CHAPTER 6
Energy, Enzymes, and Metabolism
Energy and Energy Conversions
• Energy is the capacity to do work• Potential energy is the energy of state or position; it
includes energy stored in chemical bonds• Kinetic energy is the energy of motion
• Potential energy can be converted to kinetic energy, which does work.
•
Energy Conversion figure 06-01.jpg
Kinetic Potential
First Law of Thermodynamics
• Energy cannot be created or destroyed.
Second Law of Thermodynamics• In a closed system, the quantity of energy available to do work
decreases and unusable energy increases • Usable energy = free energy (G)• Unusable energy = product of entropy (S) and absolute temperature (T)
• Total energy before transformation = enthalpy (H)
figure 06-03.jpg
Energy and Energy Conversions
• Organisms are open systems that are part of a larger closed system (universe)
Energy and Energy Conversions• Changes in free energy, total energy, temperature,
and entropy are related G = H – TS
• Exergonic reactions• Release free energy • Have a negative G
• Entropy increases, enthalpy decreases• Spontaneous
• Endergonic reactions • Take up free energy• Have a positive G
• Entropy decreases, enthalpy increases• Non-spontaneous
Reactions figure 06-05.jpg
Energy and Energy Conversions G determines equilibrium point • Exergonic reactions
• Equilibrium lies toward completion• Endergonic reacitons
• Reaction will not occur without input of energy
G-1-P G-6-P G=-1.7kcal/mol
ATP: Transferring Energy in Cells• ATP - an energy currency in cells• Hydrolysis of ATP releases free energy.
ATP: Transferring Energy in Cells• Reaction Coupling
• couples exergonic and endergonic reactions
figure 06-10.jpgCoupling Reaction
Glutamate
Enzymes: Biological Catalysts• Rates of reactions are independent of G • Determined by the activation energy • Catalysts speed reactions by lowering the activation
energy
Enzymes: Biological Catalysts• Highly specific for their substrates• Active site
• determines specificity• where catalysis takes place• enzyme–substrate complex
• Domains
Enzymes: Biological Catalysts
• In the active site, the substrate is induced into a transition state
• Transition state • temporary substrate
configuration
• Inducing & stabilizing thetransition state decreases activation energy & increases reaction rate
figure 06-15.jpgCatalytic Mechanisms
Lysozyme
Molecular Structure Determines Enzyme Function• Induced Fit
• Enzyme conformation alters upon substrate binding
Enzymes: Biological Catalysts
• Substrate concentration affects the rate of an enzyme-catalyzed reaction
Molecular Structure Determines Enzyme Function• The active sites of many enzymes contain special
reactive molecules which mediate the chemical catalysis
Metabolism and Enzyme Regulation
• Metabolic pathways • Upstream downstream sequence of reactions • Product of one reaction is a reactant for the next
• Regulation of enzymes • Feedback inhibition• Downstream products inhibit upstream enzymes
Enzyme Regulation - Competitive Inhibition
• Succinate fumarate malate OAA
• Build up of OAA inhibits succinate dehydrogenase
Enzyme Regulation - Competitive Inhibition
• Thr -Ketobutyrate Ile
• Buildup of Ile inhibits threonine dehydratase
Enzyme Regulation - Suicide Inhibitors
• Inhibitor reacts with amino acids in the active site permanently inhibiting the enzyme
• PMSF inhibits serine proteases such as trypsin
figure 06-20.jpg
Metabolism and Enzyme Regulation• Allosteric enzymes,
• reaction rate v substrate concentration is sigmoidal
Enzyme Regulation
• Allosteric inhibitors bind to sites different from the active site
• Multiple catalytic subunits may interact cooperatively
figure 06-23.jpg
Enzyme Regulation
• End product of pathway may inhibit upstream allosteric enzymes
Enzyme Regulation• pH and temperature affect enzyme activity