an introduction to metabolism
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An Introduction to Metabolism
Ch. 8AP BiologyMs. Haut
Metabolic Pathways Catabolic Pathways
Release energy by breaking down complex molecules into simpler ones
Cellular respiration provides energy for cellular work
C6H12O6 + 6O2 6CO2 + 6H2O + energy Energy released drives anabolic reactions
Metabolic Pathways Anabolic Pathways
Consume energy by building molecules Photosynthesis uses energy 6CO2 + 6H2O energy C6H12O6 + 6O2
Organisms Transform Energy
Solar Energy
(EK)
Plants (glucose)Stored in
chemical bonds(EP)
AnimalsBreak down
Sugars;Some used (EK), some stored in chemical bonds
(EP)
Energy Kinetic energy is energy
associated with motion Heat (thermal energy) is
kinetic energy associated with random movement of atoms or molecules
Potential energy is energy that matter possesses because of its location or structure Chemical energy is potential
energy available for release in a chemical reaction
Energy can be converted from one form to another
On the platform,the diver hasmore potentialenergy.
Diving convertspotentialenergy to kinetic energy.
Climbing up convertskinetic energy ofmuscle movement topotential energy.
In the water, the diver has lesspotential energy.
Laws of Thermodynamics First Law—Energy can be transferred, but never
created or destroyed Second Law—Every energy transfer results in
increased entropy (randomness in the universe) Some of the energy is converted to heat Reactions occur spontaneously
Chemical energy
Heat CO2
First law of thermodynamics Second law of thermodynamics
H2O
Free Energy Organisms live at the expense of free
energy (portion of a system’s energy available for work) acquired from the surroundings
Free energy is needed for spontaneous changes to occur
Gibbs-Helmholtz Equation
Can be used to determine if a reaction is spontaneous
Spontaneous reactions occur in systems moving from instability to stability
G = H - TSFree
energyTotal
energy
enthalpy
Temp(K)
entropy
Highenergy
Low energy
Gibbs-Helmholtz Equation
In chemical reactions, reactions absorb energy to break bonds
Energy is then released when bonds form between rearranged atoms of the product
G = H - T SMeasure of heatin the
reaction
Key Importance of G Indicates amount of energy available for work Indicates whether a reaction will occur
spontaneously (low G) G decreases as reaction approaches equilibrium G increases as reaction moves away equilibrium G = 0 when a reaction is in equilibrium
Chemical ReactionsExergonic Endergonic
Chemical products have lower G than reactants
Products store more G than reactants
Reaction releases energy Reaction requires energy input (absorbs)
G = negative value G = positive value
Spontaneous Non spontaneous
In cellular metabolism, exergonic reactions drive In cellular metabolism, exergonic reactions drive endergonic reactionsendergonic reactions
Rate of Reactions G indicates spontaneity not speed of reaction Spontaneous reactions will occur if it releases free
energy (- G ), but may occur too slowly to be effective in living cells Can leave sucrose in sterile water for yrs. with hydrolysis
occuring; add sucrase and reaction will hydrolyze in seconds
Biochemical reactions require enzymes to speed up and control reaction rates
ATP couples exergonic reactions to endergonic reactions
A cell does three main kinds of work: Mechanical Transport Chemical
To do work, cells manage energy resources by energy coupling, the use of an exergonic process to drive an endergonic one
ATP Powers Cellular WorkUnstable Bonds—can release energy when broken
Energy transferred toanother molecule (phos-phorylated intermediate) with the phosphate
Less stable More stable
LE 8-11
NH2
Glu
P i
P i
P i
P i
Glu NH3
P
P
P
ATPADP
Motor proteinMechanical work: ATP phosphorylates motor proteins
Protein moved
Membraneprotein
Solute
Transport work: ATP phosphorylates transport proteins
Solute transported
Chemical work: ATP phosphorylates key reactants
Reactants: Glutamic acidand ammonia
Product (glutamine)made
+ +
+
The Regeneration of ATP ATP is a renewable resource that is regenerated by addition
of a phosphate group to ADP• The energy to phosphorylate ADP comes from catabolic
reactions in the cell• The chemical potential energy temporarily stored in ATP
drives most cellular work
Pi
ADP
Energy for cellular work(endergonic, energy-consuming processes)
Energy from catabolism(energonic, energy-yielding processes)
ATP
+ Pi
ADP
Energy for cellular work(endergonic, energy-consuming processes)
Energy from catabolism(energonic, energy-yielding processes)
ATP
+
Enzymes Catalyst—chemical agent that speeds up a chemical
reaction without being consumed by the reaction
Hydrolysis of sucrose by the enzyme sucrase is an example of an enzyme-catalyzed reaction
SucroseC12H22O11
GlucoseC6H12O6
FructoseC6H12O6
The Activation Energy Barrier Every chemical reaction
between molecules involves bond breaking and bond forming
The initial energy needed to start a chemical reaction is called the free energy of activation, or activation energy (EA)
Activation energy is often supplied in the form of heat from the surroundings
Transition state
C D
A B
EA
Products
C D
A B
G < O
Progress of the reaction
Reactants
C D
A B
Free
ene
rgy
Enzymes Catalytic proteins that speed up metabolic
reactions by lowering energy barriers
1. Reactants must absorb energy to reach transition state (unstable)
2. Rxn occurs and energy is released as new bonds form to make products
3. G for overall rxn is difference b/w G of products and G of reactants
Substrate Specificity of Enzymes Substrate—reactant that an enzyme acts Substrate binds to the active site on the enzyme Induced fit of a substrate brings chemical groups of
the active site into positions that enhance their ability to catalyze the reaction
Induced Fit Model of Enzymatic Reactions
Enzyme-substratecomplex
Substrates
Enzyme
Products
Substrates enter active site; enzymechanges shape so its active siteembraces the substrates (induced fit).
Substrates held inactive site by weakinteractions, such ashydrogen bonds andionic bonds.
Active site (and R groups ofits amino acids) can lower EAand speed up a reaction by• acting as a template for
substrate orientation,• stressing the substrates
and stabilizing thetransition state,
• providing a favorablemicroenvironment,
• participating directly in thecatalytic reaction.
Substrates areconverted intoproducts.
Products arereleased.
Activesite is
availablefor two new
substratemolecules.
How do Enzymes Work? Active site holds 2 or more reactants in the
proper position to react Induced fit may distort chemical bonds so
less thermal energy is needed to break them
Active site may provide micro-environment that aids a reaction (localized pH)
Side chains of amino acids in active site may participate in reaction
Enzyme Activity
A cell’s physical and chemical environment affects enzyme activity
Each enzyme has optimal environmental conditions that favor the most active enzyme conformation
Effects of Temperature
Optimal temp. allows greatest number of molecular collisions without denaturing the enzyme
Reaction rate when temperature Kinetic energy increases and collisions increases Beyond optimal temperature, reaction rate slows
Too low, collisions b/w substrate and active site don’t occur fast enough
Too high, agitation disrupts weak bonds of the tertiary structure of enzyme (enzyme unfolds)
Effects of pH
Optimal pH range for most enzymes is pH 6 – 8
Beyond optimal pH, reaction rate slows Too low (acidic) H+ ions interact with
amino acid side-chains and disrupt weak bonds of the tertiary structure of enzyme
Too high (basic) OH- ions interact with amino acid side-chains and disrupt weak bonds of the tertiary structure of enzyme
Cofactors Small non-protein molecules that are required
for proper enzyme catalysis Inorganic—Zn, Fe, Cu Coenzymes—vitamins
Effects of Substrate Concentration The higher the
[substrate], the faster the rate (up to a limit)
If [substrate] high enough, enzyme is saturated with substrate Reaction rate depends on
how fast the active site can convert substrate to product
When reaction is saturated with substrate, you can speed up reaction rate by adding more enzyme
Effects of Enzyme Inhibitors
Competitive inhibitors—chemicals that resemble an enzyme’s normal substrate and compete with it for the active site Blocks active site from
substrate If reversible, can be
overcome by increasing substrate concentration
Substrate
Active site
Enzyme
Competitiveinhibitor
Normal binding
Competitive inhibition
Noncompetitive inhibitorNoncompetitive inhibition
A substrate canbind normally to the
active site of anenzyme.
A competitiveinhibitor mimics the
substrate, competingfor the active site.
A noncompetitiveinhibitor binds to the
enzyme away from theactive site, altering the
conformation of theenzyme so that its
active site no longerfunctions.
Substrate
Active site
Enzyme
Competitiveinhibitor
Normal binding
Competitive inhibition
Noncompetitive inhibitorNoncompetitive inhibition
A substrate canbind normally to the
active site of anenzyme.
A competitiveinhibitor mimics the
substrate, competingfor the active site.
A noncompetitiveinhibitor binds to the
enzyme away from theactive site, altering the
conformation of theenzyme so that its
active site no longerfunctions.
Competitive Inhibitor
Effects of Enzyme Inhibitors
Noncompetitive inhibitors—chemicals that bind to another part (allosteric site)of an enzyme Causes enzyme to change
shape and prevents substrate from fitting in active site
Essential mechanism in cell’s regulating metabolic reactions
Substrate
Active site
Enzyme
Competitiveinhibitor
Normal binding
Competitive inhibition
Noncompetitive inhibitorNoncompetitive inhibition
A substrate canbind normally to the
active site of anenzyme.
A competitiveinhibitor mimics the
substrate, competingfor the active site.
A noncompetitiveinhibitor binds to the
enzyme away from theactive site, altering the
conformation of theenzyme so that its
active site no longerfunctions.
Substrate
Active site
Enzyme
Competitiveinhibitor
Normal binding
Competitive inhibition
Noncompetitive inhibitorNoncompetitive inhibition
A substrate canbind normally to the
active site of anenzyme.
A competitiveinhibitor mimics the
substrate, competingfor the active site.
A noncompetitiveinhibitor binds to the
enzyme away from theactive site, altering the
conformation of theenzyme so that its
active site no longerfunctions.
Allosteric site
Negative Feedback
Metabolic Control often Depends on Allosteric Regulation Allosteric enzymes have 2 conformations,
catalytically active and inactive Binding of an activator to the allosteric site
stabilizes active conformation Binding of an inhibitor (noncompetitive) to the
allosteric site stabilizes inactive conformation
Control of Metabolism
In feedback inhibition, the end product of a metabolic pathway shuts down the pathway
Feedback inhibition prevents a cell from wasting chemical resources by synthesizing more product than is needed
Active siteavailable
Initial substrate(threonine)
Threoninein active site
Enzyme 1(threoninedeaminase)
Enzyme 2
Intermediate A
Isoleucineused up bycell
Feedbackinhibition Active site of
enzyme 1 can’tbindtheoninepathway off
Isoleucinebinds toallostericsite
Enzyme 3
Intermediate B
Enzyme 4
Intermediate C
Enzyme 5
Intermediate D
End product(isoleucine)
Mitochondria,sites of cellular respiration
1 µm
Specific Localization of Enzymes Within the Cell
Structures within the cell help bring order to metabolic pathways
Some enzymes act as structural components of membranes
Some enzymes reside in specific organelles, such as enzymes for cellular respiration being located in mitochondria
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