Enzymes
Jan 06, 2018
Enzymes
• Enzyme: a macromolecule (usually a protein) that acts as a catalyst; a chemical agent that speeds up a reaction without being consumed (used up) in the reaction.
• Every chemical reaction between molecules involves breaking some bonds and creating others.
• To reach a point where bonds can be broken, reactant molecules must absorb energy from their environment which causes them to enter an unstable state (transition state).
• Once the new bonds are formed, the resulting product is again stable and energy is released to the environment.
• Activation energy: the energy needed to start a chemical reaction- to get the molecules to a place where bonds can be broken- to the transition state
2 types of chemical reactions
• Exergonic: net release of free energy, the free energy of the reactants is greater than the free energy of the products
• Example: cellular respiration• C6H12O6 + 6O2 6CO2 + 6H2O + energy
Figure 6.6a
(a) Exergonic reaction: energy released, spontaneous
Amount ofenergy
released(G 0)
Reactants
ProductsEnergy
Progress of the reaction
Free
ene
rgy
• Exergonic reactions are said to be spontaneous, meaning that is is energetically favorable NOT that it happens quickly.
• Endergonic: absorbs free energy from the surroundings, there is a net increase in energy. The products have more energy than the reactants.
• The products, in a sense store energy.
• Example?
• Photosynthesis• The required energy comes from the sun• 6CO2 + 6H2O + energy C6H12O6 + 6O2
Figure 6.6b
(b) Endergonic reaction: energy required, nonspontaneous
Amount ofenergy
required(G 0)
Reactants
Products
Energy
Progress of the reaction
Free
ene
rgy
Figure 6.6 (a) Exergonic reaction: energy released, spontaneous
(b) Endergonic reaction: energy required,nonspontaneous
Amount ofenergy
released(G 0)
Amount ofenergy
required(G 0)
Reactants
ProductsEnergy
Progress of the reaction
Reactants
Products
Energy
Progress of the reaction
Free
ene
rgy
Free
ene
rgy
Figure 6.12
Transition state
Reactants
Progress of the reaction
Products
G 0
Free
ene
rgy
A
A
A
B
C
D
B
B
C
D
C D
EA
• Draw a similar diagram graphing the progress of an endergonic reaction in which:
•EF + GH EG + FH
•Exergonic• = •Spontaneous• ≠ • fast
Figure 6.12
Transition state
Reactants
Progress of the reaction
Products
G 0
Free
ene
rgy
A
A
A
B
C
D
B
B
C
D
C D
EA
• Adding heat is one way to provide energy to reach the transition state
• Molecules speed up, have more and more forceful collisions with one another
• Some reactions have a small enough activation energy (the amount of energy need to reach the transition state) that they can occur at room temperature
Problems with using heat to speed a biological reaction.
Can you think of any?
• Heat denatures proteins
• All reactions would be accelerated
Enter the enzyme
Enzymes
• Catalyze (speed up) reactions by lowering the activation energy of a reaction
• They cannot change the ΔG for a reaction• They cannot change an endergonic reaction to
an exergonic one.
Figure 6.13
Products
G is unaffectedby enzyme
Reactants
Progress of the reaction
Free
ene
rgy
EA withenzymeis lower
EA withoutenzyme
Course of reactionwithoutenzyme
Course of reactionwith enzyme
Things to know• Enzymes are substrate specific• They bind with their substrate(s) into an
enzyme-substrate complex• Binding occurs at an active site • Remember what we learned about protein
and their ligands, there is some subtle movement in the enzyme after binding that enhances the “fit” of the enzyme to its substrate. Induced fit
Figure 6.14
Enzyme-substratecomplex
Enzyme
Substrate
Active site
Mechanisms of lowering activation energy
1. The active site may provide a template so that multiple substrates can come together in the right orientation
2. The enzyme might stretch on the substrate weakening bonds
3. The active site might provide a microenvironment that is more favorable
4. The enzyme might be directly involved in the chemical reaction, but not consumed.
Figure 6.15-4
Substrates
Enzyme
Substrates areconverted toproducts.
Products arereleased.
Products
Enzyme-substratecomplex
Substrates areheld in active site byweak interactions.
Substrates enteractive site.
Activesite is
availablefor new
substrates.
5
43
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The rate of a chemical reaction
• Depends on:– Concentration of substrate– Amount of enzyme present– Once saturation occurs, the speed of the reaction
itself limits the rate unless…you can add more enzyme
– Local conditions
Local conditions
• Enzymes have optimum conditions; the conditions under which a particular enzyme works best– Temperature– pH– Cofactors- nonprotein helpers bound to the
enzyme.
Cofactors
• Nonprotein “helpers”• May be bound
– Tightly/ permanently– Loosely/ reversibly
• If the cofactor is organic, called a coenzyme• Example:
– vitamins
Enzyme inhibitors
• Competitive inhibitors– Reduce the productivity of enzymes by blocking
substrates from entering active sites– “mimic” the normal substrate molecule
• Noncompetitive inhibitors– Impede enzymatic reactions by binding to another
part of the enzyme– Changes the shape of the enzyme so that the
active site is less effective
Figure 6.17
(b) Competitive inhibition (c) Noncompetitive inhibition
(a) Normal binding
Competitiveinhibitor
Noncompetitiveinhibitor
Substrate
Enzyme
Active site
Regulation of enzyme activity
• Allosteric regulation= the binding of a regulatory molecule to an enzyme at one site that affects the function of the enzyme at a different site– Inhibition OR– Stimulation
• Cooperativity= a type of allosteric activation in which the enzyme response is amplified by the substrate itself
• Feedback inhibition= the end product of a metabolic pathway acts as a inhibitor of an enzyme within that path.
Figure 6.19 Active site available
Intermediate A
End product(isoleucine)
Intermediate B
Intermediate C
Intermediate D
Enzyme 2
Enzyme 3
Enzyme 4
Enzyme 5
Feedbackinhibition
Isoleucinebinds toallostericsite.
Isoleucineused up bycell
Enzyme 1(threoninedeaminase)
Threoninein active site
Learning objectives
• SWBAT: Create a model that represents the change in free energy of a reaction and the activation energy to carry out that reaction
• SWBAT: Describe how enzymes influence both the change in free energy and the activation energy of a reaction.
• Enzyme video• https://paul-andersen.squarespace.com/048-
enyzmes