Download Membrane Proteins

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‫‪222Cell Biology‬‬
Lecture 5
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The plasma membrane
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Each cell must interact with its environment in a
number of ways.
Each cell needs to obtain oxygen and other
nutrients (carbohydrates, amino acids, lipid
molecules, mineral ions, etc.) from the
environment, maintain water balance with its
surroundings, and remove waste materials from
the cell.
The Plasma membrane separates a cell from its
environment.
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The plasma membrane
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The plasma membrane functions
1.
2.
3.
4.
5.
The phospholipid bilayer provides the cell with a structure that
separates the outside from the inside of the cell.
The integrity of the membrane is necessary for life functions.
Because of the nature of the phospholipid, many molecules cannot
move across the membrane without help.
Maintains the cell's environment by regulating materials that enter
or leave the cell.
The plasma membrane is differentially, or selectively, permeable.
Some materials enter and leave easily through the membrane,
some with the assistance of membrane molecules, and some
prohibited. Provides mechanisms for cell-to-cell communication.
Provides mechanisms for a cell to recognize "self" versus "nonself" (foreign materials), important to the immune system,
development and defense of the organism through genetically
unique cell recognition markers
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The Fluid Mosaic Membrane Structure
1.
2.
3.
4.
5.
6.
The structure and function of a membrane depends of its molecular
composition
The foundation of the membrane is its phospholipid bilayer, with
a number of associated proteins.
Membranes also contain carbohydrates (glycoproteins and
proteoglycans) and glycolipids.
The resultant membrane structure (proteins scattered throughout
the fluid phospholipid layers) resembles a mosaic, hence the name
“Fluid Mosaic Membrane"
Membrane molecules are manufactured in the endoplasmic
reticulum and distributed by Golgi vesicles
The orientation of membranes is determined at the manufacturing
site. Molecules on the inside of the ER and Golgi vesicles become
exterior membrane molecules.
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Membrane Phospholipids
1.
2.
3.
Phospholipids have both hydrophilic (polar) and hydrophobic (non polar)
regions (in other words, they are amphipathic).
The fatty acid "tails" of the two phospholipid layers are oriented towards
each other so that the hydrophilic "heads", which contain the "charged"
phosphate portion, face out to the environment as well as into the
cytoplasm of the cell's interior, where they can form hydrogen bonds with
surrounding water molecules.
The phospholipid molecules of a membrane provide for its physical integrity
Exterior
Cytoplasm
Phospholipid Bilayer
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Phospholipid Movements
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A membrane is held together, for the most part, by hydrophobic
interactions within the phospholipid bilayer.
Because individual phospholipid molecules are not bonded to each
other, a membrane is flexible, or "fluid", particularly to lateral
movement of the fatty acids.
Phospholipid molecules easily move along the plane of the membrane;
reversing exterior – interior position (or flip-flopping) is less common.
Unsaturated/Saturated
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Phospholipid Movements
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Cholesterol, found in membranes of many animal cells, reduces
fluid movement of the phospholipids, helping to maintain
membrane integrity.
The saturation of fatty acids affects membrane fluidity – the
more saturated, the less movement
Membranes will also solidify as temperature decreases,
reducing function.
The saturation of fatty acids will affect the temperature at
which the membrane "solidifies" (just as it does with fats and
oils).
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Membrane Proteins
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Interspersed throughout a membrane's phospholipid layer
are a number of amphipathic proteins.
The hydrophobic regions of the proteins are within the
fatty acid regions of the phospholipids and hydrophilic
regions of the proteins are at the interior and exterior
aqueous interfaces of the membrane.
This orientation is important to how the membrane
proteins function.
The membrane is also associated with a network of
supporting cytoskeletal filaments, some of which help
shape the cell and some help anchor proteins within the
membrane.
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Protein Mobility
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Many proteins within the membrane are mobile; studies of
fused mouse and human cells show that proteins from the
two cells are intermixed within an hour of fusion
Membrane proteins are divided into two categories,
integral and peripheral, location depending on their
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Membrane Proteins
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Integral (Transmembrane) Proteins
o
Proteins that go through the membrane are called integral
or transmembrane proteins.
They have hydrophobic (non-polar amino acids with alpha
helix coiling) regions within the interior of the membrane
and hydrophilic regions at either membrane surface
o
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Peripheral Proteins
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Are attached to the surface of the membrane, often to the
exterior hydrophilic regions of the transmembrane proteins.
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On the interior surface, peripheral proteins typically are held in
position by the cytoskeleton.
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On the exterior, proteins may attach to the extracellular matrix.
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Peripheral proteins help give animal cell membranes strength.
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The different proteins contribute to the "sidedness" of
membranes so that the interior and exterior sides of membranes
have different properties that affect membrane function.
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Anchoring proteins
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Other proteins have non-polar α helix regions that fix the protein into specific
regions of the phospholipid bilayers. Such proteins are called anchoring proteins
The protein receptors at neuromuscular junctions on muscle cells are anchored
proteins
Anchor proteins can attach to the fibrous network of the cytoskeleton to give
shape and strength to some cells.
Some membrane lipid regions, called lipid rafts, are also specialized to help
anchor proteins within a specific region of a membrane.
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Membrane Protein Functions
1-Transport Proteins
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Transport Proteins are transmembrane proteins that serve as carriers for
specific substances that need to pass through the membrane by providing a
hydrophilic channel or pore.
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Transport proteins have binding sites that attract specific molecules. Most
of our ions, amino acids, sugars and other small nutrient molecules are
moved through transport proteins.
When a molecule binds to the carrier protein, the protein shape changes
moving the substance through the membrane. This process may require
energy (ATP), and the ATP complex is then a part of the transport protein.
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Membrane Protein Functions
2- Enzymatic Proteins
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Many enzymes are embedded in membranes, which attract reacting molecules to
the membrane surface.
The active site of the enzyme will be oriented in the membrane for the substrate
to bind
Enzymes needed for metabolic pathways can be aligned adjacent to each other to
act like an assembly line for the reactions, minimizing the need for intermediates
to diffuse through the cytoplasm of the cell.
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Membrane Protein Functions
3- Signal Transduction (Receptor) Proteins:
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Signal transduction proteins have attachment sites for chemical
messengers, such as hormones.
The signal molecule, when it attaches to the receptor promotes a
conformational change that relays the message into the cell to trigger
some cell activity
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Membrane Protein Functions
4-Attachment Proteins
-Attachment proteins attach
to the cytoskeleton or extracellular matrix to help maintain
cell shape (particularly for animal cells)
5-Recognition (Identity) Proteins
-Glycoproteins serve as
surface receptors for cell recognition and identification. They are
important to the immune system.
6-Cell Adhesion (Intercellular Joining) Proteins
- Special membrane proteins are responsible for the cell junctions (tight junctions,
desmosomes and gap junctions.
- They permit cells to adhere to each other
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Membrane Carbohydrates
Glycoprotein
complex with
long
Polysacchari
de
‫معقد جاليكىبزوتيه‬
‫مع‬
‫سلسلت طىيلت مه‬
‫السكاكز المتعددة‬
Collagen fiber
‫ليفت كىالجيه‬
Intgrine
‫اوتيجزيه‬
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Glycoproteins and glycolipids
are also important to membrane
structure and function.
Glycolipids function as recognition
signals for cell-to-cell interactions.
Glycoproteins, with their
oligosaccharides portions, are
critical for a cell to be recognized
by other cells and by protein
molecules, and for cell-to-cell
adhesion
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Functions of cell membranes:
Cell membranes are selectively permeable membranes i.e allow passage
of certain substances but not others
1. Transport
Passive transport :
a) Do not require energy
b) Cells do not perform work
c) Driving force is the concentration gradient.
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Types of passive transport
1. Simple diffusion : movement of molecules from an area of higher
concentration to an area of lower concentration untill equilibrium e.g
diffusion of gases (O2 and CO2) in our lungs and RBCs.
2. Facilitated diffusion : a type of passive diffusion provides a channel for
sugars and H2O. Binds its passenger, changes shape and release it to the
other side.
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Facilitated Diffusion
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Most molecules cannot move freely through the membrane, but can pass through
membranes if a gradient exists, with the help of membrane transport proteins.
No energy is involved, so it is still a passive process.
Transport proteins are specific, and are limited in number in membranes.
The rate of movement of materials is dependent on the availability of transport
proteins as well as the concentration of the substance to be moved.
Amino acids, monosaccharides and ions move through membranes via facilitated
diffusion.
It move through a hydrophilic protein channel or pore of the transport protein
Ion channel proteins are common in membranes.
Much water movement through membranes also involves facilitated diffusion. There
are special channel proteins, called aquaporins that facilitate the movement of water
at a rate needed for cell activities.
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Some transport proteins have channels with gates. The gate opens to let
the target molecule pass through when it receives an electrical or
chemical signal. For example, neurotransmitter chemicals serve as signal
molecules to open the gates for sodium to flow into the nerve cell.
Facilitated diffusion also occurs with carrier molecules, substances to
which the target molecule to be transported temporarily binds, resulting
in a conformational change that moves the target substance through the
membrane
Potassium Ion Channel
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Protein Glucose Carrier Proteins
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Active transport of a solute across
a membrane
Solute binding
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Phosphorylation
Transport
Protein reversion
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Moving Materials Through Membranes
There are a number of ways to move materials through membranes:
1. Simple diffusion
2. Facilitated diffusion
3. Active transport
- These ways are used to move small quantities of substances
- Simple and facilitated diffusion are means of passive transport
- Active transport uses energy to move substances against a gradient.
- Larger volumes are moved by exocytosis or endocytosis
- Endocytosis and exocytosis, which involve extensive membrane
rearrangements, are also energy consuming.
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1- Active Transport
Active transport requires energy
and uses a variety of transport
mechanisms.
- Uniports move one substance
in one direction.
- Symports move two substances
in the same direction
- Antiports move two substances
in opposite directions.
Symports and antiports are also
known as coupled transporters
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Membrane Interactions with the Environment
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Larger substances, including macromolecules, require changes in
membrane shape and/or the fusion of membranes to move into or out of
cells. Such substances move into the cell by endocytosis and from the cell
by exocytosis
1- Exocytosis
 Materials can be exported from the cell by fusing vesicles with the plasma
membrane, a process called exocytosis
 Materials for export are packaged in a Golgi body and the vesicles formed
travel along the cytoskeleton until they reach the plasma membrane.
 Once the vesicle membrane and plasma membrane fuse, the contents of
the vesicle are freed from the cell for example : insulin,
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2. Endocytosis
 Substances that enter the cell using membrane modifications move by
endocytosis.
 There are three endocytosis processes: phagocytosis, receptor-mediated
endocytosis and pinocytosis
A. Phagocytosis
 Phagocytosis occurs when membrane pseudopodia surround and engulf
particulate objects, packaging them in a membrane-bounded vacuole.
 Phagocytosis is used for solid large objects, such as prey engulfed by
Amoeba, and bacteria by white blood cells.
B. Pinocytosis (cell drinking)
 In pinocytosis, the plasma membrane invaginates, substances "fall" in
cavity, the membrane seals over and the molecules in the fluid will be
moved into the cell enclosed vesicle.
 Once in the cytoplasm, the vesicle membrane degrade, the substances
release into the cytoplasm
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Membrane Interactions with the Environment
C. Receptor mediated endocytosis:
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Highly specific receptor proteins in the membrane attract the substance to
be moved into the cell.
The receptor proteins are attached to specific substances in the membrane
creating a membrane depression in that area called a coated pit
The cytoplasmic side of the coated pit is coated with specific proteins,
called clathrins
When sufficient target molecules have been attracted, the pocket will be
pinched off forming a clathrin-coated vesicle in the cytoplasm.
Molecules that bind to receptor sites are called ligands. (It’s a general term
that simply means something that attaches to a receptor.)
Receptor-mediated endocytosis is an effective means of moving desired
materials into cells. e.g absorption of cholesterol.
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Three kinds of endocytosis
Phagocytosis
Pinocytosis
Receptor-mediated endocytosis
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Plasma membrane markers
Special proteins or enzymes found in plasma membrane like:
- Adenosine triphosphate transporting enzymes (Na+/K+ transporting
ATPase ) , which found in all plasma membranes
- Endoplasmic reticulum membrane contains glucose 6-phosphorylase
- Inner mitochondrial membranes contains succinate dehydrogenase
Transport through membrane occurs via:
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Rotation
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Crossing
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Conformation change
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Pore formation
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