The Working Cell ch 5
HOW CELLS USE ENERGY, ENZYMES and MEMBRANES
Fire flies
Glow: light energy
attract mates
attract prey
Luciferase + ATP + Oxygen converts Luciferin into a chemical that emits energy.
Enzymes control a cells chemical reactions by reducing the energy needed [activation energy EA] for a reaction to occur.
SO FIREFLIES NEED ENERGY AND ENZYMES TO PRODUCE LIGHT.
Energy
capacity to do work
Kinetic (motion)
heat (molecular movement)
light (powers photosynthesis
Potential (position)
chemical energy (arrangement of nuclei and electrons)
ex. sugar
E Transformations
kinetic
potential
------------>
sugar
protein
nucleic acids
lipids
to
--OR-to
potential
kinetic
Laws of Thermodynamics
1) 1st law _________E-transferred or transformed (but not created or destroyed)
2) 2nd law __________E transfer or transformation makes universe more disordered (raises entropy)
closed system--isolated from surrounding
open system--E transferred between system and surroundings
evolution of complex life forms from simple forms does not violate the second law.
E taken from surroundings( raises entropy of universe)
ex.
*maintain highly ordered structure by raising entropy of environment
*take in complex high energy molecules as food--extract energy (create, maintain order)
*return to the environment simpler low energy molecules (CO
+ H O) and heat
2
2
Free Energy (Delta G)
+
--------->
+ E ----------->
+ E -- Delta G
+Delta G
exergonic--products have less free energy than reactants
endergonic--products have more free energy than reactants
ATP and Cellular Work
ATP--immediate source of energy
mechanical work
chromosome movement, mitosis/meiosis
transport work
chemical work
Structure of ATP
adenine
ribose
OOI
I
--P--O~~~P--O~~~P--O-II
II
O
O
OI
II
O
unstable bonds
H O + ATP ---------> ADP + P 2
G
7.3 K/cal / mol
How ATP Works
ATP is hydrolyzed, the phosphate group is transferred to another molecule WHICH BECOMES MORE REACTIVE
A--P---P---P
Glu + NH
--------> Gln
3
Glu + ATP ---------> Glu-+ ADP
Glu-+ NH
---------> Gln + P
A---P---P
Making new ATP
ADP +
+ E ---------> ATP
from cell respiration)
ATP used and regenerated continually by cells
7
10 molecules / sec / cell
-------->
-------->
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ENERGY
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-Delta G
7.3 K/cal / mol
Delta G=
+ 7.3 K/cal / mol
ATP and Cellular Work
ATP--immediate source of energy
mechanical work
chromosome movement, mitosis/meiosis
transport work
chemical work
Structure of ATP
adenine
ribose
OOI
I
--P--O~~~P--O~~~P--O-II
II
O
O
OI
II
O
unstable bonds
H O + ATP ---------> ADP + P 2
G
7.3 K/cal / mol
Enzymes
free E change indicates a reaction that is spontaneous from one that is not
--Delta G
spontaneous reactions may be too s l
enzymes speed up and control rates
o
w
catalyst--accelerates reaction without being permanently changed in the process, and can therefore be used over
and over
Enzymes---lower activation energy
usually proteins
very specific
Free
Energy
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Transition state
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reactants
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products
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progress of reaction------------------>
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Transition state
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products
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reactants
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progress of reaction------------------->
Specificity of Enzymes
substrate-the substance an enzyme acts on
--Delta G
Free
Energy
+Delta G
Enzyme + Substrate --------> Enzyme substrate complex----------> Product + Enzyme
Active Site
place on the enzyme where the substrate fits
*pocket or groove
*changes shape with contact by substrates
*determines enzyme specificity
induced fit-change in shape of active site induced by substrate [old “lock and key hypothesis”]
Catalytic Cycle of Enzymes
Step 1) substrate binds to active site
H-bonds, ionic bonds
Step 2) induced fit of active site around substrate
side chains of a few A.A. catalyze conversion of substrate ----> product
Step 3) product departs active site
Lowering activation E / Speeding up reaction rate
1) active site properly aligns reactants so they may react (two or more reactants)
2) induced fit may distort the substrates’ chemical bonds; therefore, less E needed to form / break bonds
3) active site produces a micro environment conductive to a particular reaction (A.A. side chains)
initial substrate concentration partly determines rate of reaction
higher concentration--faster reaction (up to a limit)
temp may affect the speed of a reaction
if the substrate concentration is high enough enzyme is saturated (all sites filled)
saturated--speed depends upon individual enzyme
unsaturated--slower
Factors affecting enzyme activity
1) enzymes have optimal conditions
*Temperature
greatest number of collisions without denaturing enzyme (35-40 most humans)
37 C
40 C
45 C
*pH
optimal pH for most enzymes is 6-8 (some exist for extremes--pepsin in stomach pH 2)
pH 7
2)
pH 3
Ionic concentration
ions can interfere with ionic bonds within the enzyme (most enzymes can’t tolerate high salt)
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2) Cofactors
non-protein molecules required by many enzymes
complete active site
some inorganic Zn, Fe, Cu
some organic--coenzymes (most vitamins)
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+
----------->
vitamin C
unable to catalyze
able to catalyze
3) Inhibitors
chemicals that selectively inhibit enzyme activity
irreversible--inhibitor attaches with covalent bonds
reversible--inhibitor attaches with weak bonds
competitive inhibitors--chemicals that resemble the enzyme’s normal substrate, therefore compare for the
active site
*block active site
*if reversible, can be overcome by a raise in substrate concentration
antifreeze / ethanol
non-competitive inhibitors--inhibitor does not enter active site, binds to another location
*causes enzyme to change shape---active site altered
<--active site (correct)
<--active site (damaged)
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inhibitor
DDT / many antibiotics
penicillin--cell wall of bacteria
selective activation
substrates (won’t fit)
substrates fit—reaction occurs
selective inhibition and activation essential mechanisms for metabolic control
Allosteric Regulation
allosteric--specific receptor other than active site
*two or more peptide chains
*2 conformations
active / inactive
bind an activator--locks enzyme in the active shape
active sites
activator
<=====>
active
inactive
concentrations of activators and inhibitors can control enzyme activity ex. ATP /ADP
presence of ADP changes enzyme to ATP SYNTHESIS activity
presence of ATP changes enzyme to inactivate further ATP SYNTHESIS
cooperativity--substrate locks enzyme in active shape
inhibitor
Control of Metabolism
metabolism regulated by controlling enzyme activity
feedback inhibition
end product inhibits an enzyme within a pathway
enzyme
1
2
3
4
5
threonine---> A ---> B ---> C ---> D---> isoleucine (end product--allosteric inhibitor of enzyme)
/\ ______________________I
prevents the cell from wasting chemical resources
Multi-enzyme complex
enzymes assembled for steps of a metabolic pathway
within or along cell structures
fixed on a membrane, dissolved within an organelle
high salt
Membrane Structure
artificial membranes
fluid mosaic model
phospholipid bilayer
embedded proteins
some drift, some tethered
unsaturated fatty acids and embedded cholesterol enhance fluidity
proteins determine membrane function
surface carbohydrates
cell--cell recognition
sorting cells into tissues
bonded to protein / lipids
membrane protein functions
transport (ATP)
enzymes
cell-cell adhesion
Traffic of small molecules
selective permeability
non-polar + O
lipid
permeability
2
small polar--H O, CO
2
large polar-glucose
ions
Na+, H+
transport proteins
integral proteins, specific, receptor site
avoids contact with lipid bilayer
tunnels, physical movement
anti port
2 solutes opposite direction
Na+
Ca++
symport--2 solutes same time, same direction
uniport single solute
1. diffusion and passive transport
*concentration gradient
2
*net movement
*diffusion--movement down a concentration gradient
random molecular motion, no ATP energy required
*passive transport
diffusion across a membrane
2. osmosis
*diffusion-H O---across selectively permeable membrane
2
hyperosmotic solution
hypoosomotic
solution
selectively permeable membrane
osmotic pressure--amount of (energy) force needs to prevent H O movement
2
H O-------> until equilibrium reached
water balance
maintaining correct solute / H O concentrations
2
3. facilitated diffusion
proteins help solutes cross a cell membrane
transport proteins
specific
saturation possible
inhibited by similar solutes
do not catalyze reactions
facilitated diffusion
4. active transport
energy requiting
against concentration gradient
maintains strong gradients
Na+, K+, Mg+, Ca+, Clsodium potassium pump
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ion transport
anions and cations unequally distributed across plasma membrane; voltages across the membrane
membrane potential
measured voltage range-50mv to -200mv (inside--compared to outside cell)
this difference represents potential E
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cotransport
ATP powered pump transports one solute
indirectly drives transport of two solutes against concentration gradient
Transport of LARGE molecules
exocytosis
endocytosis