enzymes
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Enzymes. Forms of Energy. Energy is the capacity to cause change Energy exists in various forms, some of which can perform work Kinetic energy is energy associated with motion Heat (thermal energy) is kinetic energy associated with random movement of atoms or molecules - PowerPoint PPT PresentationTRANSCRIPT
Enzymes
Forms of EnergyEnergy is the capacity to cause changeEnergy exists in various forms, some of which can
perform workKinetic 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 structureChemical energy is potential energy available for
release in a chemical reactionEnergy can be converted from one form to another
The free-energy change of a reaction tells us whether the reaction occurs spontaneouslyBiologists want to know which reactions occur
spontaneously and which require input of energyTo do so, they need to determine energy changes
that occur in chemical reactionsA living system’s free energy is energy that can
do work when temperature and pressure are uniform, as in a living cell
Free EnergyThe change in free energy (∆G) during a process
is related to the change in enthalpy, or change in total energy (∆H), and change in entropy (T∆S):
∆G = ∆H - T∆S• Only processes with a negative ∆G are
spontaneous• Spontaneous processes can be harnessed to
perform work
Free Energy, Stability, and EquilibriumFree energy is a measure of a system’s instability,
its tendency to change to a more stable stateDuring a spontaneous change, free energy
decreases and the stability of a system increasesEquilibrium is a state of maximum stabilityA process is spontaneous and can perform work
only when it is moving toward equilibriumThe concept of free energy can be applied to the
chemistry of life’s processes
Exergonic and Endergonic Reactions in MetabolismAn exergonic reaction proceeds with a net
release of free energy and is spontaneousAn endergonic reaction absorbs free energy from
its surroundings and is nonspontaneous
LE 8-6a
Reactants
EnergyProducts
Progress of the reaction
Amount ofenergyreleased(G < 0)
Free
ene
rgy
Exergonic reaction: energy released
LE 8-6b
ReactantsEnergy
Products
Progress of the reaction
Amount ofenergyrequired(G > 0)
Free
ene
rgy
Endergonic reaction: energy required
Catalysts• So what’s a cell got to do to reduce
activation energy?– get help! … chemical help… ENZYMES
G
Call in the ENZYMES!
Enzymes vocabularysubstrate
• reactant which binds to enzyme• enzyme-substrate complex: temporary association
product • end result of reaction
active site • enzyme’s catalytic site; substrate fits into active site
substrate
enzyme
productsactive site
Enzymes speed up metabolic reactions by lowering energy barriersA catalyst is a chemical agent that speeds up a
reaction without being consumed by the reactionAn enzyme is a catalytic proteinHydrolysis of sucrose by the enzyme sucrase is
an example of an enzyme-catalyzed reaction
LE 8-13
SucroseC12H22O11
GlucoseC6H12O6
FructoseC6H12O6
The Activation Energy BarrierEvery chemical reaction between molecules
involves bond breaking and bond formingThe 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
LE 8-14
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
How Enzymes Lower the EA BarrierEnzymes catalyze reactions by lowering the EA
barrierEnzymes do not affect the change in free-energy
(∆G); instead, they hasten reactions that would occur eventually
LE 8-15
Course ofreactionwithoutenzyme
EA
without enzyme
G is unaffectedby enzyme
Progress of the reaction
Free
ene
rgy
EA withenzymeis lower
Course ofreactionwith enzyme
Reactants
Products
Substrate Specificity of EnzymesThe reactant that an enzyme acts on is called the
enzyme’s substrate The enzyme binds to its substrate, forming an
enzyme-substrate complexThe active site is the region on the enzyme where
the substrate bindsInduced fit of a substrate brings chemical groups
of the active site into positions that enhance their ability to catalyze the reaction
Catalysis in the Enzyme’s Active SiteIn an enzymatic reaction, the substrate
binds to the active siteThe active site can lower an EA barrier by
Orienting substrates correctlyStraining substrate bondsProviding a favorable microenvironmentCovalently bonding to the substrate
LE 8-17
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 EA
and speed up a reaction by• acting as a template for substrate orientation,• stressing the substrates and stabilizing the transition state,• providing a favorable microenvironment,• participating directly in the catalytic reaction.
Substrates areconverted intoproducts.
Products arereleased.
Activesite is
availablefor two new
substratemolecules.
Effects of Local Conditions on Enzyme ActivityAn enzyme’s activity can be affected by:
General environmental factors, such as temperature and pH
Chemicals that specifically influence the enzyme
Each enzyme has an optimal temperature in which it can function
Each enzyme has an optimal pH in which it can function
CofactorsCofactors are nonprotein enzyme helpersCoenzymes are organic cofactors
Enzyme InhibitorsCompetitive inhibitors bind to the active site of
an enzyme, competing with the substrateNoncompetitive inhibitors bind to another part of
an enzyme, causing the enzyme to change shape and making the active site less effective
LE 8-19Substrate
Active site
Enzyme
Competitiveinhibitor
Normal binding
Competitive inhibition
Noncompetitive inhibitor
Noncompetitive 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.
Regulation of enzyme activity helps control metabolismChemical chaos would result if a cell’s metabolic
pathways were not tightly regulatedTo regulate metabolic pathways, the cell switches
on or off the genes that encode specific enzymes
Allosteric Regulation of EnzymesAllosteric regulation is the term used to describe
cases where a protein’s function at one site is affected by binding of a regulatory molecule at another site
Allosteric regulation may either inhibit or stimulate an enzyme’s activity
Most allosterically regulated enzymes are made from polypeptide subunits
Each enzyme has active and inactive formsThe binding of an activator stabilizes the active form
of the enzymeThe binding of an inhibitor stabilizes the inactive form
of the enzyme
LE 8-20a
Allosteric enzymewith four subunits
Regulatorysite (oneof four) Active form
ActivatorStabilized active form
Active site(one of four)
Allosteric activatorstabilizes active form.
Non-functionalactive site
Inactive form Inhibitor Stabilized inactive form
Allosteric inhibitorstabilizes inactive form.
Oscillation
Allosteric activators and inhibitors
Cooperativity is a form of allosteric regulation that can amplify enzyme activity
In cooperativity, binding by a substrate to one active site stabilizes favorable conformational changes at all other subunits
LE 8-20b
Substrate
Binding of one substrate molecule toactive site of one subunit locks allsubunits in active conformation.
Cooperativity another type of allosteric activation
Stabilized active formInactive form
Feedback InhibitionIn feedback inhibition, the end product of a
metabolic pathway shuts down the pathwayFeedback inhibition prevents a cell from
wasting chemical resources by synthesizing more product than is needed
LE 8-21
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)
Specific Localization of Enzymes Within the CellStructures within the cell help bring order to
metabolic pathwaysSome enzymes act as structural components of
membranesSome enzymes reside in specific organelles, such
as enzymes for cellular respiration being located in mitochondria