BIOCHEMICAL REGULATION

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BIOCHEMISTRY
PART 2 BIOCHEMICAL REGULATION DR SAMEER FATANI
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Plasma membrane structure and metabolite
transport Structure and membrane
transport Introduction Biological membranes
from eukaryotic and prokaryotic cells have many
properties in common such as same chemical
components and similar structural organization.
The major differences could be in specific
lipids, protein and carbohydrate
components. The mammalian membranes have
trilaminar appearance with overall width of 7-10
nm. Intracellular membranes are usually thinner
than plasma membrane.
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Functions of the membranes Membranes are (very
dynamic structures) usually undergo specific
movement to give the cells and subcellular
structures the ability to adjust their shape and
change the position. However, the chemical
components, proteins and lipids are involved in
this dynamic role. The cellular membranes can
control the composition of the cells by
-preventing a variety of molecules to enter the
cells - allowing the movement of specific
molecules from one side to another by selective
transport systems.
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Chemical composition of the membranes Lipids and
proteins are the two major components of all
membranes. The amount of each varies greatly in
different membranes (12.2). Protein ranges from
about 20 (as in myelin sheath ) to over 70 (as
in inner membrane of mitochondria). Intracellular
membranes have a high percentage of protein due
to the large number of enzymatic activities
available. Membranes contain small amount
various polysaccharide in the form of
glycoprotein and glycolipid, but contain no free
carbohydrate.
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MEMBRANE LIPIDS There are three major types of
lipids in biological membranes 1-
Glycerophospholipids are the most abundant
lipids of membranes. Two types of basic
structure Basic structure number 1 Phosphatidic
acid where different hydroxyl containing
compounds (such as choline, ethanolamine, or
serine) are esterified to its phosphate group to
form glycerophospholipids.
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Basic structure for glycerophospholipids
Any hydroxyl containing compound e.g. Choline,
Ethanolamine, Serine, Inositol
Esterification
Phosphate group
Glycerophospholipid
Esterification
Phosphatidic acid
Glycerol molecule
Esterification at C1 C2 of gly.
Two long chain fatty acids Sat. FA at C1 Unsat.
FA at C2
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• the most common glycerophospholipids
• Ethanolamine glycerophospholpid or cephalin
• Choline glycerophospholipid or Lecithin
• Diphosphatidylglycerol or Cardiolipin
  inner mitochondrial
• 
  membrane

in most membranes
Another basic structure for glycerophospholipids P
lasmalogen (Glycerol Ether phospholipids)
contain an alkyl group or unsaturated ether
instead of fatty acid in C1 Of glycerol molecule

• Glycerophospholipids are amphipathic, contain
  both
• Polar end or head, due to the charged phosphate
• and subsitutions on phosphate
• Non polar tail, due to hydrophobic hydrocarbon
• chains of fatty acyl groups.

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2- Sphingolipids amino alcohols. The basic
structure sphingosine and dihydrosphingosine e.g.
Ceramide has Unsat. F.A. in amide linkage on the
amino group e.g. sphingomyelin has
phosphorylcholine esterified to the 1-OH,
(abundant in mammalian tissues) -
Glycossphingolipids no phosphate group, contain
a sugar molecule Attached by glycosidic linkage
to the C1 of sphingosine. e. g. Ganglioside most
complex glycosphingolipids, contain
Oligosaccharide with one or more residues of
sialic acid

• 3- Cholesterol is an important component of
  plasma
• Membranes.
• - Composed of four fused rings and branched
  hydrocarbon chain (C8)
• attached to D rings. (fig. 12.16D5).
• - Compact, rigid , and hydrophobic molecule.
• It has a polar hydroxy group (HO) at C3
• cellular function cholesterol can alter the
  fluidity of membranes
• and participates in controlling the
  microstructure of plasma
• membranes.

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• the amount of lipids in cell membranes
• Plasma membrane has the greatest variation in
  percentage
• of lipid composition, because the quantity of
  cholesterol
• can be affected by the nutritional state of
  animal.
• The same intra cellular membranes of certain
  tissue in
• different species have very similar classes of
  lipids
• e.g. Myelin membranes rich in sphingolipids
  with high
• glycosphingolipids, while intracellular
  membranes
• primarily contain glycerophospholipids
  and little
• sphingolipids
• The constancy of composition of different
  membranes
• indicates the relationship between lipids and
  the specific
• functions of individual membranes.

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• MEMBRANE BROTEINS
• Peripheral proteins located on the surface
• of membrane . E.g. electrostatin (electrostatic
• binding)
• Integral proteins (IP)
• Two types
• 1- proteolipids hydrophobic lipoproteins.
• - soluble in chloroform and methanol but
• not water).
• - present in many membranes, particularly
• myelin (50 or protein component)
• - e.g. lipophilin (in brain myelin).
• 2- Glycoproteins contain carbohydrates,
• present mainly in plasma membranes.
• e.g. glycophoryn.
• IP contain bound lipids (if removed lead to

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• functions of membrane proteins
• Have a role in transmembrane movement of
  molecules
• Act as receptors for binding hormones and growth
  factors
• In many cells (e.g. neurons) membrane proteins
  have structural
• role to maintain the shape of the cell.

• MEMBRANE CARBOHYDRATES
• In membranes, carbohydrates are present as
  oligosaccharides attached covalently
• to proteins to form glycoproteins
• or to lipids to form glycolipids
• Their sugar include glucose, galactose, mannose,
  N-acetylgalactosamine, N-acetylglucsamine, and
  sailic acid.
• Carbohydrate is located on the exterior side of
  plasma membrane or luminal side of the
  endoplasmic reticulum.
• Functions of plasma carbohydrates
• cell-cell recognition
• Adhesion
• - And receptor action

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• STRUCTURE OF BIOLOGICAL MEMBRANES
• Fluid Mosaic Model of Biological Membranes
• Introduction. All biological membranes have a
  bilayer arrangement of lipids. Amphipathic lipids
  and cholesterol are oriented so that their
  hydrophobic portions interact to minimize their
  contact with water or polar groups. (fig.
  12.22--- D5).
• Fluid mosaic model for membranes in which
• Some proteins (intrinsic) are actually immersed
  in the lipid bilayer. While others (extrinsic)
  are loosely attached to the surface.
• -therefore, it was suggested that some proteins
  span the lipid bilayer and are in contact with
  the aqueous environment on both sides. Fig.
  12.22--- D5).
• The characteristics of lipid bilayer has a
  relationship with the properties of the cellular
  membranes, including fluidity, flexibility that
  permits changes of shape and form, ability to
  self heal and impermeability.

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• Asymmetric distribution of lipid
• Lipid components are distributed in asymmetric
  manner across the biological membranes.
• Each layer of the bilayer has different
  composition with respect to individual
  glycerophospholipids and sphingolipids.
• E.g. the asymmetric distribution of lipids in
  human erythrocyte membrane
• Sphingomyelin is predomenantly in outer layer,
  whereas phosphatidyl ethanolamine is
  predomenantly in the inner layer, in contrast
  cholesterol is equally distributed on both
  layers.
• Asymmetry of lipids is maintained by specific
  membrane proteins, termed lipid transporters
• Flipase catalyzes the inward transport
• Flopase catalyzes the outward transport
• Scramblase mixes phospholipids between the two
  layers.

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Movement of molecules through membranes The
lipid nature of biological membranes strongly
restricts the type of molecules that diffuse
from one side to another. Inorganic ions and
charged organic molecules do not diffuse at a
significant rate because of their attraction to
water molecules And also due to the hydrophobic
environment of Lipid membranes. The size of
large molecules such as proteins and nucleic
acids preclude significant diffusion. To
overcome this and to transport such molecules
across membranes , a variety of specialized
channels and transporters are involved. Mediated
transport system (facilitated) Membrane
channels membranes of most cells contain
specific channels (pores), these channels permit
rapid movement of Specific molecules or ions
from one side of a membrane to the other. The
substances can diffuse in both directions of the
membrane Via an aqueous holes. Channel protein do
not bind the molecules or ions to be
transported. The channels have some degree of
specificity, based on size and charge of
substance.
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Transporters the transporters translocate the
molecule or ion across the membrane by binding to
physically moving the substance (no chemical
reactions occurs). Types of transporters 1-
passive transport (facilitated diffusion) the
transporters move the substrate only down their
concentration gradient. 2- Active transport
transporters move the substrate against its
concentration gradient, and require some form of
energy. With both channels and transporters the
molecule is unchanged after translocation across
the membrane. A major difference between
channels and transporters is the rate of
substrate translocation. Group translocation
involves not only movement of a substance across
a membrane but also a chemical modification of
the substrate during the process. e.g. Uptake of
sugars by bacteria involves transport and then
phosphorylation of the sugar before release into
the cytosol.
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• Characteristics of membrane transport systems
• The proteins or protein complexes involved in
  transport systems have a number of
  characteristics (fig. 12.5--- D5)
• They facilitate the movement of a molecule or
  molecules through the lipid bilayer at a rate
  that is significantly faster than can be
  accounted by simple diffusion.
• As the concentration of the substance to be
  translocated increases, the initial rate of
  transport increases but reaches maximum when the
  substance saturates the protein transporter.
  Simple diffusion does not demonstrate saturation
  kinetics.
• most transporters have structural and stereo
  specificity for the substance transported.
• Structural analogs of the substrate inhibit
  competitively and reagents that react with
  specific groups on proteins are noncompetitive
  inhibitors.

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Four common steps in the transport of solute
molecules (fig. 12.32---- D5) 1- recognition by
the transporter f appropriate solute from a
variety of solutes in the aqueous
environment. 2- translocation of solute across
the membrane. 3- release of solute by the
transporter, and 4- recovery of transporter to
its original condition to accept Another solute
molecule. The above four steps for the
movement of single solute Molecule by a
transporter. There are systems that move
two Molecules simultaneously in one direction,
called (symport mechanisms). Two molecules in
opposite directions, (antiport mechanism).
Single molecule in one direction (uniport
mechanism).
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Energetics of membrane transport systems the
change in free energy when an unchanged molecules
Moves from concentration C1 to concentration C2
on the other side of a membrane is given by the
following Eq. ?G 2.3RT log (C2/C1) A
facilitated transport system is one in which ?G
is negative and the movement of solute occurs
spontaneously, without the need for a driving
force. When ?G is positive, as would be the
case if C2 is larger than C1, coupled input of
energy from some source is required for movement
of the solute and the process is called active
transport. Active transport is driven by either
hydrolysis of ATP to ADP or utilization of an
electrochemical gradient of Na or H across the
membrane.
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• CHANNELS AND PORES
• Channels and pores in membranes function
  differentialy
• Channels and pores are intrinsic membrane
  proteins and are
• differentiated on the basis of their degree of
  specificity for
• Molecules crossing the membrane.
• - Channels are selective for specific inorganic
  cations and anions,
• Whereas pores are not selective, permitting
  organic and
• inorganic molecules to pass through the
  membrane.
• e.g. Na channel permits movement of Na at rate
  ten times
• greater than K.
• The mechanisms of opening and closing the
  channels
• Opening and closing of membrane channels involves
  
• a conformational change in the channel protein,
  in turn
• this conformational change is controlled by the
  transmembrane
• Potential (these channels called voltage-gated
  channels).
• e.g. in Na channel, depolarization of the
  membrane lead to

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Sodium channel Voltage-sensetive Na channels
mediate rapid increase In intracellular Na
following depolarization of the plasma Membrane
in nerve and muscle cells. There are four repeat
homology units in the channel. Each With six
transmembrane a helices. One membrane segment
has a positively charged amino acid at every
third position and may serve as a voltage sensor.
A mechanical shift of this region due to a
change in the membrane potential may lead to a
conformational change in the protein, resulting
In the opening of the channel. Nicotinic-Acetyl
choline channel (nAChR) The nicotinic-acetylcholin
e channel (acetylcholine receptor), is An example
of a chemically regulated channel, in which
binding Of acetylcholine opens the channel and
allowing selective Cations to move across the
membrane. the nicotinic-acetylcholine receptor
is inhibited by several deadly Neurotoxins
including d-tubocurarine.
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PASSIVE MEDIATED TRANSPORT SYSTEMS Passive
mediated transport (facilitated diffusion) is a
mechanism for translocation of solutes through
cell membranes (from higher to lower
concentration) without expenditure of metabolic
energy. Glucose transport is facilitated Eight
members of superfamily of membrane proteins that
mediate D-glucose transport have been reported in
mammalian cells, and are expressed in a
tissue-specific manner. The glucose transporters
are designated as GLUT1, GLUT2, and so on. All
have 12 hydrophobic segments considered to be the
transmembrane regions. Most are in the plasma
membrane where direction of movement of glucose
is usually out to in. GLUT2, however, may be
responsible for glucose export from liver
cells. GLUT5 of sarcolemmal membranes of skeletal
muscle transport fructose preferentially. GLUT4
is an insulin-responsive transporter. Several
sugar analogs as well as phoretin are competitive
inhibitors.
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Cl- and HCO transport system An anion
transporter in erythrocytes and kidneys involves
the antiport movement of Cl- and HCO3-. This
transporter called the Na-independent Cl- -
HCO- exchanger. The direction of the flow is
reversible and depends on the concentration
gradients of the ions across the membrane. The
transporter is important in adjusting the
erythrocyte HCO3- concentration in arterial and
venous blood. Mitochondria contain a number of
transport systems The inner mitochondrial
membrane contains antiport systems for exchange
of anions between cytosol and mitochondrial
matrix, including 1- a transporter for exchange
of ADP and ATP, 2- a symport transporter for
phosphate and H, 3- a dicarboxylate carrier that
catalyzes an exchange of malate for phosphate,
and 4- a translocator for exchange of aspartate
and glutamate. These transporters mediate
passive exchange of metabolites down their
concentration gradient.
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• ACTIVE MEDIATED TRANSPORT SYSTEMS
• Characteristics
• - they require utilization of energy to
  translocate solutes.
• - Saturation kinetics, specificity, and
  inhibatory.
• Classification
• 1- Primary active transporters if they require
  ATP as the energy source.
• Types of primary active transport
• - P type transporters ( if he protein
  phosphorylated and dephosphorylated during the
  transport activity).
• V vacuole type responsible for acidification
  (proton pumps) of the interior of lysosomes,
  endosomes..
• - F type transporters are involved in ATP
  synthesis.
• 2- Secondary active transporters if a
  transmembrane chemical gradient of Na or H is
  required for translocation of sugars and amino
  acids.
• Translocation of Na and K is by primary active
  transport
• All mammalian cells contain a Na - K
  transporter, of type P, which utilizes ATP to
  drive the translocation.

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Na/K - exchanging ATPase In all plasma
membranes the enzyme ATPase will be activated
and catalyzes the reaction
ATP
Na K ADP Pi

Mg2 -The function of this enzyme and reaction
is to translocate Na and K across the
biological membrane (3 Na ions moving out and 2
K ions into the cell). The enzyme has high
activities in excitable tissues such as muscle
and nerves. -Antiport process
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Sodium/ glucose cotransporter (Secondary active
transport systems) Transport of D-glucose is
driven by movement of Na. The Na
electrochemical gradient across the plasma
membrane Is an energy source for active symport
(moving two molecules in One direction) movement
with Na of sugars, amino acids, and Ca. In the
process, two Na are moving by passive
facilitated transport Down the electrochemical
gradient and glucose in the same time will Be
carried along against its concentration gradient.
Na gradient is maintained by the Na/K
-exchange ATPase Amino acids are translocated
by luminal epithelial cells of intestines by
Na/amino acid cotransporters by a symport
mechanism. Symport movement of molecule
utilizing the Na gradient involves Cooperative
interaction of Na ions and another molecule
translocated On the protein. A conformational
change of the protein occurs following
Association of both ligands. Which moves them
the necessary distance In the transporter to
bring them into contact with cytosolic
environment . Dissociation of Na ion from the
transporter because of the low intracellular Na
concentration leads to a return of the protein to
its original conformation And a decrease in
affinity and release of the other ligand (fig.
12.51----D5)
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IONOPHORES Its a class of antibiotics of
bacterial origin that facilitate the Movement of
monovalent and divalent inorganic ions across The
biological and synthetic membranes. Two major
groups - Mobile carriers ionophores can diffuse
in the biological membrane and carry an ion
across a membrane. - Channel formers ionophores
can create a channel that transverses The
membrane and through which ions can diffuse
e.g. Valinomycin transport K by an
electrogenic uniport mechanism that creates an
electrochemical gradient across a membrane as it
carries a positive charged K.
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