Reminder: Have you received the lab material

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Reminder Have you received the lab material?
If so, dont forget to prepare for the quiz.
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Cellular Regulation of Enzymes 1). Allosteric
interaction modulators cause
conformation change in the enzyme 2). Regulation
by addition and removal of chemical groups
3). Proteolytic Activation Some enzymes
are activated by proteolytic cleavage
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• Proteolytic cleavage - breakage of peptide
• bonds by peptidases or proteases.
• Zymogen inactive form of enzyme, that can be
  activated by the removal of one or more short
  pieces of peptide sequences

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Many digestive enzymes are regulated this way.
For. Example, chymotrypsinogen ?
?-Chymotrypsin ? ?-chymotrypsin   The activation
process is completed by same type of enzymes or
even the active form of the same enzyme.
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Activation of Chymotrypsin
chymotrypsinogen (inactive)
trypsin
?-chymotrypsin (active)
chymotrypsin
Ser - Arg and Thr - Asn 14 15
147 148
?-chymotrypsin (active)
The three pieces are held together by inter chain
disulfide bonds.
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Structure and Functions of Biomembranes (Chapter
s 7 and 8)
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• Functions of membranes
• Physical barrier - protection and structure
• 2. Transportation channels and transport
  proteins selectively allow substances to pass
  through
• 3.  Receiving electrical and chemical signals
• 4.  Specific functions, such as electron
  transport,
• enzymatic reactions
• 5. Cell-cell communication

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•  
• Membrane Components and Structure
•  Lipid bilayer with proteins embedded in it
• - Fluid Mosaic Model.
• The membrane structure has elasticity and
• fluidity because of the lipids.
• The diversity of functions is mostly due to
• the membrane proteins.
• 5 carbohydrate in glycolipids and glycopro-
• teins.

 
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Lipid Bilayer   Hydrophilic polar heads of
phospholipids stick out the hydrophobic
nonpolar tails hide inside.
This is a random arrangement with lowest free
energy level and is most stable.
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Fluid mosaic model of membranes was suggested by
Jonathan Singer and Garth Nicholson in 1972.
carbohydrates
a-helix protein
cholesterol
phospholipids
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Membrane lipids Phospholipids (phosphotidyl
serine, phosphotidyl choline, phosphotidyl
inositol and phosphotidyletha- nolamine). Sphin
golipids sphingomyelin forming myelin
sheath Glycolipids often serves as cell surface
markers (antigens responsible for ABO
types). Sterols - makes membranes strong and
rigid. The major one is cholesterol.
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Phospholipid (phosphotidyl Choline)
Sphingolipid
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Fatty acids in the membrane structure All
between 12C 20C in length. Table 7-2 Name
C number Double bunds Palmitate 16 0 St
earate 18 0 Oleate 18 1 Linoleate 18 2 Li
nolenate 18 3 Arachidonate 20 4 Unsaturated
fatty acids increase the fluidity of the
membrane.
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• Membranes are not static.
• Layers move over each other based on percent of
  unsaturated fatty acids.

lateral diffusion
flip-flop rare
rotation
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To prove, use fluorescence recovery after
photobleaching
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Membrane can function properly only in the fluid
state. (Mobility and conformation change of
proteins are important for receptor binding,
signal transduction, cell-cell interaction) Tran
sition temperature (Tm) The temperature at
which the membrane gels (freezes). Below Tm,
membrane function will be impaired.
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Fig. 7-12 Effect of fatty acid composition on
membrane fluidity.
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The fluidity of lipid membrane is also influenced
by the chain length of fatty acids
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Sterols decrease the membrane fluidity at high
temperature and increase it at low
temperature. They make the lipid bilayer
stronger also make the membrane less permeable
by plugging the small spaces between
phospho- lipids.
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Content of sterols (cholesterol) varies between
different membranes.

• Animal cells contain 3 cholesterol in
• mitochondria membrane, and up tp 50 in
• plasma membrane.
• Plant and bacteria cells do not contain
• cholesterol.

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• Homeoviscous adaptation
• - Regulation of membrane fluidity according to
• the environmental temperature.
• In micrococcus, drop in temperature causes
  increased proportion of shorter fatty acids.
• In E. coli, a drop in temperature triggers the
  synthesis of desaturase, an enzyme that
  introduces double bonds to the hydrocarbon
  chains.
• In hibernating animals, incorporation of
  unsaturated fatty acids increases as body
  temperature falls.

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• Membrane Proteins
• Membranes from different cells have
• different biological properties and functions.
  
• This diversity is mostly due to different
• membrane proteins.

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• The ratio of protein ranges 20-80, depending
• on the source of the membrane.
• Mitochondrial membrane 76
• myelin sheath 18 (high the lipid content for
  
• better insulation).
•  

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• Two types of membrane proteins
• Peripheral membrane proteins with hydrophilic
  a.a facing external and hydrophobic ones in the
  central region
• Some proteins span the lipid bilayer - integral
  proteins (less than 10) mostly serves as
  transport proteins or channels.
• Both types of proteins can move around.

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• Functions of membrane proteins
• Enzymes
• - Na/K ATPase, adenylate cyclase, lipases
• Transport proteins - Glut4
• Channels several ?-helices form a lumen
• Receptors often glycoporteins
• Intercellular communication also
  glycoproteins

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Asymmetry of membranes

• Reflected in asymmetry of proteins with precise
  orientations

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• Specialized attachments and structures in
  proteins
• that confer stability in the membrane
• No flipping of proteins
• Separates two distinct environments

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Movement within the membrane

• All proteins and lipids are mobile within the
  membrane
• Thermal motion permits free rotation
• Exchanging places 107 times per second
• Reality while all are mobile, most lipids
  mobile within a limited range
• There are protein-rich regions and lipid-rich
  regions
• Some lipids may be tightly bound to integral
• proteins in the membrane, which limits mobility

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Plasma membrane

• Selectively permeable
• Allows maintenance of a relatively constant
  internal environment
• Relatively impermeable to most water soluble
  molecules
• Essential to have a means to transport these
  across the membrane
• Specific for particular cell types due to
  specific requirements
• Organelles may require specific molecules as well
• Different than the rest of the cellmay require
  specific transporters

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Semi-permeability

• Pure phospholipid membrane
• Permeable to gases,
• ( O2 and CO2),
• small uncharged polar molecules (ethanol -
• lipid soluble)

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Membrane Transport

• Passive transport cost no energy
• - simple diffusion
• - facilitated diffusion
• - osmosis
• Active transport
• - energy consuming
• - aided by proteins
• - bulk transport

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• Simple diffusion (Passive diffusion )
• Unaided by transport proteins
• No energy cost
• Little specificityany small hydrophobic molecule
• Down the chemical concentration gradient
• Exp. water, gases and some ions Na, K, Cl-

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Factors that affect simple diffusion a, The
magnitude of the concentration gradient The
net rate of transport is directly proportional to
the concentration difference between the
two sides of the membrane.   v inward
P?S   ?S concentration gradient P
permeability coefficient
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b. Size of the molecule. Smaller molecules
are easier to cross the membrane. c. Lipid
solubility Alcohol, steroids and some
vitamins can easily diffuse through the
membrane. d. Permeability of the membrane
Lipid bilayer is generally impermeable to
ions. Ion gradient is important for cellular
function.
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e. The surface area of the permeable membrane
In the area that are specialized for diffusion
(digestive tract), the cell membrane form
many tiny folds called moicrovilli.
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• Osmosis, a special case of diffusion
• Water diffuses from its high concentration
• to low concentration.
• Whenever there is a concentration gradient
• for solute , there is a concentration gradient
• for water.
• Water tent to diffuse across the membrane
• towards the region with high solute
• concentration.  

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• For osmosis to occur the membrane must be
• more permeable to water than to solutes.
•  
• Osmotic pressure - the force of water
• movement into a solution.
• The higher the solute concentration in a
• solution, the higher the osmotic pressure.
•  

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• Osmolarity / tonicity
• Solute concentration in an aqueous solution is
  -
•  
• Any solution that has same solute concentration
  as
• intracellular fluid and cause no osmotic flow
  of water,
• is called isoosmotic or isotonic.
• Exp. normal saline (0.9 of NaCl).
•  
• Solutions with higher solute concentration than
  
• cytosol are called hyperosmotic or
  hypertonic.
• Solutions with lower osmotic pressure than
  cytosol
• are called hypoosmotic or hypotonic solution.

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• Facilitated Diffusion (also passive)
• Mediated by carrier proteins and channels
• Also driven by concentration gradient.

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Carrier Proteins (transporters or
permeases) Carrier proteins have alternating
conformations
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• Carrier proteins behave like enzymes
• Facilitated diffusion has transport maximum,
• carriers can be saturated.
• Carriers are specific.
• GLUT transport glucose only.
• Facilitated diffusion follow Michaelis-Menten
• Kinetics

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Carrier proteins can transport one or two solutes
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• Primary transport or uniport
• one type of solute is transported,
• Secondery transport
• Symport same direction (glucose and Na)
• Antiport opposite direction (Na and K).

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• Channel proteins facilitate diffusion by forming
• hydrophilic transmembrane channels
• Several alpha helices line side by side to a
  lumen.-
• Ion channels Na, K, Ca, Cl-
• Opening of the channels are controlled
• Voltage gated or ligand gated.
• Water channels (aquaporins or AQPs)
• AQPs on collecting tubules of the kidneys are
• regulated by ADH (antidiuretic hormone)
• ADH increases the permeability of AQPs.

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• Active transport
• Transport is against concentration gradient
•  
• Energy is required thermodynamically
  unfavorable,
• must couple with an exergonic reaction.
• Purposes
• - import (nutrients) and export (waste products)
• certain materials regardless of their
  intracellular
• or extracellular concentrations
• - maintain concentration gradient for certain
  ions
•   For exp., N/K pump

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• Coupling of transport and energy source
• 1). ATPase ATP hydrolyzed to provide energy
• P-type The enzyme go through reversible
• phosphorylation
• Exp. NA/K pump
• V-type (vesicle) produce proton gradient
• F-type (factor) found in mitochondria,
• chloroplasts and bacteria contain
• F0 (transmembrane) and F1(ATPase)
• Also produce H gradient
• ABC type (ATP-binding cassette)

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• Na-K ATPase Pump
• For animal cells, the distribution of Na and
  K
• is strictly controlled.
• Na concentration is higher outside of the
  cells
• than the inside of the cells. K distribution
  is
• just opposite.
• Na and K ions are constantly pumped against
• their concentration gradients by Na-K
• ATPase Pump.
• When 3 Na are being pumped out, 2 K ions
  are pumped in.

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• The transportation work is coupled with the
• hydrolysis of ATP molecules. When ATP
• molecules are cleaved by ATPase, energy is
• extracted for pumping Na and K ions.
• Other molecules that are actively transported
• across cell membranes include sugar, a.a., H
  
• etc. Nutrient molecule are absorbed into the
  cells
• line intestinal walls through active
  transport.
•  

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Bulk transport- Endocytosis/Exocytosis (Or
vesicular transport), is also an active mechanism
that requires energy, when cells need to import
or export large quantity of materials.
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•  
• Cell Signaling
• The process that cell communicate with each
  other.
•  
• Four major ways of cell signaling
• 1. Paracrine signaling
• Cells within the same organ secrete the
• regulatory molecules that target on the
• neighbor cell.
• Paracrine regulation is considered to be local.

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•  2.    Autocrine signaling
• Cells secrete regulatory molecules that act on
  themselves.
• It usually works as a self amplification
• mechanism.
•  
• Endocrine signaling
• Hormones produced by endocrine glands enter
• blood and reach the target cells.

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In all the above cases, there have to be
receptors on the target cell membrane for the
regulatory molecules to bind to.
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• 4.    Synaptic signaling
• Refers to the communication between neurons or
  between neuron and target cells.
• (Axon dendrite)
• Or communication between a motor neuron
• and the target muscle.
• Neurotransmitters are secreted by one neuron
  and diffuse through the synaptic gap to reach
  another neuron or muscle cell.

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  5.    Gap junction A special case, where
plasma membrane is fused between two cells, ions
and regulatory molecules can directly travel
between the adjoining cells.