energy and enzymes tie a favorable rxn with
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Energy and Enzymes Today�s Topics • Coupled Reactions
– ATP
• How do Enzymes Work? – Activation energy – Active site
• Enzyme Regulation – Competitive & Allosteric
• Enzymes are Linked in Metabolic Pathways
• Oxidation and Reduction September 27, 2021 2
Tie a favorable rxn with An otherwise unfavorable rxn COUPLED Reactions
ATP
Adenine
3 phosphate groups Ribose
4
ΔG = +5.5 kcal/mole
1.
ΔG = -7.3 kcal/mole
2. ATP+ H2O ADP + Pi
Favorable or unfavorable ?
Favorable or unfavorable ?
Coupled Reactions
Each step is favorable
Figure 3.13 Enzymes Lower the Energy Barrier Ea
Products
Time course of reaction
Free
ene
rgy
(G)
ΔG
Ea
Reactants
Catalyzed reaction
Uncatalyzed reaction
2
Substrates enter active site.
Substrates are held in active site by weak interactions.
Substrates Enzyme-substrate complex
Active site is available for new substrates.
Enzyme
Products are released.
Products
The active site lowers EA.
Substrates are converted to products.
1 2
3
4 5
6 OH
HO +
HO
Polar Nonpolar
Charged The active site has a specific 3-D Shape
With a specific arrangement of functional groups
Figure 8.16 (b)
Enzyme- substrate complex
Enzymes cause Bond Strain
• Physical
• Chemical
Figure 8.13
A B
C D Transition state
A
C
B
D
A
C
B ∆G < 0
D
Products
Free
ene
rgy EA
Reactants
Progress of the reaction
Asp 52
Glu35
Chemical bond strain: Two charged amino acids pull on the bonds
Lysozyme example
Physical bond strain: Binding to the enzyme flattens the zig-zag shape, which stretches the bonds
Figure 3.14 Enzyme Action Active site
Sucrose
Fructose
Glucose
Water
Enzyme (sucrase)
O
OO
O
OO
O
O
OH
HO
H2O
O O4. Enzymes contribute to reactions but are not consumed in them
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Principal Ways of Regulating
Enzymes
• Competitive Inhibition
• Non-competitive (Allosteric) Inhibition
• Covalent Modification (phosphorylation)
-
HO
OH
HO
OH
+
HO
OH
HO
OH
I
S1 S2
S & I bind to same site
Competitive Inhibitors: • bind to active site �unproductively� and block true substrates� access
I
S1 S2
Depends on relative concentration
Allosteric Inhibitors distort the enzyme so the substrates no longer fit
�other����site�
(Can also have Positive Regulators that help the enzyme work
better)
Covalent regulators: Phosphorylation
Enzymes are specific, so each chemical reaction in a cell usually has its own enzyme
Enzyme 1
Reaction 1 Starting molecule
Enzyme 2 B
Reaction 2 C
Enzyme 3
Reaction 3 D
Product
A
4
Metabolism ofCofactors and Vitamins
Nucleotide Metabolism
CarbohydrateMetabolism
LipidMetabolism
Amino AcidMetabolism
ATPProduction
Metabolism ofOther Substances
Enzymes control metabolic pathways
Each step catalyzed by a different enzyme
Feedback Loops Initial substrate (threonine)
Active site available
Threonine in active site
Enzyme 1 (threonine deaminase) Isoleucine
used up by cell
Feedback inhibition
Active site no longer available; pathway is halted
Intermediate A
Enzyme 2
Intermediate B
Enzyme 3
Isoleucine binds to allosteric site.
Intermediate C
Enzyme 4
Intermediate D
Enzyme 5
End product (isoleucine)
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Atoms bonded in High Potential Energy Configuration
Atoms bonded in Low Potential Energy Configuration
Chemical Products have a lower Potential Energy than Reactants
Energy is Released
If one thing gets oxidized, another becomes reduced
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CH4
H
H
H
H C O O O O O C
H H
Methane (reducing agent)
Oxygen (oxidizing agent)
Carbon dioxide Water
+ 2O2 CO2 + Energy + 2 H2O
becomes oxidized
becomes reduced
Reactants Products
Sometimes an electron Is only partially lost or gained
Change the degree of electron sharing in covalent bonds
Oxidation, Reduction, and Energy
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