1. Metabolism: Energy & Enzymes
CHAPTER 6
Metabolism: Energy & Enzymes

2. I. Energy & the Cell A. Energy - capacity to perform work
1. Kinetic - energy of motion; that is doing work
a. Heat - movement of molecules
2. Potential - stored energy
a. Due to location or arrangement
3. Chemical - potential energy of molecule
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3. II. Laws of Thermodynamics
A. Thermodynamics - study of energy transformations that occur in matter
1. Free Energy - energy in a system available for doing work
a. First Law of Thermodynamics
Total amount of energy in the universe is constant.
 Energy cannot be created or destroyed but can be changed from one form into another
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4. B. 2nd Law of Thermodynamics
1. Energy changes are accompanied by a loss of useable energy and an increase in disorganization.
a. Entropy = amount of disorder
Example = heat
2. Total entropy of the universe is increasing
a. If a particular system becomes more ordered, then its surroundings become more disordered
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5. 3. ∆G = Change in Free Energy
a. Measured at 25oC, 1 atm, pH 7.0, [1 mole/liter]
Initial Reactants Final Products
A B C D
Gi Gf
∆G = Gf - Gi
b. If ∆G is (-) reaction is exergonic
c. If ∆G is (+) reaction is endergonic
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6. III. Reactions & Energy
Metabolism - sum of chemical reactions in a cell.
1. Involves turning reactants into products.
B. Endergonic Reactions
1. Require a net input of energy
2. Start with reactants with less free energy.
3. Yield products that are high in free energy. Thus, products store energy (∆G is positive)
a. Energy stored in covalent bonds
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8. Carbohydrate Synthesis
Example of endergonic reactions
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9. C. Exergonic Reactions 1. Release energy
2. Start with reactants whose bonds contain more energy
3. Yield products that are lower in free energy (∆G is negative)
4. Releases energy to surroundings a. Example = burning wood
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11. Carbohydrate Metabolism
Example of exergonic reactions
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12. IV. ATP Shuttles Energy in Cell
A. Cells couple exergonic reactions with endergonic ones
1. First obtain energy from an exergonic reaction
i.e. Glucose  ATP
2. Then use energy of ATP to drive an endergonic reaction
B. Such energy coupling is crucial to cells’ functioning
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13. C. Structure of ATP (A-P-P~P)
1. ATP is composed of 3 parts:
a. adenine, a nitrogenous base
b. ribose, a 5-carbon sugar
c. a chain of 3 phosphate groups
2. The covalent bonds connecting the 2nd & 3rd phosphate groups are unstable (symbolized by ~)
a. These can be readily broken by hydrolysis
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14. Structure of ATP

15. D. Functioning of ATP 1. 3 things happen when bond breaks:
a. phosphate is removed
b. ATP becomes ADP
c. energy is released
2. Then 3rd phosphate is transferred to another molecule. This is called phosphorylation.
a. Gives energy to new molecule
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16. The ATP Cycle
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17. E. 3 Functions of ATP 1. Chemical Work
a. Supplies energy to synthesize macromolecules
2. Transport Work
a. Gives energy to pump substances across cell membrane
3. Mechanical Work
a. Supplies energy to move muscles, cilia, flagella, chromosomes
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18. F. Coupled Reactions
a. In coupled reactions, the energy released by an exergonic reaction is used to drive an endergonic reaction.
b. ATP breakdown is often coupled to cellular reactions that require an input of energy.
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19. Coupled Reactions
Figure 6.4
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20. “A” is Initial Reactant
V. Metabolic Pathways
A. Reactions usually occur in a series of linked reactions
1. Products of one reaction become reactants of a later reaction
Example:
AB C D E FG
“G” is End Product
“A” is Initial Reactant
Intermediates
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21. VI. Enzymes A. Protein molecules that serve as biological catalysts
B. Increase the rate of a reaction without itself being changed
C. Reactants in enzymatic reactions are called substrates.
E. Each enzyme accelerates a specific reaction.
E1 E2 E3 E4 E5 E6
A  B  C  D  E  F  G
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22. VII. How Enzymes Work A. Energy of Activation (Ea)
1. Amount of energy reactants must absorb to start chemical reaction
a. Example = energy needed to break bond between 2nd & rd phosphate in ATP
 ATP molecules don’t spontaneously break down in cells
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23. (*** Jumping bean analogy here)
2. Enzymes speed up reactions by lowering the energy activation barrier
a. They do this by bringing the substrates into contact with one another
(*** Jumping bean analogy here)
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24. Energy of Activation
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25. B. Enzymes are very selective
1. Enzymes have a unique 3- dimensional shape which determines their specificity
2. Only one small part of the enzyme, called the active site, binds with the substrate.
a. Each enzyme recognizes only one specific substrate
b. Denatured enzymes lose activity due to change in their shapes
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26. E + S ----> ES ----> E + P
C. Binding Mechanisms
1. “Lock & key” idea (old idea)
a. Reactive portion of substrate & active site of enzyme must fit together like a key.
b. Come close enough to bond temporarily to form an enzyme- substrate complex (ES)
E + S ----> ES ----> E + P
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27. 2. Induced-fit hypothesis
a. Many enzymes undergo conformational changes during bonding, which improve the fit and make the ES more reactive
b. Enzyme & substrate must be able to make numerous & precise weak bonds with each other
c. After reaction the product is released and active site returns to original state.
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28. Induced Fit Model
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29. Degradation vs. Synthesis
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30. D. Factors Affecting Enzymes :
1. Substrate Concentration
a. Molecules must collide to react
b. Enzyme activity increases with substrate concentration
c. More collisions between substrate molecules and the enzyme
d. As more substrate molecules fill enzyme active sites, more product can be made per unit time.
e. Eventually all sites are filled and rate cannot increase any more.
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31. a. Affects molecular motion
2. Temperature
a. Affects molecular motion
b. Optimal temperature produces highest rate of contact between substrate and enzyme
c. Higher temperatures denature the enzyme, altering its shape
d. Humans: oC optimum
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32. Effect of Temperature on Enzyme Activity
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33. 3. Salt concentration & pH
a. Salt ions interfere with chemical bonds in proteins
b. Extra hydrogen ions can interfere with bonding patterns
 Optimal pH is usually near 7, between 6 and 8
 Outside this range, normal functioning may be impaired
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34. Effect of pH on Enzyme Activity
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35. ● which enzymes are present ● concentration of enzymes in cell
4. Enzyme Concentration
a. Cells regulate:
● which enzymes are present
● concentration of enzymes in cell
● which enzymes are active or inactive
5. Phosphorylation
a. Enzymes known as kinases activate enzymes by phosphorylating them (adding a phosphate group)
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36. Phosphorylation
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37. E. Cofactors & Coenzymes
1. Many enzymes need inorganic ions or nonprotein helpers in order to function.
a. The inorganic ions are called cofactors.
Examples: zinc, iron, copper
b. If the cofactor is an organic nonprotein molecule it is called a coenzyme.
Vitamins needed for synthesis of some coenzymes.
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38. F. Enzyme inhibition 1. Inhibitor
a. Chemical that interferes with enzyme’s activity
b. There are 2 types:
 Competitive inhibitors
 Noncompetitive inhibitors
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39. 2. Competitive Inhibitor
a. Resembles enzyme’s normal substrate
b. Competes with substrate for active site on enzyme
 Blocks substrate from entering
 Prevents enzyme from acting
c. This is usually reversible
 Depends on concentration
d. Example = CO poisoning
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40. 3. Noncompetitive Inhibitor
a. Does not enter active site
b. Binds to enzyme at some other site called the allosteric site.
 Binding changes shape of enzyme so that active site no longer fits substrate
 Called an allosteric change
 Enzyme exists in two forms:
 active &  inactive
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41. 4. Reversibility a. Irreversible
 When covalent bonds form between enzyme & inhibitor
b. Examples:
Cyanide blocks ATP production
Penicillin lethal to bacteria
Mercury and lead
Nerve gases
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42. 5. Importance of Inhibitors
a. Inhibitor is sometimes the substance the reaction produces
 Example:
When cell has too much ATP, ATP acts as a noncompetitive inhibitor:
 prevents its own production
b. This is called feedback inhibition, or negative feedback.
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43. Feedback Inhibition
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44. VIII. Oxidation-Reduction Reactions
A. Electrons pass from one molecule to another.
1. Oxidation is the loss of electrons
a. The molecule loses an electron has been oxidized.
2. Reduction is the gain of electrons
b. The molecule gaining an electron has been reduced.
Both take place at the same time. One molecule accepts electron given up by another.
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45. B. Summary of Redox
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46. C. Photosynthesis 1. Equation for Photosynthesis
6CO2 + 6H2O + energy --> C6H12O6 + 6O2
a. Water is oxidized since it loses hydrogen atoms
b. Carbon dioxide is reduced since it gains hydrogen atoms
c. NADP+ is a coenzyme that accepts electrons and passes them to reactions that form glucose
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47. D. Cellular Respiration
1. Equation for Aerobic Respiration
C6H12O6 + 6O2 --> 6CO2 + 6H2O + ATP
a. Glucose is oxidized since it loses hydrogen atoms
b. Oxygen is reduced since it gains hydrogen atoms
c. NAD+ is a coenzyme that accepts hydrogen ions and electrons
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48. E. Electron transport chains
1. Ordered groups of molecules embedded in membranes of the mitochondria and chloroplasts
a. Electrons are passed along membrane-bound carriers
 Everytime an electron is transferred to a new carrier, energy is released
 The energy is used to produce ATP
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49. Electron Transport Chain
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50. c. Establishes an electrochemical (H+) gradient across the membrane.
b. Electron energy is used to pump hydrogen ions (H+) to one side of membrane.
c. Establishes an electrochemical (H+) gradient across the membrane.
d. Special ATP synthase complexes span the membrane. Each one allows H+ ions to flow down their gradient. The flow of ions provides energy to produce ATP from ADP.
♣ This is called chemiosmosis.
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51. Chemiosmosis
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