1. Metabolism: Energy and Enzymes

2. Chapter Outline I) Cells and the Flow of Energy
II) Metabolic Reactions and Energy Transformations
III) Metabolic Pathways and Enzymes

3. I) Cells and the Flow of Energy
Energy is the ability to do work or bring about a change
Cells use acquired energy to maintain their organization, grow, develop and reproduce
There are two fundamental types of energy

4. Potential Energy
Potential = stored energy waiting to do work  its capacity to do work is not being used at this moment (e.g. gravitational, elastic, chemical forms)

5. Kinetic Energy
Kinetic = energy at work or the energy of movement (e.g. light, heat, mechanical forms)

6. 1) Cells and the Flow of Energy
Under the right conditions, energy can be transformed from one type to another (potential < kinetic) and from one form to another (e.g. chemical> thermal)
These energy transformations follow the Laws of Thermodynamics

7. Energy Transformations

8. The First Law of Thermodynamics
The First Law of Thermodynamics (sometimes called the Law of Conservation of Energy) states the amount of energy in any process is constant
Energy can not be created nor destroyed by ordinary processes, only transformed from one form to another form, and between potential and kinetic types
With each transformation within any system, some energy is released to the surroundings as “useless” heat energy (which can not do useful work)

9. “useless”
Heat
energy
“useless”
Heat
energy

11. The Second Law of Thermodynamics
The Second Law of Thermodynamics (sometimes called the Entropy Law) states that the amount of “useful” energy decreases when energy transformations occur (because “useless” heat energy is released)
Therefore, there is a tendency for all systems to reach the lowest possible “useful” energy level
Entropy is the term for the measure of the amount of disorder (loss of higher “useful” energy) in a system
Therefore, there is a tendency for all systems to reach the highest possible entropy level

12. “Useful” Energy = Chemical Energy
High
Low
“Useless” Energy = Heat Energy
Low
High
Entropy = State of Disorder
Low
High

13. As an example, plants trap light energy from the sun in the process of photosynthesis (anabolism) to produce carbohydrates
Plants use the carbohydrates to provide energy to maintain themselves (i.e. growth, reproduction, homeostasis, etc)
High useful
energy
Low useful
energy

14. High useful energy
Low useful energy
Plants lose their energy when heterotrophic organisms feed on them through digestion (catabolism)
Plants become disordered (increase in entropy) as they pass through the digestive system of an organism as the latter processes the plants for nutrients in order to maintain the organization of its cells

15. What is the Source of all Energy on Earth?
Energy must always be put into a system in order to sustain it, because all of the chemical reactions that occur in cells are decreasing the amount of “useful” energy and increasing the amount of “useless” energy
Where does this input of energy come from?

16. The Sun
The Sun constantly loses useful light energy and increases in entropy, in order to maintain life on Earth

17. II) Metabolic Reactions and Energy Transformations
Metabolism = the sum of all of the chemical reactions in a cell
Anabolism = reactions that synthesize complex molecules from simple molecules
Catabolism = reactions that breakdown complex molecules into simple molecules

19. Chemical Reactions
Reactants = Substrate = the substances that start a chemical reaction
Products = the substances that form as a result of a chemical reaction
Reactants  Products
Energy is released or absorbed in chemical reactions

20. Chemical Reactions and Energy
Reactants  Products
Chemical reactions proceed spontaneously if there is an increase in entropy (= decrease in “useful” energy) as the reaction proceeds to products
The “useful” energy of the products < the “useful” energy of the reactants

21. Chemical Reactions and Energy
Gibb’s Free Energy (Δ G)
= amount of energy available after a chemical reaction to do work
= “useful” energy of products –
“useful” energy of the reactants

22. Exergonic Reactions Chemical reactions that release energy
The amount of “useful” energy in the products is less than that of the reactants
Gibbs Free Energy (Δ G) is negative
e.g. cellular respiration Δ G = -686 kcal

23. : Cellular Respiration
(CH2O)n

24. Endergonic Reactions
Chemical reactions that require an input of energy to proceed to products
The amount of “useful” energy in the products is greater than that of the reactants
Gibbs Free Energy (Δ G) is positive
e.g. photosynthesis Δ G = +686 kcal

25. : Photosynthesis
+ O2

26. complex
simple

27. Coupled Reactions
Where do endergonic reactions get there input energy from ?
From exergonic reactions that they are coupled to.

28. Coupled Reactions
How is the energy transferred from the exergonic reaction to the endergonic reaction?
By the energy carrier molecule ATP.

29. Adenosine Triphosphate (ATP)
Cells do not directly use forms of energy : the energy of chemical fuel molecules (e.g. glucose) must be transformed into ATP in an exergonic chemical reaction
ATP is then used to provide the energy to complete an endergonic chemical reaction.
There are a number of additional energy carrier molecules involved in cell metabolic processes such as photosynthesis and cell respiration: NAD+, NADP+, FAD and the Cytochromes.

31. Adenosine Triphosphate (ATP)
The second and third phosphate bonds of ATP are unstable. When this phosphate bond is broken by hydrolysis, energy is released (an exergonic reaction). This released energy is just perfect for the amount of energy needed for many cell reactions. This is why we call ATP an energy carrier. It "carries" the energy needed to do the cell work.
The product of the hydrolysis of ATP is a molecule of ADP and a free phosphate molecule (Pi ). Much of the energy released when ATP is broken is in the form of less useful heat energy.

34. III) Metabolic Pathways and Enzymes
Metabolic Pathway = series of linked chemical reactions that begins with a particular reactant and terminates with an end product

35. Controlling Metabolic Pathways
Metabolic pathways must be tightly regulated for our cells to function:
1) Cells couple endergonic chemical reactions with exergonic chemical reactions
2) Cells have energy-carrier molecules that capture energy released in exergonic reactions and transport it to endergonic reactions
3) Cells regulate chemical reactions using enzymes, which are protein catalysts

36. Chemical Reactions, Activation Energy and Enzymes
All reactant molecules are in constant motion
For any chemical reaction to get proceed, the reactants must come together at the right bonding place at the right time
No matter how "spontaneous" a chemical reaction is, some energy is needed to get the reaction started
This energy is called the activation energy

37. : Cellular Respiration
(CH2O)n

38. Chemical Reactions, Activation Energy and Enzymes
Reactants molecules will come together randomly, but far too slowly at normal earth temperatures
Catalysts speed up the rate of chemical reactions by lowering the activation energy
The catalyst is not part of the chemical reaction. It is neither a reactant nor a product
A catalyst facilitates the reaction and is never consumed in the reaction
In living organisms, we use a special class of catalysts, called enzymes

40. Enzymes Enzymes are globular proteins with tertiary structure
There is a place on the surface of the enzyme where reactant molecules can bind this "notch" is the active site
The active site has a precise size, shape, and electrical charge that exactly complements the reactant molecules enzymes are highly specific to their reactant molecules
Therefore, each chemical reaction that occurs in cells has its own enzyme

41. Mechanism of Enzyme Action
When a reactant molecule binds to the enzyme, it "fits" into the active site of the enzyme  induced fit
This binding temporarily distorts the reactant molecules  transition state
In the transition state, the bonds of the reactant molecules are more easily broken (lowered activation energy), thereby promoting the reaction
Once the reaction occurs, the active site is altered, releasing the product
The enzyme is unaffected by the reaction.

43. Factors Affecting Reaction Rate
1) Substrate Concentration
2) Enzyme Concentration
3) Temperature
4) pH
5) Presence of Competitive Inhibitor
6) Presence of Allosteric Inhibitor
7) Presence of Enzyme Cofactors
8) Presence of Metal Ions

44. 1) Effect of Substrate Concentration on Reaction Rate
1) At lower substrate concentrations, the active sites on most of the enzyme molecules are not filled
2) At higher substrate concentrations, there are more collisions between the substrate and enzyme molecules  faster reaction rate
3) The maximum reaction rate is reached when the active sites are almost continuously filled 3 2 1

45. 2) Effect of Enzyme Concentration on Reaction Rate
1) If there is insufficient enzyme present, the reaction will not proceed as fast as it otherwise would because there is not enough enzyme for all of the reactant molecules
2) As the amount of enzyme is increased, the rate of reaction increases
3) If there are more enzyme molecules than are needed, adding additional enzyme will not increase the rate 3 2 1

46. 3) Effect of Temperature on Enzyme Activity

47. 4) Effect of pH on Enzyme Activity

48. 5) Effect of Competitive Inhibitor on Reaction Rate

49. Types of Competitive Inhibitors
e.g. cyanide competes with oxygen for the active site of the enzyme cytochrome oxidase
e.g. penicillin competes for the active site of an enzyme unique to bacteria

51. 6) Effect of Allosteric Inhibitor on Reaction Rate

52. Allosteric Inhibition

53. 7) Effect of Enzyme Cofactors on Reaction Rate
Cofactors = inorganic molecule required by enzyme for proper functioning of enzyme
e.g. copper (Cu+), zinc (Zn++), iron (Fe++), magnesium (Mg++), potassium (K+), and calcium (Ca++) ions
The cofactors bind to the enzyme and participate in the reaction by removing electrons, protons , or chemical groups from the substrate.

54. 7) Effect of Enzyme Coenzymes on Reaction Rate
Coenzymes = organic (non- protein) molecule required for proper functioning of enzyme
e.g. NAD, FAD, vitamin complexes
Coenzymes often remove electrons from the substrate and pass them to other molecules
Often the electron is added to a proton to form a hydrogen atom before it is passed
In this way, coenzymes serve to carry energy in the form of electrons (or hydrogen atoms) from one compound to another

55. 8)Presence of Metal Ions
The addition of heavy metal ions such as mercury, lead and arsenic to an enzyme-mediated reaction decreases the reaction rate
The heavy metal ions denature the enzyme by destroying the 3-dimensional shape of the active site