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How do transport/carrier proteins rotate during facilitated diffusion?

How do transport/carrier proteins rotate during facilitated diffusion?



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In my book, it is written that when a molecule binds to the carrier protein then the protein rotates and releases the molecule inside the cell. I want to know how is this possible. Wouldn't the surrounding phospholipid molecules or other proteins hinder its rotation?

EDIT

The rotation axis is in the plane of membrane

Source : http://ncert.nic.in/ncerts/l/kebo111.pdf (page 177, Third paragraph)


"How do transport/carrier proteins rotate during facilitated diffusion?"

They don't. The explanation in the PDF you linked…

Figure 11.1 shows an extracellular molecule bound to the transport protein; the transport protein then rotates and releases the molecule inside the cell.

… is an oversimplification, to say the least (you could simply say that this is plain wrong). What really happens is way more complex.

We call it conformational change.

According to Alberts, Molecular Biology of the Cell (2002):

Each type of carrier protein has one or more specific binding sites for its solute (substrate). It transfers the solute across the lipid bilayer by undergoing reversible conformational changes that alternately expose the solute-binding site first on one side of the membrane and then on the other. (emphasis mine)

This is an image that explains way better the process, from the same chapter in Alberts (the legend provides a detailed explanation):

Figure 11-11A model for the molecular mechanism of action of the bacterial lactose permease (A) A view from the cytosol of the proposed arrangement of the 12 predicted transmembrane helices in the membrane. The loops that connect the helices on either side of the membrane are omitted for clarity. A glutamic acid on helix X binds H+, and amino acids contributed by helices IV and V bind lactose. (B) During a transport cycle, the carrier flips between two conformational states: in one, the H+ -and lactose-binding sites are accessible to the extracellular space (1 and 2); in the other, they are exposed to the cytosol (3 and 4). Unloading of the solutes on the cytosolic face (3 → 4) is favored because the lactose-binding site is partly disrupted and a positive charge contributed by an arginine on helix IX displaces the H+ from the glutamic acid on helix X. (A, adapted from H.R. Kaback and J. Wu, Accts. Chem. Res. 32:805-813, 1999.)

Thus, as you can see, we have conformational states: one of them exposes the binding site to the extracellular space, while the other one exposes the binding site to the cytosol. Just like what happens when an enzyme changes conformation, the 3D changes in the protein structure are quite small, and it's hydrophobic regions keep in the same place in both states.

Source: Alberts, B., Johnson, A. and Lewis, J. (2002). Molecular biology of the cell. 1st ed. New York: Garland Science..


4.3A: Facilitated Transport

  • Contributed by Boundless
  • General Microbiology at Boundless

Facilitated transport is a type of passive transport. Unlike simple diffusion where materials pass through a membrane without the help of proteins, in facilitated transport, also called facilitated diffusion, materials diffuse across the plasma membrane with the help of membrane proteins. A concentration gradient exists that would allow these materials to diffuse into the cell without expending cellular energy. However, these materials are ions or polar molecules that are repelled by the hydrophobic parts of the cell membrane. Facilitated transport proteins shield these materials from the repulsive force of the membrane, allowing them to diffuse into the cell.

The material being transported is first attached to protein or glycoprotein receptors on the exterior surface of the plasma membrane. This allows the material that is needed by the cell to be removed from the extracellular fluid. The substances are then passed to specific integral proteins that facilitate their passage. Some of these integral proteins are collections of beta-pleated sheets that form a channel through the phospholipid bilayer. Others are carrier proteins which bind with the substance and aid its diffusion through the membrane.


Uptake and Transport of Hormones

Carrier proteins transfer an ion or a molecule from one side of a membrane to the other. They are specific to each ion or molecular species. While the movement of the ion or molecule itself is passive, the motive force for the carrier protein is provided by the proton or pH gradient and/or electrical gradient, which are set up by the proton pumps (see below). Carrier proteins may act as “symporters,” transferring two different species across the membrane in the same direction, or as “antiporters,” which transfer one species in one direction and the other in the opposite direction.


What Are Examples of Facilitated Diffusion?

Examples of facilitated diffusion are the passing of K+ ions through a membrane with an aid of a potassium transport protein and the passing of glucose and amino acids with the aid of proteins called permeases. Retinol binding protein acts as a water-soluble carrier for retinol and fatty acids.

Diffusion is movement of molecules across a membrane. There are three main types of diffusion: simple, channel and facilitated types. Particles normally move from areas of high concentration to areas of low concentration along the concentration gradient. Prokaryotic cells demonstrate simple diffusion, whereas facilitated diffusion only happens in more complex eukaryotic cells.

Simple diffusion requires no energy and happens linearly. The more particles added to the solution, the higher the concentration becomes. Channel diffusion involves proteins letting molecules through a membrane.

In a facilitated diffusion, an integral protein changes its conformation to let a passing molecule through, as in the case of permeases proteins. Other protein carriers, such as retinol binding proteins, do not change. Carrier or transport proteins stay in their places in the membrane. Transport proteins are tailored to let only particular molecules through.

During facilitated diffusion, unlike other types, individual molecules can travel against the concentration gradient, but the net movement is as in other types of diffusion. As opposed to simple diffusion, saturation can be reached during facilitated diffusion.


Examples of Facilitated Diffusion

A number of important molecules undergo facilitated diffusion to move between cells and subcellular organelles.

Glucose Transporter

When food is digested, there is a high concentration of glucose within the small intestine. This is transported through the membranes of the cells of the alimentary canal, towards the endothelial cells lining blood capillaries. Thereafter, glucose is transported throughout the body by the circulatory system. When blood flows through tissues that need energy, glucose traverses the endothelial cell membranes again and enters cells with low glucose concentration. Occasionally, when blood sugar levels drop, the movement can occur in reverse – from body tissues into blood circulation. For instance, hepatic cells can generate glucose even from non-carbohydrate sources to maintain a basal blood sugar concentration and prevent hypoglycemia.

The glucose transporter that facilitates this movement is a carrier protein that has two major conformational structures. While the exact three-dimensional structure is not known, the binding of glucose probably causes a conformational change that makes the binding site face the interior of the cell. When glucose is released into the cell, the transporter returns to its original conformation.

Ion Channels

Ion channels have been extensively studied in excitatory cells like neurons and muscle fibers since the movement of ions across the membrane is an integral part of their function. These channel proteins form pores on the lipid bilayer that can be either in the open or closed conformation, depending on the electrical potential of the cell and the binding of ligands. In this sense, these proteins are called ‘gated’ channels.

The presence of ion pumps in most cells ensures that the ionic composition of the extracellular fluid is different from the cytosol. The resting potential of any cell is driven by this process, with an excess of sodium ions in the extracellular region and an excess of potassium ions within the cell. The electrical and concentration gradient generated in this manner is used for the propagation of action potentials along neurons and the contractility of muscle cells.

When a small change in the voltage of a cell occurs, sodium ion channels open and allow the rapid ingress of sodium ions into the cell. This, in turn, induces the opening of potassium ion channels, allowing these ions to move outward, demonstrating that the diffusion of one substance can occur independently of another. In a few milliseconds, a region in the cell membrane can undergo large changes in voltage – from -75 mV to +30 mV.

The binding of neurotransmitters like acetylcholine to receptors on muscle cells changes the permeability of ligand-gated ion channels. The transmembrane channel is made of multiple subunits arranged like a closed cylinder. The binding of the ligand (acetylcholine) alters the conformation of the hydrophobic side chains that block the central passage. This leads to the rapid influx of sodium ions into the muscle cell. The change in the electric potential of the cell further results in the opening of calcium ion channels, which then lead to the contraction of the muscle fiber.

Aquaporins

Like other transmembrane proteins, aquaporins have not been fully characterized. However, it is known that there are many such channels for the rapid passage of water molecules in nearly every cell. These highly conserved proteins are present in bacteria, plants, fungi and animals. Mutations in the proteins forming aquaporins can lead to diseases like diabetes insipidus.


What is Active Transport

Active transport refers to the transport of molecules across the plasma membrane against the concentration gradient by using energy. Transmembrane carrier proteins are involved in active transport. Two types of active transports can be identified in a cell. They are primary active transport and secondary active transport. Primary active transport directly uses the metabolic energy in the form of ATP to transport molecules across the membrane. The carrier proteins that transport molecules by primary active transport are always coupled with ATPase. The most common example of primary active transport is the sodium-potassium pump. It moves three Na+ ions into the cell while moving two K+ ions out of the cell. Sodium-potassium pump helps in maintaining the cell potential. The sodium-potassium pump is shown in figure 2.

Figure 2: Sodium-Potassium Pump

The secondary active transport relies on the electrochemical gradient of the ions in either side of the plasma membrane to transport molecules. That means secondary active transport uses the energy released by transporting one type of molecules through its concentration gradient to transport another type of molecule against the concentration gradient. Therefore, transmembrane proteins involved in the secondary active transport are called cotransporters. The two types of cotransporters are symporters and antiporters. Symporterstransport both molecules in the same direction. Sodium-glucose cotransporter is a type of symporter. Antiporterstransport the two types of molecules to opposite directions. The sodium-calcium exchanger is an example of antiporter.


Differences Between Carrier and Channel Proteins

Mechanism

▶ Carrier proteins transfer solutes across the biological membrane by binding to the solute and alternate between two conformations. Their mechanism is similar to enzyme-substrate reactions following Michaelis-Menten equation (however, they do not change the substrate, i.e., the solute).

▶ Channel proteins interact the least with the solute they transfer.

Nature of Solute

▶ Carrier proteins transfer both polar and nonpolar solutes across the biological membrane.

▶ Channel proteins transfer only small and polar solutes across the biological membrane.

Specificity

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▶ Specificity of carrier proteins is due to the specific binding sites to which the solute molecules bind.

▶ Specificity of channel proteins is due to an ion selectivity filter.

Ion selectivity filter: In simplest words, it can be defined as the narrowest part of the pore which will only allow the passage of specific molecules with a particular size and charge to pass through.

Rate of Transfer of Solute

▶ The rate of transfer of solute by carrier proteins is about 10 4 ions per second.

▶ As the proteins do not flip from one conformation to the other, the rate of transfer of solute by channel proteins is much higher, i.e., 10 8 ions per second.

Nature of Transport

▶ Carrier proteins usually transport molecules against the concentration gradient to do so, they require energy. This energy can be supplied to it either by hydrolysis of ATP (known as active transport) or can be coupled with the transfer of another solute molecule (known as facilitated diffusion).

Examples of Carrier Protein-mediated Active Transport

1. Na + /K + ATPase: It plays an important role in the uptake of glucose by the cell. Three Na + ions are pumped out of the cell, and two K + ions are pumped inside the cell. This takes place against the concentration gradient, and 1 ATP is consumed for this process. These Na + ions help to bring glucose inside the cell (discussed below).

2. SR Ca 2+ ATPase: It is present in the Sarcoplasmic Reticulum (abbreviated as SR specialized Endoplasmic Reticulum present in muscle cells). Two Ca 2+ ions are transported from the cytosol into the SR. This step consumes 1 ATP and is required for the contraction of muscles.

Examples of Carrier Protein-mediated Facilitated Diffusion

1. Symport: Co-transport of molecules or ions in the same direction of the biological membrane.

E.g. Na + -driven glucose pump: It is present in the intestinal epithelial cells. Glucose is taken up by the intestinal cells with this pump. Here, the glucose moves against its concentration gradient. The energy to perform this is supplied by the movement of Na + ions into the cells this movement is down its concentration gradient and is favored.

2. Antiport: Co-transport of molecules or ions in the opposite direction of the biological membrane.

E.g. Na + /Ca 2+ exchanger: Here, Na + ions move down their concentration gradients, and this provides energy for the movement of Ca 2+ against its concentration gradient. Three Na + ions are transported inside the cell, and one Ca 2+ ion is pumped out of the cell.

Channel proteins always transport molecules down the concentration gradient by a process of diffusion and, hence, mediate passive transport.

Examples of Channel Protein-mediated Passive Transport

Ion channels are not continuously open and are said to be gated, which open only in response to specific stimulus.

1. Voltage-gated Channels: These ion channels are activated when there is a potential difference generated across the biological membrane. An example is voltage-dependent calcium channels, which are found on the cell membrane of neurons, glial cells, and muscle cells. These channels are activated when there is a potential difference across the membrane and cause an influx of Ca 2+ ions inside the cells. They may play a role in neurotransmission, muscle relaxation, gene expression, etc., depending on the type of cell on which they are present.

2. Ligand-gated Channels: Nicotinic Acetylcholine Receptor (nAchR) channel is usually found in neuromuscular junctions. When the nAchR channel binds to the neurotransmitter, acetylcholine (ligand), the closed channels open and allow the influx of Na + ions, thus helping in the contraction of muscles.

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Active transport

Active transport is the movement of substances across the membrane in combination with a carrier protein against energy gradients: uphill. It requires an additional source of energy derived from the cell. There are two major mechanisms of active membrane transport: primary and secondary active transport.

Active transport occurs only through the lipid layer of the cell membrane where the transported substance combines with a specific carrier protein. It requires energy derived directly from the breakdown of adenosine triphosphate (ATP) or another high-energy phosphate compound (creatine phosphate). This leads to the conformational change in the carrier and it pumps the carried substance across the membrane. The most important example of a primary active transport is the sodium-potassium (Na + -K + ) pump.

Sodium-potassium (Na + -K + ) pump

It is a transport process that pumps sodium ions outward of the cell through the cell membrane and at the same time pumps potassium ions from the outside to the inside of the cell against their concentration gradient.

The concentration of K + inside the cell is 30 times higher than that outside, and the reverse is true of Na + . because Na + and K + leak slowly but continuously through leakage channels in the plasma membrane along their concentration gradient, the Na + -K + pump operates more or less continuously to drive Na + out of the cell and pump K + back into the cell, against their concentration gradient.

In Na + -K + pump, the carrier protein is a complex of two separate globular proteins: a larger one called the “a” subunit, and a smaller one, the “b” subunit. The larger protein has three specific features:

  1. It has 3 receptor sites for binding sodium ions on the surface facing the inside of the cell.
  2. It has 2 receptor sites for potassium ions on the outside surface.
  3. It has an ATPase activity.

When potassium ions bind on the outside of the carrier protein and the sodium ions bind on the inside, the ATPase function of the protein becomes activated. This then cleaves one molecule of ATP, splitting it and liberating a high-energy phosphate bond. This energy causes a conformational change in the carrier molecule and extruding the sodium ions to the outside and potassium ions to the inside.

Electrogenic nature of the Na + -K + pump

The fact that the Na + -K + pump moves three Na + ions to the exterior for every two K + ions to the interior creates positivity outside the cell and negativity on the inside (membrane potential). Na + -K + pump is said to be electrogenic because it creates an electrical potential across the cell membrane, which is a basic requirement for the functions of excitable tissues.

Importance of Na + -K + pump

  1. Na + -K + pump is responsible for maintaining the sodium & potassium concentration differences across the cell membrane.
  2. Maintenance of intracellular potassium is necessary for protein metabolism.
  3. It maintains a negative electrical voltage inside the cells.
  4. It keeps the osmotic equilibrium and controlling cell volume.

Secondary active transport

In this type of transport, there is a carrier existing in the lipid layer of the membrane, which has one site for one sodium ion and the other site may be used by one molecule of glucose, galactose or amino acids. The two sites must be occupied at the same time before the carrier can act. As in primary active transport, the molecules move against their electrochemical gradient. The energy supplied for this process comes from the movement of sodium along its electrochemical gradient.

All secondary active transport systems are coupled systems, they move more than one substance at a time. If the two transported substances are moved in the same direction, the system is a symport system (Co-transport). If the transported substances cross the membrane in opposite directions, the system is an antiport system (Counter-transport).

Co-Transport

Glucose and amino acids are transported into most cells against large concentration gradients by the co-transport mechanism. The excess sodium outside the membrane always attempts to diffuse to the interior, uses the stored energy to pull other substances along with it through the membrane. This is achieved by means of a carrier protein that serves as an attachment point for both the sodium ion and the substance to be co-transported.

Counter-transport

There are two especially important counter-transport mechanisms (transport in a direction opposite to the primary ion), they are sodium-calcium counter-transport and sodium-hydrogen counter-transport.


Endocytosis

Endocytosis takes up particles into the cell by invaginating the cell membrane, resulting in the release of the material inside of the cell.

Learning Objectives

Describe endocytosis, including phagocytosis, pinocytosis, and receptor-mediated endocytosis.

Key Takeaways

Key Points

  • Endocytosis consists of phagocytosis, pinocytosis, and receptor -mediated endocytosis.
  • Endocytosis takes particles into the cell that are too large to passively cross the cell membrane.
  • Phagocytosis is the taking in of large food particles, while pinocytosis takes in liquid particles.
  • Receptor-mediated endocytosis uses special receptor proteins to help carry large particles across the cell membrane.

Key Terms

  • endosome: An endocytic vacuole through which molecules internalized during endocytosis pass en route to lysosomes
  • neutrophil: A cell, especially a white blood cell that consumes foreign invaders in the blood.

Endocytosis

Endocytosis is a type of active transport that moves particles, such as large molecules, parts of cells, and even whole cells, into a cell. There are different variations of endocytosis, but all share a common characteristic: the plasma membrane of the cell invaginates, forming a pocket around the target particle. The pocket pinches off, resulting in the particle being contained in a newly-created intracellular vesicle formed from the plasma membrane.

Phagocytosis

Phagocytosis: In phagocytosis, the cell membrane surrounds the particle and engulfs it.

Phagocytosis (the condition of “cell eating”) is the process by which large particles, such as cells or relatively large particles, are taken in by a cell. For example, when microorganisms invade the human body, a type of white blood cell called a neutrophil will remove the invaders through this process, surrounding and engulfing the microorganism, which is then destroyed by the neutrophil.

In preparation for phagocytosis, a portion of the inward-facing surface of the plasma membrane becomes coated with a protein called clathrin, which stabilizes this section of the membrane. The coated portion of the membrane then extends from the body of the cell and surrounds the particle, eventually enclosing it. Once the vesicle containing the particle is enclosed within the cell, the clathrin disengages from the membrane and the vesicle merges with a lysosome for the breakdown of the material in the newly-formed compartment ( endosome ). When accessible nutrients from the degradation of the vesicular contents have been extracted, the newly-formed endosome merges with the plasma membrane and releases its contents into the extracellular fluid. The endosomal membrane again becomes part of the plasma membrane.

Pinocytosis

Pinocytosis: In pinocytosis, the cell membrane invaginates, surrounds a small volume of fluid, and pinches off.

A variation of endocytosis is called pinocytosis. This literally means “cell drinking” and was named at a time when the assumption was that the cell was purposefully taking in extracellular fluid. In reality, this is a process that takes in molecules, including water, which the cell needs from the extracellular fluid. Pinocytosis results in a much smaller vesicle than does phagocytosis, and the vesicle does not need to merge with a lysosome.

Potocytosis, a variant of pinocytosis, is a process that uses a coating protein, called caveolin, on the cytoplasmic side of the plasma membrane, which performs a similar function to clathrin. The cavities in the plasma membrane that form the vacuoles have membrane receptors and lipid rafts in addition to caveolin. The vacuoles or vesicles formed in caveolae (singular caveola) are smaller than those in pinocytosis. Potocytosis is used to bring small molecules into the cell and to transport these molecules through the cell for their release on the other side of the cell, a process called transcytosis.

Receptor-mediated Endocytosis

Receptor-Mediated Endocytosis: In receptor-mediated endocytosis, uptake of substances by the cell is targeted to a single type of substance that binds to the receptor on the external surface of the cell membrane.

A targeted variation of endocytosis, known as receptor-mediated endocytosis, employs receptor proteins in the plasma membrane that have a specific binding affinity for certain substances. In receptor-mediated endocytosis, as in phagocytosis, clathrin is attached to the cytoplasmic side of the plasma membrane. If uptake of a compound is dependent on receptor-mediated endocytosis and the process is ineffective, the material will not be removed from the tissue fluids or blood. Instead, it will stay in those fluids and increase in concentration. Some human diseases are caused by the failure of receptor-mediated endocytosis. For example, the form of cholesterol termed low-density lipoprotein or LDL (also referred to as “bad” cholesterol) is removed from the blood by receptor-mediated endocytosis. In the human genetic disease familial hypercholesterolemia, the LDL receptors are defective or missing entirely. People with this condition have life-threatening levels of cholesterol in their blood, because their cells cannot clear LDL particles from their blood.

Although receptor-mediated endocytosis is designed to bring specific substances that are normally found in the extracellular fluid into the cell, other substances may gain entry into the cell at the same site. Flu viruses, diphtheria, and cholera toxin all have sites that cross-react with normal receptor-binding sites and gain entry into cells.


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