1. Chemical Reactions in Cells
Energetics, Enzymes and Metabolic Reactions

2. Energy Energy is the capacity for work or change.
Kinetic Energy = energy of movement
Potential Energy = stored energy
1st Law of Thermodynamics
Energy can be transferred and transformed from one form to another but it cannot be created or destroyed.

3. Energy 2nd Law of Thermodynamics
Energy transfer or transformation increases the entropy of the universe
Increase in entropy = randomness
Energy conversions result in a loss of useful energy
usable

4. Free Energy = Energy Useful for Change
Spontaneity of a reaction depends on free
energy change G G
reaction = Gproducts – Greactants
If is negative, free energy is released and the reaction is spontaneous G
If is positive, free energy is consumed G

5. Free Energy = Energy Useful for Change
Free Energy change depends on changes in
total energy (enthalpy)
entropy (unusable energy, disorder) H S
In living systems, entropy changes have substantial influence
when is positive, and the term is large, a negative value predicts a spontaneous reaction S T G G H S T = –

6. Chemical Reactions
Involve the breaking and formation of chemical bonds
Reactants are converted to products.
Two types of reactions based on energy use:
Exergonic– free energy released
Endergonic – free energy consumed

7. Burning glucose (sugar): an exergonic reaction
Exergonic Reactions
Burning glucose (sugar): an exergonic reaction
Reactants changed to transition-state species
Energy content of molecules
high
low
Activation energy needed to ignite glucose
Glucose + O2 G
Energy released by burning glucose
C O2 + H2O
Progress of reaction

8. Energy content of molecules
Endergonic Reactions
Photosynthesis: an endergonic reaction
Energy content of molecules
high
low
Glucose
Activation energy from light captured by photosynthesis
Net energy captured by synthesizing glucose G
CO2 + H2O
Progress of reaction

9. Applying Your Knowledge
Endergonic Reaction
Exergonic Reaction
A. Which type of reaction would be spontaneous?
B. For which type of reaction will the products have a higher energy than the reactants?
C. Which type of reaction releases energy?
D. Which type of reaction would have a positive value for G?

10. ATP Provides Energy for Cellular Reactions
+ H2O
+ Pi + free energy

11. Short-Term Energy Storage
Chemical Energy is stored in the bonds of ATP
ATP = adenosine triphosphate
ADP = adenosine diphosphate
to store energy
ADP + Phosphate + Energy ATP
to release energy
ATP  ADP + Phosphate + Energy

12. Coupled Reactions
Pairing of an Exergonic reaction, often involving ATP, with an Endergonic reaction
Note that overall free energy change is negative

13. Metabolic Reactions Anabolic Catabolic
link simple molecules to produce complex molecules (eg. dehydration synthesis of starch)
require energy
Catabolic
break down complex molecules to release simple ones (eg. hydrolysis of starch sugars)
release energy stored in chemical bonds

14. Metabolic Pathways F G B A C E D Pathway 1 Pathway 2 Initial Reactants
Intermediates
Final Products D B E C A
Enzyme 1
Enzyme 2
Enzyme 3
Enzyme 4
Pathway 1 F G
Pathway 2
Enzyme 5
Enzyme 6

15. Enzymes Assist in Biological Reactions
Enzymes are biological catalysts.
biological: composed of protein or, rarely, RNA
catalyst: speeds up a reaction without being changed by the reaction

16. Properties of Enzymes
Enzymes speed up biological reactions by lowering the activation energy for the reaction.
Enzymes provide a surface where the catalysis takes place
The reaction reaches equilibrium more rapidly
– The value of and the ratio of reactants and products at equilibrium is the same as for an uncatalyzed reaction G

17. Activation Energy: Controls Rate of Reaction
Amount of energy required for reaction to occur
transition state
Energy content of molecules
high
low
Activation energy without catalyst G
Activation energy with catalyst
Progress of reaction

18. Properties of Enzymes
Enzymes are SPECIFIC for the reactants (substrates) in the reactions that they catalyze.
Only substrates that fit the active site of the enzyme can bind and complete the reaction
active site: region on enzyme where substrates bind

19. Enzyme-Substrate Interactions
1 Substrates enter active site
Active Site
2 Shape change promotes reaction
Enzyme
Product released; enzyme ready again
induced fit

20. Chemical Events at Active Sites
Enzymes hold substrates in the proper orientation for the reaction to take place

21. Chemical Events at Active Sites
Enzymes induce strain in the substrate to produce a transition state favorable to reaction
Active site provides a microenvironment that favors the chemical reaction

22. Chemical Events at Active Sites
Active site directly participates in the reaction
covalent bonding can occur between enzyme and substrate
R groups of the enzyme’s amino acids can temporarily add chemical groups to the substrates

23. Molecules that Assist Enzymes
Cofactors: inorganic ions that bind to enzymes, eg. zinc
Coenzymes: small organic factors that temporarily bind to enzymes, eg. biotin, NAD, ATP
Prosthetic groups: non-protein factors that are permanently bound an enzyme, eg. heme

24. Factors Influencing Reaction Rate
Rate no longer increases since the active sites of all enzymes are saturated with substrate
Substrate Concentration
Rate is more rapid
Rate is proportional to substrate concentration

25. Factors Influencing Reaction Rate
Competitive Inhibitors: Bind at the active site, compete for binding with substrate
Irreversible: form covalent bond with amino acids in the active site
DIPF

26. Factors Influencing Reaction Rate
Competitive Inhibitors: Bind at the active site, compete for binding with substrate
Reversible: molecule similar to substrate occupies active site but does not undergo reaction

27. Factors Influencing Reaction Rate
Non-Competitive Inhibitors: Bind to a different site, cause a conformational change in the enzyme that alters the active site
Reversible

28. Factors Influencing Reaction Rate
Allosteric Regulation
Conversion between active and inactive forms of an enzyme due to binding of regulatory molecules at an allosteric site
Activators stabilize the active form
Allosteric inhibitors stabilize the inactive form

29. Factors Influencing Reaction Rate
Allosteric Regulation
Cooperativity: a substrate causing induced fit in one enzyme subunit can cause a change to the active form in all the other subunits

30. Enzyme Regulation: Feedback Inhibition
Commitment step
CH2 C
COOH
CH3
NH3 H
CH3 C
COOH OH
NH3 H A B C D
Enz. 1
Enz. 2
Enz. 3
Enz. 4
Enz. 5
Feedback Inhibition Isoleucine allosterically inhibits enzyme 1
Threonine (substrate)
Isoleucine (end product)
Feedback Inhibition: The product of a pathway inhibits an initial step in the pathway to decrease its own production

31. Properties of Enzymes
Three dimensional structure of an enzyme preserves its ACTIVE SITE
Conditions that can affect three dimensional structure include: heat, pH (acid/base balance) and other chemicals (salt, charged ions)

32. Effects of Temperature and pH on Enzymatic Activity
fewer collisions between enzyme and substrate
enzyme unfolds (denatures)
enzyme unfolds (denatures)

33. Applying Your Knowledge
Active Site
Activation Energy
Allosteric Site
Commitment step
Induced fit
Where can an inhibitor bind to stabilize the inactive form of an enzyme?
Where do the substrates bind?
Enzymes (raise or lower) the (1, 2, 3, 4 or 5) of a reaction.
What is the model for a shape change caused by substrate binding to the enzyme?