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