1. Chapter 6: Metabolism – Energy and Enzymes (Outline)
Forms of Energy
Two Laws of Thermodynamics
Cells and Entropy
Metabolic Reactions
ATP – Energy for Cells
Metabolic Pathways and Enzymes
Energy of Activation
Enzyme-Substrate Complex
Redox reactions

2. Forms of Energy
The capability to do work or produce an effect and is divided into two types
Kinetic- the energy of motion, such as waves, electrons, atoms, molecules, substances and objects
Mechanical energy of motion
Electrical (e.g. lightning)
Thermal (i.e. geothermal energy)
Radiant (e.g. visible light, x-rays, gamma rays and radio waves)

3. Forms of Energy Potential:
Stored energy or energy that is "waiting to happen"
Chemical - energy stored in the bonds of atoms and molecules
Nuclear – energy stored in the nucleus of an atom (the energy that holds the nucleus together)
Stored mechanical energy (e.g. compressed springs)

4. Laws of Thermodynamics
First law:
Law of conservation of energy
Energy cannot be created or destroyed, but
Energy CAN be changed from one form to another

5. Laws of Thermodynamics
Second law:
Energy cannot be changed from one form to another without a loss of usable energy
Therefore, no process requiring a conversion of energy is ever 100% efficient
Carbohydrate Metabolism
Carbohydrate Synthesis

6. Cells and Entropy Law of entropy:
The spontaneous movement from order to disorder (randomness)
Waste energy goes to increase disorder
Entropy – a measure of the degree of a system’s disorder which increases over time
Cars rust, dead trees decay, buildings collapse; all these are examples of entropy in action

7. Cells and Entropy

8. Energy Flow in Living Things

9. Metabolic Reactions and Energy Transformations
Metabolism:
Sum of cellular chemical reactions in a cell
Reactants participate in a reaction
Products form as result of a reaction
Free energy – the amount of energy available to perform work
Concept of free energy was developed by Gibbs
For a reaction to occur spontaneously free energy must decrease - the products must have less free energy than the reactants

10. Metabolic Reactions & Energy Transformations
Energy Changes in Metabolic Reactions
Exergonic Reactions - products have less free energy than reactants (i.e. spontaneous and releases energy)
Example: ATP breakdown
Endergonic Reactions - products have more free energy than reactants (i.e. not spontaneous and requires energy)
Example: Muscle contraction

11. ATP: Energy for Cells Adenosine triphosphate (ATP) Composed of
High energy compound used to drive metabolic reactions
Constantly being generated from adenosine diphosphate (ADP) and a molecule of inorganic phosphate
Composed of
Adenine, ribose (C5 sugar), and 3 phosphate groups
Biological advantages to the use of ATP
It provides a common energy currency used in many types of reactions
When ATP becomes ADP + ℗ the amount of energy released is sufficient for biological function with little waste of energy (~ 7.3 kcal per molecule)
ATP breakdown can be coupled to endergonic reactions that prevents energy waste

12. The ATP Cycle

13. ATP and Coupled Reactions
Energy released by an exergonic reaction is captured in ATP
That ATP is used to drive an endergonic reaction
Occur in the same place, at the same time, why?
Because the energy released by the hydrolysis of ATP is higher than the energy consumed by the endergonic reaction

14. Coupled Reactions A cell has two ways to couple ATP hydrolysis
ATP is used to energize a reactant
ATP is used change the shape of a reactant
Both can be achieved by transferring ℗ to the reactant so that the product is phosphorylated
Example:
Ion movement across the plasma membrane of a cell through carrier proteins
Attachment of amino acids to a growing polypeptide chain

15. Coupled Reactions – Muscle Contraction

16. Enzymes A  B  C  D  E  F  G E1 E2 E3 E4 E5 E6
Protein molecules that function as catalysts, however ribozymes are made of RNA not proteins
The reactants of an enzymatically accelerated reaction are called substrates
Each enzyme accelerates a specific reaction
Each reaction in a metabolic pathway requires a unique and specific enzyme
End product will not appear unless ALL enzymes are present and functional
E1 E2 E3 E4 E5 E6
A  B  C  D  E  F  G

17. “A” is Initial Reactant
Metabolic Pathways
Reactions usually occur in a sequence
Products of an earlier reaction become reactants of a later reaction
Such linked reactions form a metabolic pathway
Begins with a particular reactant,
Proceeds through several intermediates, and
Terminates with a particular end product
AB C D E FG
“A” is Initial Reactant
“G” is End Product
Intermediates

18. Energy of Activation
Reactants are often “reluctant” to participate in the reaction
Energy must be added to at least one reactant to initiate the reaction
Energy of activation - minimum amount of energy required to trigger a chemical reaction
Enzyme Operation:
Enzymes operate by lowering the energy of activation
Accomplished by bringing the substrates into contact with one another under mild conditions
Does not get consumed by the reaction nor does it alter the equilibrium of the reaction, thus it remains intact

19. Energy of activation (Ea)

20. Enzyme-Substrate Complex
The “lock and key” model of enzyme activity
The active site is made up of amino acids and has a very specific shape
The enzyme and substrate slot together to form a complex, as a key slots into a lock
This complex reduces the activation energy for the reaction
Then the products no longer fit into the active site and are released allowing another substrate in

21. Enzyme-Substrate Complex
Induced fit model
The active site complexes with the substrates by weak interactions, such as hydrogen bonds, etc
Causes active site to change shape
Shape change forces substrates together, initiating bond
Some enzymes participate in the reaction (e.g. Trypsin – active site contains 3 amino acids with R groups interacting with the peptide bond (to break the bond and introduce H2O

22. Synthesis vs. Degradation
Enzyme complexes with two substrate molecules
Substrates are joined together and released as single product molecule
Degradation:
Enzyme complexes with a single substrate molecule
Substrate is broken apart into two product molecules

23. Enzymatic action

24. Naming enzymes Enzyme names usually end in ase
Many enzyme names have 3 parts: (substrate name) (type of reaction) (ase)
Substrate
Enzyme
Lipid
Lipase
Urea
Urease
Maltose
Maltase
Cellulose
Cellulase
Lactose
Lactase
Sucrose
Sucrase

25. Factors Affecting Enzyme Activity
Substrate concentration
Enzyme activity increases with substrate concentration
More collisions between substrate molecules and the enzyme
When the active sites of the enzyme are filled, with increasing substrate, the enzyme’s rate of activity cannot increase any more
Amount of active enzyme can also increase or limit the rate of an enzymatic reaction

26. Factors Affecting Enzyme Activity
Temperature
Enzyme activity increases with temperature
Warmer temperatures cause more effective collisions between enzyme and substrate
However, hot temperatures destroy enzymes, how?
Denaturation – enzyme’s shape changes and it can no longer bind its substrate efficiently
However, there are exceptions such as
Some prokaryotes living in hot springs
Coat color pattern in Siamese cats

27. Factors Affecting Enzyme Activity
Optimal pH
Most enzymes are optimized for a particular pH
Changes in pH can make and break intra- and intermolecular bonds, and/or hydrogen bonds thus changing the globular shape of the enzyme
pH change can alter the ionization of R side chains
Under extreme conditions of pH, the enzyme becomes inactive (due to altered shape)
Example: Pepsin (stomach) and trypsin (small intestine)

28. Effect of Temperature and pH

29. Enzyme Cofactors
Many enzymes require additional help in catalyzing their reaction from a coenzyme or cofactor
Cofactor is used to refer to inorganic metallic ions such as zinc, copper and iron, which is required by an enzyme
Coenzyme is a non-protein organic molecule
Many of the coenzymes are derived from vitamins
Enzyme activity decreases if vitamin is not available (i.e. vitamin-deficient disorder)
Niacin deficiency results in skin disease (pellagra), while riboflavin deficiency results in cracks at the corner of mouth

30. Enzyme Inhibition Competitive inhibition
Substrate and the inhibitor are both able to bind to the active site
If a similar molecule is present, it will compete with the real substrate for the active sites
Product will form only when the substrate, not the inhibitor, is at the active site
This will regulate the amount of product

31. Enzyme Inhibition Noncompetitive inhibition
Noncompetitive inhibitors are considered to be substances which when added to the enzyme change the enzyme in a way that it cannot accept the substrate
The inhibitor binds to another location (allosteric site) on the enzyme and inactivates the enzyme molecule
Both competitive and noncompetitive inhibition are examples of feedback inhibition

32. Competitive and Noncompetitive Inhibitors

33. Feedback Inhibition
The end product of a pathway inhibits the pathway’s first enzyme
The metabolic pathway is shut down when the end product of the pathway is bound to an allosteric site on the first enzyme of the pathway
Normally, enzyme inhibition is reversible and the enzyme is not damaged

34. Oxidation and Reduction
The loss of one or more electrons
Reduction
The gain of one or more electrons
Reduction-oxidation (redox) reactions
Simultaneous reaction in which one molecule is oxidized and another is reduced
Redox reactions occur during photosynthesis and cellular respiration

35. Electron Transfer in Redox Reactions