Biol 178 Lecture 13

Bio 178 Lecture 13
Energy and Metabolism
Chapter 8
Quiz Material
Questions on P 158
Chapter 8 Quiz on Text Website
Energy and Metabolism
 Free Energy
 Enzymes
Endergonic and Exergonic Reactions
Activation Energy (AE)
The energy required to start a reaction.
Why do spontaneous reactions require AE?
Existing bonds must be broken.
• Process of influencing chemical bonds in a way that
lowers the AE of a reaction, eg. Stressing bonds.
•  Speed up reactions
Importance of AE
Prevents macromolecules from breaking down
Activation Energy
Adenosine Triphosphate (ATP)
Adenosine Triphosphate (ATP)
Used to power the majority of energy-requiring processes
in cells.
Energy storage
• Electrostatic repulsion of phosphates  Unstable (low
AE to break the bonds).
• ATP  ADP + Pi + Energy (7.3 kcal/mole).
ATP hydrolysis occurs simultaneously with endergonic
reactions. If there is a net release of energy the reaction is
exergonic and will proceed.
ATP (Cntd.)
Why is ATP not used as a long-term energy storage
Too unstable - cells continually produce ATP for immediate
What are enzymes?
Biological catalysts made of protein.
Enzyme Catalysis
• Weaken bonds (apply stress)
• Bring substrates close together
 Lower activation energy of a reaction
How do Enzymes Work?
Active Site
Precisely and specifically fits the substrate - aa enzyme
side groups interact with substrate  usually stressing
Induced Fit
Enzyme adjusts its shape to fit the substrate.
Catalytic Cycle
Enzymes are not used up in the reaction.
Catalytic Cycle of an Enzyme
Multienzyme Complexes
A sequence of reactions may be controlled by a sequence of
enzymes. These may be bonded together noncovalently.
• Close proximity allows the product of one reaction to be
passed to the next in rapid sequence.
• Central control of entire sequence of reactions.
Example - Pyruvate dehydrogenase
60 protein subunits (multiple copies of 3 enzymes).
Pyruvate Dehydrogenase
Non-Protein Biological Catalysts
Catalysts made of RNA.
Types of Ribozymes
• Intramolecular
Catalyze reactions on themselves
• Intermolecular
Catalyze reactions on other molecules
Which came first - Protein or RNA?
Environmental Effects on Enzyme Catalysis
Affected by anything that alters its 3D shape:
pH, [salt], temperature, regulatory molecules
• Optimum Temperature
Temperature at which reaction rate is greatest.
• Below Optimum
Increasing temp increases substrate-enzyme collisions &
can stress bonds.
Bonds not flexible to permit induced fit  not optimum.
• Above Optimum
Effect of Temperature on Enzyme Catalysis
Environmental Effects on Enzyme Catalysis
• Optimum pH
pH at which reaction rate is greatest.
• Above/Below Optimum
Change in [H+] affects charge balance between charged
amino acids, which affects intramolecular bonding.
Effect of pH on Enzyme Catalysis