Lecture 4: Mediated Transport Flashcards
Common properties of protein mediated transport
- Involve interaction with protein carrier
- Show saturation characteristics
- Have maximum rate of transportation
- Show chemical and stereo-specificity
- Show competition
Facilitated diffusion characteristics
- Requires no energy
- Driving force is Delta[Solute]
- Works downhill
- Cannot move solute against its [ ] gradient
- Inhibited by specific poisons
Examples of facilitated diffusion
- GLUTs
- UTs
Specific characteristics of primary active transport
- Requires energy at site of transportation
- Transports solute against concentration gradient (uphill)
- Phosphorylation of carrier occurs
- Inhibited by specific and metabolic poisons
Specific characteristics of secondary active transport
- Requires energy, but NOT AT SITE of transportation
- Transports solute of interest against concentration gradient (uphill)
- Powered by another solute moving down its concentration gradient
- Specific characteristics
- No phosphorylation of carrier involved
- Inhibited by metabolic and specific poisons
ATP Binding Cassette (ABC Transporer)
- Contain aa sequences (cassette) that bind ATP
- ATP catalysis not necessarily coupled to transport
- Can behave like pumps, channels or regulators
- Multi Drug Resistance transporters, MDR
- Cystic Fibrosis Transmembrane conductance Regulator, CFTR
Mediated transport requires
- Physical interaction between transporter and transported substance
Three types of mediated transport
- Facilitated diffusion (passive)
- Primary active transport
- Secondary active transport
Active systems
- Require energy
- Move substance against its concentration gradient
Epithelial transport uses
- A variety of transport systems to cross cellular barriers
Mediated transport systems require
- Intrinsic membrane proteins that act as transporters (carriers)
- Molecule that would normally move very slowly, if at all, across the lipid bilayer
Saturation
- A maximum rate of transportation
Specificity
- Transport only one type of solute (mostly)
Competition
- Similar sized and shaped molecules can inhibit transport
Facilitated diffusion moves atoms, ions, and molecules
- Down a concentration gradient
- Via interaction with a transport protein
The driving force for movement is
- The concentration gradient of solute
- It acts to equalize the concentration inside and outside cells
- No metabolic energy is required
The rate of glucose transport into the RBC via facilitated diffusion is
- Greater than predicted from the glucose partition coefficient
Glucose entry as a function of glucose concentration
- Deviates from the diffusion law
Glucose transport has
- Maximal rate of transport (Vmax)
- Km (recall Km is an index of the affinity of the transport system for the transported substance)
Glucose entry
- Stereospecific
D-glucose enters the cell
- Rapidly
- Km = 1.5mM
D-galactose enters the cell
- More slowly
- Km = 30mM
L-glucose enters the cell
- Not at all
- Km = 3000mM
Others sugars transported via D-glucose systems include
- D-mannose
- D-xylose
- L- arabinose
- All have Km’s greater than glucose
Systems can be inhibited by specific poisons such as
- HgCl2
- Dinitrofluorobenzene
Systems are not affected by non-specific metabolic poisons such as
- Cyanide
- Dinitrophenol
Examples of facilitated diffusion
- Sugars
- Urea
Sugars (facilitated diffusion) diffuse into
- RBC
- WBC
- Myocytes
- Adipocytes
- Choroid plexus
Large family of transporters for sugars include
- GLUT (14 members currently)
- One of which is insulin dependent (GLUT 4)
Insulin increases rate of glucose transport in
- Myocytes
- Adipocytes
Urea (facilitated diffusion) found in
- Collecting duct of nephrons
- Transport urea out of duct
Amino acids (facilitated diffusion) diffuse into
- RBC & WBC
- Out of intestinal epithelium
- Wide variety of systems
- At least 3 basic types
Active transport
- Moves atoms, ions, molecules against a concentration gradient
- Helps maintain concentration differences across permeable barriers
In active transport,
- Work must be done
- Energy (ATP) is required
- Directly in primary active transport
- Secondarily in secondary active transport
Active transport shares
- All the characteristics of mediated transport systems
Primary active transport energy consumption
- 1/3 total resting energy (ATP) consumption of humans is used in pumping these ions
In primary active transport, gradients control
- Cell volume
- Membrane excitability
- Drive sodium cotransport of sugars/amino acids/nutrients
Concentration gradients in primary active transport are maintained by
- Na+/K+ ATPase
- Located in the plasma membrane
Intracellularly, most cells have a _____ concentration of K+ and a _____ concentration of Na+
- High [K+]
- Low [Na+]
Model of Na+/K+ ATPase
- 3Na+ out, 2K+ in, 1ATP consumed
- Protein: at least 4 conformational states
- ATP must bind/energize the carrier
- ATP is not hydrolyzed unless sodium and
potassium are transported
Na+/K+ ATPase is electrogenic
- Generates an electrical current in the plasma membrane
Na+/K+ ATPase pump can be reversed
- To synthesize ATP
Cardiac glycosides
- Inhibit Na+/K+ ATPase pump
- Competes with potassium for the external binding site
- Extremely useful therapeutic tool
Clinical significance of Na/K ATPase pump
- Digitalis-based drugs used to treat heart failure
- Inhibit Na/K ATPase
- Need to monitor blood potassium levels
- In hypokalemia, therapeutic dose can become toxic
- In hyperkalemia, normal dose may be ineffective
P-type ATPase
- Pump type
- Includes Ca-ATPase in plasma and mitochondrial membranes of many cells, sarcoplasmic reticulum of muscle
- H+-ATPase in many cells particularly acid producing cells of stomach
V type ATPase
- Vesicular type
- Actively accumulate hydrogen ion in organelles as lysosomes and storage granules
F type ATPase
- Cristae of inner mitochondrial membrane
- The ATP synthase uses the hydrogen gradient to make ATP
ABC transporters
- ATP Binding Cassette transporters
- Contain ATP binding sequences that hydrolyze ATP to effect transport
Cystic fibrosis transmembrane regulator (CFTR)
- Cl- efflux channel found in hepatic/pancreatic ducts and respiratory passages
- Can require more than one ATP to be bound for action
Mutation of CFTR
- Missing phenylalanine at position 508
- Leads to cystic fibrosis
Cystic fibrosis
- Failure to transport Cl-, and therefore Na+ and consequently water into ductular lumen
MDR-1 is found in
- Renal tubules
- Intestine
MDR-1 function
- Transportation of toxic by-products into the lumen
- Pumps drugs out of cells thereby decreasing their efficacy
MDR-1 examples
- Limits anti-retroviral drugs used in AIDS treatment
- Limits anti-neoplastic drugs’ duration of action
Secondary active transport
- Active transport across a biological membrane
- Movement of an ion (typically Na+ or H+) down its electrochemical gradient is linked to the uphill movement of another molecule or ion against its concentration/electrochemical gradient
Types of secondary active transport
- Co-transport (symport)
- Exchange (antiport)
- Most common type is one involving sodium transport
Characteristics of sodium co-transport systems
- Mediated system but much more specific than facilitated diffusion
- Transported molecule is moved against its concentration gradient
- Transport is metabolism-dependent but no phosphorylation of the carrier is required
3 examples of sodium co-transport systems
- Na+ is required for the transport of glucose across epithelia
- Sodium is required for the transport of calcium out of cardiac myocytes
- Hydrogen ion co-transported across intestinal epithelium with di- and tri-peptides
When Na+ is required for the transport of glucose across epithelia
- Usually two Na+ ions transported per glucose molecule transported
- Can take luminal glucose concentrations to very low levels
Clinical examples of diseases affecting sodium co-transport system (all rare autosomal recessive diseases)
- Glucose/galactose malabsorption
- Hartnup’s disease
- Cystinuria
Hartnup’s Disease
- Malabsorption of neutral AA (e.g. Try) in intestine, kidney
Cystinuria
- Malabsorption of basic or positively charged AA (Cys, Lys, Orn, Arg) transport in, kidney
Other Sodium co-transport systems
- Choline and para-amino hippuric acid (PAH) in kidney and choroid plexus
- Catecholamine reuptake by neurons
- Antiport systems, as Na:Ca, Na:H and anion exchange in many membranes as Cl:HCO3
Clinical relevance of sodium co-transport systems
- In intestine, coupling of sodium and glucose/amino acid transport explains the rationale of treating cholera with orally administered, balanced salt solutions containing sugars and amino acids
Epithelial transport
- Involves two membranes that are polarized (transportation mechanism are different)
- Net movement through two membranes (transcellular pathway)
Epithelial transport movement through two membranes also demonstrates
- Paracellular transport, that is, between cells
Exchange diffusion
- 1:1 exchange, no net movement
- Requires no metabolic energy but shows characteristics of mediated systems
- May be that antiport system as Na:Ca and Na:H
- Not 100% specific on internal binding
Mediated systems require
- Physical interaction between transporter and transported substance
Three types of mediated systems
- Facilitated diffusion
- Primary active transport
- Secondary active transport
All mediated systems show
- Specificity
- Competition
- Saturation characteristics
Some mediated systems are passive in which they
- Transport substances down their concentration gradients
- Require no energy
Active systems move substances
- Against their concentration gradient
Active systems require energy that may be consumed at
- Site of transport
- Distant location
