chemical reactions in cells energetics, enzymes and metabolic reactions
TRANSCRIPT
Chemical Reactions in CellsChemical Reactions in Cells
Energetics, Enzymes and Energetics, Enzymes and Metabolic ReactionsMetabolic Reactions
EnergyEnergy• Energy is the capacity for work or Energy is the capacity for work or
change.change.
• Kinetic Energy = energy of movementKinetic Energy = energy of movement
• Potential Energy = stored energy Potential Energy = stored energy
• 11stst Law of Thermodynamics Law of Thermodynamics– Energy can be transferred and Energy can be transferred and
transformed from one form to another but transformed from one form to another but it cannot be created or destroyed.it cannot be created or destroyed.
EnergyEnergy• 22ndnd Law of Thermodynamics Law of Thermodynamics– Energy transfer or transformation Energy transfer or transformation
increases the entropy of the universeincreases the entropy of the universe– Increase in entropy = randomnessIncrease in entropy = randomness– Energy conversions result in a loss of Energy conversions result in a loss of
useful energyuseful energy
usable usable usable
usable
Free Energy = Energy Useful for ChangeFree Energy = Energy Useful for Change
GGreactionreaction = G = Gproductsproducts – G – Greactantsreactants
•Spontaneity of a reaction depends on free Spontaneity of a reaction depends on free
energy changeenergy change GG
•If is positive, free energy is consumed If is positive, free energy is consumed GG
•If is negative, free energy is released If is negative, free energy is released and the reaction is spontaneousand the reaction is spontaneous
GG
Free Energy = Energy Useful for ChangeFree Energy = Energy Useful for Change
•Free Energy change depends on changes inFree Energy change depends on changes in
–total energy (enthalpy)total energy (enthalpy)
–entropy (unusable energy, disorder)entropy (unusable energy, disorder)
HHSS
•In living systems, entropy changes have In living systems, entropy changes have substantial influence substantial influence
–when is positive, and the term is large, when is positive, and the term is large, a negative value predicts a spontaneous a negative value predicts a spontaneous reactionreaction
SSTTGG
SS
GG HH SSTT== ––
Chemical ReactionsChemical Reactions
• Involve the breaking and formation of Involve the breaking and formation of chemical bondschemical bonds
– Reactants are converted to products. Reactants are converted to products.
– Two types of reactions based on Two types of reactions based on energy use:energy use:
• Exergonic– free energy releasedExergonic– free energy released
• Endergonic – free energy consumedEndergonic – free energy consumed
Exergonic Reactions
EnergyEnergycontentcontent
ofofmoleculesmolecules
highhigh
lowlow
Burning glucose (sugar):Burning glucose (sugar):an exergonic reactionan exergonic reaction
Progress of reactionProgress of reaction
Activation energy neededActivation energy neededto ignite glucoseto ignite glucose
Activation energy neededActivation energy neededto ignite glucoseto ignite glucose
Glucose + OGlucose + O22Glucose + OGlucose + O22
Energy released byEnergy released byburning glucoseburning glucose
Energy released byEnergy released byburning glucoseburning glucose
C OC O22 + H + H22OOC OC O22 + H + H22OO
Reactants changed to Reactants changed to transition-state speciestransition-state speciesReactants changed to Reactants changed to transition-state speciestransition-state species
GG
Endergonic Reactions
EnergyEnergycontentcontent
ofofmoleculesmolecules
highhigh
lowlow
Photosynthesis:Photosynthesis:an endergonic reactionan endergonic reaction
Progress of reactionProgress of reaction
GlucoseGlucoseGlucoseGlucose
Net energyNet energycaptured bycaptured bysynthesizingsynthesizing
glucoseglucose
COCO22 + H + H22OOCOCO22 + H + H22OO
ActivationActivationenergy fromenergy from
light capturedlight capturedby photosynthesisby photosynthesis
GG
Short-Term Energy StorageShort-Term Energy Storage
• Chemical Energy is stored in the bonds Chemical Energy is stored in the bonds of ATPof ATP– ATP = adenosine triphosphateATP = adenosine triphosphate– ADP = adenosine diphosphateADP = adenosine diphosphate– to store energyto store energy
• ADP + Phosphate + EnergyADP + Phosphate + Energy ATP ATP
– to release energyto release energy• ATP ATP ADP + Phosphate + Energy ADP + Phosphate + Energy
Coupled ReactionsCoupled Reactions Pairing of an Exergonic reaction, often Pairing of an Exergonic reaction, often
involving ATP, with an Endergonic reaction involving ATP, with an Endergonic reaction
Note that overall free energy change is negativeNote that overall free energy change is negative
Metabolic ReactionsMetabolic Reactions
• AnabolicAnabolic– link simple molecules to produce complex link simple molecules to produce complex
molecules molecules (eg. dehydration synthesis of starch) (eg. dehydration synthesis of starch)
– require energyrequire energy
• CatabolicCatabolic– break down complex molecules to release break down complex molecules to release
simple ones simple ones (eg. hydrolysis of starch(eg. hydrolysis of starch sugars) sugars)
– release energy stored in chemical bondsrelease energy stored in chemical bonds
Metabolic PathwaysMetabolic Pathways
AA BB CC DDDD EEEE
FFFF GGGG
Enzyme 1Enzyme 1 Enzyme 2Enzyme 2 Enzyme 3Enzyme 3 Enzyme 4Enzyme 4
Enzyme 5Enzyme 5 Enzyme 6Enzyme 6
InitialInitialReactantsReactants
IntermediatesIntermediates FinalFinalProductsProducts
Pathway 1Pathway 1Pathway 1Pathway 1
Pathway 2Pathway 2Pathway 2Pathway 2
Enzymes Assist in Biological ReactionsEnzymes Assist in Biological Reactions
Enzymes are biological catalysts.Enzymes are biological catalysts.
biological: composed of protein or, rarely, biological: composed of protein or, rarely, RNARNA
catalyst: speeds up a reaction without catalyst: speeds up a reaction without being changed by the reactionbeing changed by the reaction
Properties of EnzymesProperties of Enzymes
• Enzymes speed up biological reactions Enzymes speed up biological reactions by lowering the activation energy for the by lowering the activation energy for the reaction. reaction. – Enzymes provide a surface where the Enzymes provide a surface where the
catalysis takes placecatalysis takes place– The reaction reaches equilibrium more The reaction reaches equilibrium more
rapidly rapidly
–– The value of and the ratio of The value of and the ratio of reactants and products at equilibrium is reactants and products at equilibrium is the same as for an uncatalyzed reaction the same as for an uncatalyzed reaction
GG
EnergyEnergycontentcontent
ofofmoleculesmolecules
highhigh
lowlowProgress of reactionProgress of reaction
ActivationActivationenergy withoutenergy without
catalystcatalyst
ActivationActivationenergy withenergy with
catalystcatalyst
Activation Energy: Controls Rate of ReactionActivation Energy: Controls Rate of Reaction
Amount of energy required for
reaction to occurtransition state
GG
Properties of EnzymesProperties of Enzymes
• Enzymes are SPECIFIC for the Enzymes are SPECIFIC for the reactants (substrates) in the reactions reactants (substrates) in the reactions that they catalyze. that they catalyze.
• Only substrates that fit the active site of Only substrates that fit the active site of the enzyme can bind and complete the the enzyme can bind and complete the reactionreaction– active site: region on enzyme where active site: region on enzyme where
substrates bindsubstrates bind
Enzyme-Substrate InteractionsEnzyme-Substrate Interactions
SubstrateSubstrateSubstrateSubstrateSubstrateSubstrateSubstrateSubstrate
EnzymeEnzymeEnzymeEnzyme
ActiveActiveSiteSite
ActiveActiveSiteSite
11 Substrates Substrates enter active enter active sitesite
11 Substrates Substrates enter active enter active sitesite
induced fit
22 Shape change Shape change promotes reactionpromotes reaction
22 Shape change Shape change promotes reactionpromotes reaction
33 Product released;Product released;enzyme ready againenzyme ready again
33 Product released;Product released;enzyme ready againenzyme ready again
Chemical Events at Active SitesChemical Events at Active Sites
• Enzymes hold substrates in the proper Enzymes hold substrates in the proper orientation for the reaction to take placeorientation for the reaction to take place
Chemical Events at Active SitesChemical Events at Active Sites
• Enzymes induce strain in the substrate to Enzymes induce strain in the substrate to produce a transition state favorable to produce a transition state favorable to reactionreaction
• Active site provides a microenvironment Active site provides a microenvironment that favors the chemical reactionthat favors the chemical reaction
Chemical Events at Active SitesChemical Events at Active Sites
• Active site directly participates in the Active site directly participates in the reaction reaction – covalent bonding can occur between covalent bonding can occur between
enzyme and substrateenzyme and substrate– R groups of the enzyme’s amino acids can R groups of the enzyme’s amino acids can
temporarily add chemical groups to the temporarily add chemical groups to the substratessubstrates
Molecules that Assist EnzymesMolecules that Assist Enzymes
• Cofactors: Cofactors: inorganic ions that bind to inorganic ions that bind to enzymes, eg. zincenzymes, eg. zinc
• Coenzymes: Coenzymes: small organic factors that small organic factors that temporarily bind to enzymes, eg. biotin, NAD, temporarily bind to enzymes, eg. biotin, NAD, ATPATP
• Prosthetic groups: Prosthetic groups: non-protein factors that non-protein factors that are permanently bound an enzyme, eg. hemeare permanently bound an enzyme, eg. heme
Factors Influencing Reaction RateFactors Influencing Reaction Rate
• Substrate ConcentrationSubstrate Concentration
Rate is proportional to substrate
concentration
Rate is more rapid
Rate no longer increases since the active sites of all enzymes are saturated with substrate
Factors Influencing Reaction RateFactors Influencing Reaction Rate
• Competitive Inhibitors: Bind at the active Competitive Inhibitors: Bind at the active site, compete for binding with substratesite, compete for binding with substrate– Irreversible: form covalent bond with amino Irreversible: form covalent bond with amino
acids in the active siteacids in the active site
DIPF
Factors Influencing Reaction RateFactors Influencing Reaction Rate
• Competitive Inhibitors: Bind at the active Competitive Inhibitors: Bind at the active site, compete for binding with substratesite, compete for binding with substrate– Reversible: molecule similar to substrate Reversible: molecule similar to substrate
occupies active site but does not undergo occupies active site but does not undergo reactionreaction
Factors Influencing Reaction RateFactors Influencing Reaction Rate
• Non-Competitive Inhibitors: Bind to a Non-Competitive Inhibitors: Bind to a different site, cause a conformational different site, cause a conformational change in the enzyme that alters the change in the enzyme that alters the active site active site – Reversible Reversible
Factors Influencing Reaction RateFactors Influencing Reaction Rate
• Allosteric Regulation Allosteric Regulation – Conversion between Conversion between
active and inactive forms active and inactive forms of an enzyme due to of an enzyme due to binding of regulatory binding of regulatory molecules at an allosteric molecules at an allosteric sitesite• Activators stabilize the Activators stabilize the
active formactive form
• Allosteric inhibitors Allosteric inhibitors stabilize the inactive stabilize the inactive formform
Factors Influencing Reaction RateFactors Influencing Reaction Rate
• Allosteric Regulation Allosteric Regulation – Cooperativity: a substrate causing Cooperativity: a substrate causing
induced fit in one enzyme subunit can induced fit in one enzyme subunit can cause a change to the active form in all cause a change to the active form in all the other subunitsthe other subunits
Enz. 5Enz. 5Enz. 5Enz. 5DDDD
Enz. 4Enz. 4Enz. 4Enz. 4CCCC
Enz. 3Enz. 3Enz. 3Enz. 3BBBB
Enzyme Regulation: Feedback InhibitionEnzyme Regulation: Feedback Inhibition
Enz. 2Enz. 2Enz. 2Enz. 2AAAA
Feedback InhibitionFeedback InhibitionIsoleucine allosterically Isoleucine allosterically
inhibits enzyme 1inhibits enzyme 1
Enz. 1Enz. 1Enz. 1Enz. 1
CHCH33CHCH33
CCCC
CCCC
COOHCOOHCOOHCOOH
OHOHOHOH
NHNH33NHNH33HHHH
HHHHCHCH22
CHCH22
CCCC
CCCC
COOHCOOHCOOHCOOH
CHCH33CHCH33
NHNH33NHNH33HHHH
HHHH
CHCH33CHCH33
ThreonineThreonine(substrate)(substrate)ThreonineThreonine(substrate)(substrate) IsoleucineIsoleucine
(end product)(end product)IsoleucineIsoleucine
(end product)(end product)
Feedback Inhibition: The product of a pathway inhibits an initial step in the pathway to decrease its own production
Commitment step
Properties of EnzymesProperties of Enzymes
• Three dimensional structure of an Three dimensional structure of an enzyme preserves its ACTIVE SITEenzyme preserves its ACTIVE SITE
• Conditions that can affect three Conditions that can affect three dimensional structure include: heat, pH dimensional structure include: heat, pH (acid/base balance) and other chemicals (acid/base balance) and other chemicals (salt, charged ions) (salt, charged ions)