1. AP Bio Energetic of Chemical Reactions & The Role of Enzymes Lecture
Campbell & Reece, Biology 7th Edition pp
Chapter 8

2. Energy and Chemical Reactions
Exergonic rxn’ = release energy (products have less chemical energy than reactants)
ex: AB + CD  AC + DB + energy ΔG
Endergonic rxn’s = absorb energy (products have more chemical energy than reactants)
ex: AB + CD + energy  AC + DB ΔG
Activation energy: energy added to reactants to "jumpstart" the rxn
Catalysts: reduce the amount of activation energy that is needed to start the rxn.

3. The energetics of chemical reactions
Think about rolling a boulder up a hill…it takes energy input, right? And the boulder at the top of the hill now has that energy stored in it (potential). So the product ends up having MORE energy than the reactant.
Think about a boulder rolling down a hill…it has lots of energy. And the boulder at the bottom of the hill now has less (potential) energy. So the product ends up having LESS energy than the reactant.

4. Self-Quiz
Which of these biological processes is (overall) endergonic? Exergonic? –ΔG ? +ΔG ?

5. Chemical Reactions The activation energy, EA
Is the initial amount of energy needed to start a chemical reaction
Is often supplied in the form of heat from the surroundings in a system

6. The effect of enzymes on reaction rate
An enzyme catalyzes reactions
By lowering the EA barrier
Progress of the reaction
Products
Course of
reaction
without
enzyme
Reactants
with enzyme EA
EA with
is lower
∆G is unaffected
by enzyme
Free energy
Figure 8.15

7. Enzymes
Are a type of protein that acts as a catalyst, lowering the EA and speeding up chemical reactions
Substrate
(sucrose)
Enzyme
(sucrase)
Glucose OH
H O
H2O
Fructose
3 Substrate is converted
to products.
1 Active site is available for
a molecule of substrate, the
reactant on which the enzyme acts.
Substrate binds to
enzyme. 2
4 Products are released.
Figure 5.16

8. Substrate Specificity of Enzymes
The substrate (A)
Is the reactant an enzyme acts on
The enzyme
Binds to its substrate, forming an enzyme-substrate complex (C)

9. The Active Site Is the region on the enzyme where the substrate binds
Their shape (conformation) makes each enzyme substrate-specific
“Lock-and-key” fit enables reaction

10. The active site can lower an EA barrier by
Orienting substrates correctly
Straining substrate bonds
Providing a favorable microenvironment
Covalently bonding to the substrate

11. Effects of Local Conditions on Enzyme Activity
The activity of an enzyme
Is affected by general environmental factors
temperature pH
salinity
enzyme concentration
substrate concentration
presence of any inhibitors or activators.

12. Denaturation is a process in which proteins or nucleic acids lose their tertiary structure and secondary structure by application of some external stress or compound, such as a strong acid or base, a concentrated inorganic salt, an organic solvent (e.g., alcohol or chloroform), or heat.
If proteins in a living cell are denatured, this results in disruption of cell activity and possibly cell death.

13. Effects of Temperature and pH
Each enzyme
Has an optimal temperature in which it can function
Optimal temperature for
enzyme of thermophilic
Rate of reaction 20 40 80
100
Temperature (Cº)
(a) Optimal temperature for two enzymes
typical human enzyme
(heat-tolerant)
bacteria

14. Optimal pH for two enzymes
Rate of reaction
Optimal pH for pepsin
(stomach enzyme)
Optimal pH for trypsin
(intestinal enzyme) 1 2 3 4 5 6 7 8 9
Each enzyme has an optimal pH in which it can function

15. Cofactors & Coenzymes Cofactors Coenzymes
Are nonprotein enzyme helpers
Most often minerals (metal ions)
Zn 2+, Mg2+
Coenzymes
Are organic cofactors
Usually vitamins
B2, B6, B12, etc

16. Enzymes Regulate reactions
Concept 8.5: Regulation of enzyme activity helps control metabolism
A cell’s metabolic pathways
Must be tightly regulated
Enzyme 1
Enzyme 2
Enzyme 3 A B C D
Reaction 1
Reaction 2
Reaction 3
Starting molecule
Product
Biochemical pathway
Feedback inhibition of a biochemical pathway
Enzyme action and the hydrolysis of sucrose

17. Allosteric Activation and Inhibition
Many enzymes are allosterically regulated
Allosteric regulation
Is the term used to describe any case in which a protein’s function at one site is affected by binding of a regulatory molecule at another site
Enzymes can be “turned on” or “turned off” this way

18. Enzyme Inhibitors Competitive inhibitors
Bind to the active site of an enzyme, competing with the substrate
Figure 8.19
(b) Competitive inhibition
A competitive
inhibitor mimics the
substrate, competing
for the active site.
Competitive
inhibitor
A substrate can
bind normally to the
active site of an
enzyme.
Substrate
Active site
Enzyme
(a) Normal binding

19. Noncompetitive inhibitors
Bind to another part of an enzyme, changing the function
Figure 8.19
A noncompetitive
inhibitor binds to the
enzyme away from
the active site, altering
the conformation of
the enzyme so that its
active site no longer
functions.
Noncompetitive inhibitor
(c) Noncompetitive inhibition

20. Feedback inhibition In feedback inhibition
The end product of a metabolic pathway shuts down the pathway
Active site available
Isoleucine used up by cell
Feedback inhibition
Isoleucine binds to allosteric site
Active site of enzyme 1 no longer binds threonine; pathway is switched off
Initial substrate (threonine)
Threonine in active site
Enzyme 1 (threonine deaminase)
Intermediate A
Intermediate B
Intermediate C
Intermediate D
Enzyme 2
Enzyme 3
Enzyme 4
Enzyme 5
End product (isoleucine)
Figure 8.21