPase?
- ATP binding domain (N)
- Regulatory domain (A)
- Phosphoryl domain (P)
What role does phospholamban play in the function of SERCA?
Inhibits Ca2⁺ transport when in the unphosphorylated state.
What happens to phospholamban when it is phosphorylated?
It dissociates from SERCA, allowing Ca2⁺ uptake from the cytosol.
How many Ca2⁺ ions does SERCA transport for each ATP hydrolyzed?
Two Ca2⁺ ions.
What is the significance of Asp351 in the P domain of SERCA?
Site of phosphorylation required for Ca2⁺ transport.
What is the function of the permeability glycoprotein (P-glycoprotein)?
Exports toxic compounds from the cell.
What is the primary role of the cystic fibrosis transmembrane conductance regulator protein?
Transports Cl⁻ ions across the membrane of lung epithelial cells.
What is a common mutation associated with cystic fibrosis?
Deletion of Phe508 in the cystic fibrosis transmembrane conductance regulator polypeptide.
How do ABC transporters differ from P-type transporters in their mechanism?
ABC transporters induce large conformational changes driven by ATP hydrolysis, without forming a phosphorylated intermediate.
What is the three-step import model for bacterial ABC transporters?
- Step 1: Substrate carrier protein binds, inducing a conformational change.
- Step 2: ATP hydrolysis causes another conformational change, opening the substrate binding site to the cytoplasm.
- Step 3: ADP and Pi are replaced by ATP, returning the transporter to resting state.
True or False: SERCA and Na⁺–K⁺ ATPase have similar domain structures.
True.
What initiates the conformational changes in ABC transporters?
Substrate binding and ATP hydrolysis.
Fill in the blank: The Na⁺–K⁺ ATPase is a _______ active transporter.
primary
What is the role of protein kinase A and Ca2⁺/calmodulin kinase II in relation to phospholamban?
They phosphorylate phospholamban, leading to SERCA activation.
What type of protein is SERCA?
P-type primary active transporter.
What is the effect of epinephrine signaling on muscle contraction?
Improves recovery time between contractions, increasing contraction rate.
What are the two main types of primary active transporters?
- P-type transporters
- ABC transporters
What happens during the four-step model of Ca2⁺ transport by SERCA?
- Step 1: H⁺ exchanged for Ca2⁺.
- Step 2: ADP dissociates, and Ca2⁺ exits.
- Step 3: ATP binds, sealing off the opening.
- Step 4: Dephosphorylation returns protein to resting state.
What type of gradient does the substrate binding site depend on for accessibility?
An uphill substrate concentration gradient
This gradient limits accessibility to one side of the membrane at any given time.
What must occur to ensure directional movement of the substrate after ATP hydrolysis?
A favorable energy process must occur
This ensures the substrate moves out of the chamber and into the cytosol.
What analogy is used to describe the function of ABC transporters?
An airlock mechanism
Only one door is open at a time to prevent equilibration across an impermeable barrier.
What is the function of secondary active transporters?
They depend on the stored potential energy from a concentration gradient
This energy is generated by ATP hydrolysis or redox energy.
Which protein is an example of a secondary active transporter in E. coli?
Lactose permease
It functions as a symporter, using a proton gradient to transport lactose.
What triggers the initial conformational change in lactose permease?
The binding of lactose and H⁺ molecules to the substrate binding sites
This binding converts the outward conformation to the inward conformation.
What is the role of the Na⁺–I− symporter in thyroid gland cells?
It imports iodide ions (I−) for thyroid hormone synthesis
It transports one iodide ion for every two sodium ions.
What is a consequence of insufficient dietary iodine in humans?
Development of goiter
This is a massively enlarged thyroid gland protruding from the neck.
What is the primary active transporter that establishes the Na⁺ gradient in thyroid cells?
Na⁺–K⁺ ATPase
This transporter provides the potential energy for the Na⁺–I− symporter.
How is radioactive iodine (131I) used in clinical practice?
As a diagnostic tool and treatment for thyroid abnormalities
It helps visualize the thyroid gland and treat conditions like hypothyroidism and hyperthyroidism.
What type of substances require membrane transport proteins to cross biological membranes?
Polar substances
They cannot cross the hydrophobic region of the lipid bilayer by simple diffusion.
What are the two types of membrane transport proteins?
Passive transporters and active transporters
Passive transporters allow movement down a concentration gradient, while active transporters require energy.
What is the main purpose of facilitated diffusion?
To allow selective passage of small molecules down their concentration gradient
This is achieved using a membrane protein.
What is the structural motif of porins?
β-barrel structure
Porins are abundant in the outer membrane of bacteria, mitochondria, and chloroplasts.
What does the term ‘sarcoplasmic reticulum’ refer to?
An organelle that stores Ca2⁺ ions
These ions are released when muscle fibers are stimulated.
What is the function of aquaporins?
Transporting water molecules across hydrophobic cell membranes
They are passive membrane transport proteins.
What is a P-type transporter?
An active membrane transport protein that uses energy from ATP hydrolysis
It drives large conformational changes and pumps molecules across the membrane.
What is the role of the Ca2⁺-ATPase?
Transporting Ca2⁺ ions from the cytoplasm into the sarcoplasmic reticulum
This process promotes muscle relaxation.
What does the Na⁺−K⁺ ATPase do?
Maintains an Na⁺ gradient across a membrane
It is a P-type transporter.
What is the mechanism of action of serotonin-selective reuptake inhibitors?
They inhibit serotonin transporter proteins
This leads to increased serotonin levels in the neuronal synapse.
What is the primary function of an antiporter?
Moves molecules across a membrane in opposite directions
It is a type of active membrane transport protein.
What happens to thyroid hormone synthesis if there is insufficient iodide uptake?
It can lead to low levels of T4 and T3
Iodine is essential for the biosynthesis of these hormones.
