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Energy, Enzymes, and Metabolism. 8 Energy, Enzymes, and Metabolism. 8.1 What Physical Principles Underlie Biological Energy Transformations? 8.2 What Is the Role of ATP in Biochemical Energetics? 8.3 What Are Enzymes? 8.4 How Do Enzymes Work? 8.5 How Are Enzyme Activities Regulated?.
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8 Energy, Enzymes, and Metabolism • 8.1 What Physical Principles Underlie Biological Energy Transformations? • 8.2 What Is the Role of ATP in Biochemical Energetics? • 8.3 What Are Enzymes? • 8.4 How Do Enzymes Work? • 8.5 How Are Enzyme Activities Regulated?
8 Energy, Enzymes, and Metabolism Many laundry aids have been developed that include various enzymes to hydrolyze proteins, fats, and starches to remove a variety of stains. Opening Question: How are enzymes used in other industrial processes?
8.1 What Physical Principles Underlie Biological Energy Transformations? • A chemical reaction occurs when atoms have enough energy to combine or change bonding partners. • sucrose + H2O → glucose + fructose • reactants products
8.1 What Physical Principles Underlie Biological Energy Transformations? • Metabolism: the sum total of all chemical reactions occurring in a biological system at a given time. • Metabolic reactions involve energy changes.
8.1 What Physical Principles Underlie Biological Energy Transformations? • Energy is the capacity to do work, or the capacity for change. • In biochemical reactions, energy changes are associated with changes in the composition and properties of molecules.
8.1 What Physical Principles Underlie Biological Energy Transformations? • All forms of energy are either: • Potential energy—energy stored as chemical bonds, concentration gradient, charge imbalance, etc. • Kinetic energy—the energy of movement. • Energy can be converted from one form to another.
8.1 What Physical Principles Underlie Biological Energy Transformations? • Two types of metabolism: • Anabolic reactions: complex molecules are made from simple molecules, and energy input is required. • Catabolic reactions: complex molecules are broken down to simpler ones, and energy is released.
8.1 What Physical Principles Underlie Biological Energy Transformations? • Catabolic and anabolic reactions are often linked. • The energy released in catabolic reactions is used to drive anabolic reactions—to do biological work.
8.1 What Physical Principles Underlie Biological Energy Transformations? • The laws of thermodynamics apply to all matter and all energy transformations in the universe. • They help us to understand how cells harvest and transform energy to sustain life.
8.1 What Physical Principles Underlie Biological Energy Transformations? • First law of thermodynamics: energy is neither created nor destroyed. • When energy is converted from one form to another, the total energy before and after the conversion is the same.
8.1 What Physical Principles Underlie Biological Energy Transformations? • Second law of thermodynamics: when energy is converted from one form to another, some of that energy becomes unavailable to do work. • No energy transformation is 100 percent efficient; some energy is lost to disorder.
8.1 What Physical Principles Underlie Biological Energy Transformations? • Entropy is a measure of the disorder in a system. • It takes energy to impose order on a system. Unless energy is applied to a system, it will be randomly arranged or disordered.
8.1 What Physical Principles Underlie Biological Energy Transformations? • In any system: • Total energy = usable energy + unusable energy • H = G + TS • enthalpy (H) = free energy (G) + entropy (S) • G = H – TS • (T = absolute temperature)
8.1 What Physical Principles Underlie Biological Energy Transformations? • Free energy (G) is the usable energy that can do work. • Change in energy can be measured in calories or joules. • Change in free energy (ΔG) in a reaction is the difference in free energy between the products and the reactants.
8.1 What Physical Principles Underlie Biological Energy Transformations? • ΔG = ΔH – TΔS • If ΔG is negative, free energy is released. • If ΔG is positive, free energy is required. • If free energy is not available, the reaction does not occur.
8.1 What Physical Principles Underlie Biological Energy Transformations? • Magnitude of ΔG depends on: • ΔH—total energy added (ΔH > 0) or released (ΔH < 0). • ΔS—change in entropy. Large changes in entropy make ΔG more negative.
8.1 What Physical Principles Underlie Biological Energy Transformations? • If a chemical reaction increases entropy, the products will be more disordered. • Example: In hydrolysis of a protein into its component amino acids, ΔS is positive.
8.1 What Physical Principles Underlie Biological Energy Transformations? • Second law of thermodynamics: disorder tends to increase because of energy transformations. • Living organisms must have a constant supply of energy to maintain order.
8.1 What Physical Principles Underlie Biological Energy Transformations? • Exergonic reactions release free energy (–ΔG). • Catabolism: complexity decreases (generates disorder).
8.1 What Physical Principles Underlie Biological Energy Transformations? • Endergonic reactions consume free energy (+ΔG) • Anabolism: complexity (order) increases.
8.1 What Physical Principles Underlie Biological Energy Transformations? • In principle, chemical reactions can run in both directions. • At chemical equilibrium, ΔG = 0 A B • The concentrations of A and B determine which direction will be favored.
8.1 What Physical Principles Underlie Biological Energy Transformations? • Every reaction has a specific equilibrium point. • ΔG is related to the point of equilibrium: the further towards completion the point of equilibrium is, the more free energy is released. • ΔG values near zero are characteristic of readily reversible reactions.
8.1 What Physical Principles Underlie Biological Energy Transformations? • ΔG also depends on the beginning concentrations of reactants and products, temperature, pressure, and pH. • ΔG is determined under standard conditions: 25°C, one atmosphere pressure, one molar (1M) solutions, and pH 7.
8.2 What Is the Role of ATP in Biochemical Energetics? • ATP (adenosine triphosphate) captures and transfers free energy. • ATP releases a large amount of energy when hydrolyzed. • ATP canphosphorylate, or donate phosphate groups, to other molecules.
8.2 What Is the Role of ATP in Biochemical Energetics? • Hydrolysis of ATP yields free energy: • ATP + H2OADP + Pi + free energy • ΔG = –7.3 to –14 kcal/mol (exergonic)
8.2 What Is the Role of ATP in Biochemical Energetics? • Two characteristics of ATP account for the free energy released: • Phosphate groups have a negative charge and repel each other—the energy needed to get them close enough to bond is stored in the P~O bond. • The free energy of the P~O bond is much higher than the energy of the O—H bond that forms after hydrolysis.
8.2 What Is the Role of ATP in Biochemical Energetics? • Bioluminescence is an endergonic reaction driven by ATP hydrolysis:
8.2 What Is the Role of ATP in Biochemical Energetics? • The formation of ATP is endergonic: • ADP + Pi + free energy ATP + H2O • Formation and hydrolysis of ATP couples exergonic and endergonic reactions.
8.2 What Is the Role of ATP in Biochemical Energetics? • Coupling of exergonic and endergonic reactions is very common in metabolism. • Hydrolysis of ATP releases free energy to drive an endergonic reaction.
Figure 8.7 Coupling of ATP Hydrolysis to an Endergonic Reaction
8.2 What Is the Role of ATP in Biochemical Energetics? • An active cell needs to produce millions of molecules of ATP per second. • An ATP is typically consumed within a second of its formation. • Each ATP molecule undergoes about 10,000 cycles of synthesis and hydrolysis every day!
8.3 What Are Enzymes? • Catalysts speed up the rate of a reaction. • The catalyst is not altered by the reactions. • Most biological catalysts are enzymes (proteins) that act as a framework in which reactions can take place.
8.3 What Are Enzymes? • Some reactions are slow because of an energy barrier—the amount of energy required to start the reaction, called activation energy (Ea). • Activation energy puts the reactants in a reactive mode called the transition state.
8.3 What Are Enzymes? • Activation energy changes the reactants into unstable forms with higher free energy—transition state intermediates. • Activation energy can come from heating the system—the reactants have more kinetic energy. • Enzymes and ribozymes lower the energy barrier by bringing the reactants together.
8.3 What Are Enzymes? • Enzymes and ribozymes are highly specific. • Reactants are called substrates. • Substrate molecules bind to the active site of the enzyme. • The three-dimensional shape of the enzyme determines the specificity.
8.3 What Are Enzymes? • The enzyme–substrate complex (ES) is held together by hydrogen bonds, electrical attraction, or covalent bonds. • E + S ESE + P • The enzyme may change while bound to the substrate but returns to its original form.
8.3 What Are Enzymes? • The dissociation constant (KD) is a measure of the affinity of two molecules. • The lower the KD, the tighter the binding. • For enzymes and their substrates, KD values range from 10–5 to 10–6 M. This favors the formation of ES.
8.3 What Are Enzymes? • Enzymes lower the energy barrier for reactions. • The final equilibrium does not change, and ΔG does not change. • Enzymes can increase reaction rates by 1 million to 1017 times!
