Bio 178 Lecture 13
Energy and Metabolism
http://www.colorado.edu/epob/academics/web_resources/cartoons/atp.html
Reading
•
Chapter 8
Quiz Material
•
Questions on P 158
•
Chapter 8 Quiz on Text Website
(www.mhhe.com/raven7)
Outline
•
Energy and Metabolism
 Free Energy
 ATP
 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.
Catalysis
• 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
spontaneously.
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).
Mechanism
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
molecule?
Too unstable - cells continually produce ATP for immediate
use.
Enzymes
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
bonds.
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
Description
A sequence of reactions may be controlled by a sequence of
enzymes. These may be bonded together noncovalently.
Advantages
• 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
Ribozymes
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
Temperature
• 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
Denaturation.
Effect of Temperature on Enzyme Catalysis
Environmental Effects on Enzyme Catalysis
pH
• 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