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 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 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 – T  S 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 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 jpg Coupling 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 the transition state decreases activation energy & increases reaction rate

figure jpg Catalytic 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 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 jpg

Enzyme Regulation End product of pathway may inhibit upstream allosteric enzymes

Enzyme Regulation pH and temperature affect enzyme activity