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Ch. 8 An Introduction to Metabolism
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A organism’s metabolism is subject to thermodynamic laws
• The totality of an organism’s chemical reactions is called metabolism.
• We can picture a cell’s metabolism as an elaborate road map of the thousands of chemical reactions that occur in a cell, arranged as intersecting metabolic pathways
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Metabolic pathways
• A metabolic pathway begins with a specific molecule, which is then altered in a series of defined steps, resulting in a certain product.
• Each step of a pathway is catalyzed by a specific enzyme
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Metabolic pathways
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Metabolic pathways
• Enzymes catalyze reactions in intersecting metabolic pathways, which may be catabolic (breaking down molecules, releasing energy) or anabolic (building molecules, consuming energy).
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Bioenergetics
• Energy is the capacity to cause change; some forms of energy do work by moving matter.
• Kinetic energy is associated with motion and includes thermal energy (heat) associated with random motions of atoms or molecules.
• Potential energy is related to the location or structure of matter and includes chemical energy possessed by a molecule due to its structure
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bioenergetics
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bioenergetics
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Thermodynamics
• The first law of thermodynamics, conservation of energy, states that energy cannot be created or destroyed, only transferred or transformed
• The second law of thermodynamics states that spontaneous processes, those requiring no outside input of energy, increase the entropy (disorder) of the universe
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First law of thermodynamics:
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Second law of thermodynamics
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Free energy
• A living system’s free energy is energy that can do work under cellular conditions.
• The change in free energy (∆G) during a biological process is related directly to enthalpy change (∆H) and to the change in entropy (∆S): ∆G=∆H - T∆S.• Organisms live at the expense of free energy
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Free energy
• During a spontaneous change, free energy decreases and that stability of a system increases.
• At maximum stability, the system is at equilibrium and can do no work.
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Spontaneous reaction
• In an exergonic (spontaneous) chemical reaction, the products have less free energy than the reactants (-∆G)
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Spontaneous reactions
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Endergonic reactions
• Endergonic (nonspontaneous) reactions require an input of energy (+∆G).
• The addition of starting materials and the removal of end products prevent metabolism from reaching equilibrium
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Endergonic reactions
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Coupling and ATP
• ATP is the cell’s energy shuttle. Hydrolysis of its terminal phosphate yields ADP and P and ᵢreleases free energy.
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Coupling and ATP
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Coupling and ATP
• Through energy coupling, the exergonic process of ATP hydrolysis drives endergonic reactions by transfer of a phosphate group to specific reactants, forming a phosphorylated intermediate that is more reactive
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Coupling and ATP
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Coupling and ATP
• ATP hydrolysis (sometimes with protein phosphorylation) also causes changes in the shape and binding affinities of transport and motor proteins
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Coupling and ATP
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Coupling and ATP
• Catabolic pathways drive regeneration of ATP from ADP and P .ᵢ
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Coupling and ATP
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Catabolic pathways drive regeneration of ATP
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Enzymes speed up metabolic reactions
• In a chemical reaction, the energy necessary to break the bonds of the reactants is the activation energy, E .ᴀ
• Enzymes lower the E barrierᴀ
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Enzymes lower the E barrierᴀ
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Enzyme action
• Each type of enzyme has a unique active site that combines specifically with its substrate(s), the reactant molecule(s) on which it acts.
• The enzyme changes shape slightly when it binds the substrate(s) (induced fit).
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Enzyme action
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Enzyme action
• The active site can lower the E barrier by ᴀorienting substrates correctly, straining their bonds, providing a favorable microenvironment, or even covalently bonding with the substrate• Each enzyme has optimal temperature and pH. Inhibitors reduce enzyme function. A competitive inhibitor binds to the acive site, whereas a noncompetitive inhibitor binds to a different site on the enzyme
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Enzyme action
• Natural selection, acting on organisms with mutant genes encoding altered enzymes, is a major evolutionary force responsible for the diverse array of enzymes found in organisms
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Enzyme regulation
• Many enzymes are subject to allosteric regulation: regulatory molecules, either activators or inhibitors, bind to specific regulatory sites, affecting the shape and function of the enzyme
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Allosteric regulation
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Allosteric regulation
• In cooperativity, binding of one substrate molecule can stimulate binding or activity at other active sites.
• In feedback inhibition, the end product of a metabolic pathway allosterically inhibits the enzyme for a previous step in the pathway
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Feedback inhibition
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Enzyme regulation
• Some enzymes are grouped into complexes, some are incorporated into membranes, and some are contained inside organelles, increasing efficiency of metabolic processes