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Introduction to Metabolism - Chapter 6 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Overview: The Energy of Life • The living cell is a miniature chemical factory where thousands of reactions occur • The cell extracts energy and applies energy to perform work • Some organisms even convert energy to light, as in bioluminescence Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Concept 6.1: An organism’s metabolism transforms matter and energy, subject to the laws of thermodynamics • Metabolism is the totality of an organism’s chemical reactions • Metabolism is an emergent property of life that arises from interactions between molecules within the cell Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Organization of the Chemistry of Life into Metabolic Pathways • A metabolic pathway begins with a specific molecule and ends with a product • Each step is catalyzed by a specific enzyme Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Enzyme 1 A B Reaction 1 Starting molecule Enzyme 2 Enzyme 3 D C Reaction 2 Reaction 3 Product • Catabolic pathways release energy by breaking down complex molecules into simpler compounds • Anabolic pathways consume energy to build complex molecules from simpler ones • Bioenergetics is the study of how organisms manage their energy resources Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • Kinetic energy is energy associated with motion – Heat (thermal energy) is kinetic energy associated with random movement of atoms or molecules • Potential energy is energy that matter possesses because of its location or structure – Chemical energy is potential energy available for release in a chemical reaction • Energy can be converted from one form to another Animation: Energy Concepts Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings On the platform, the diver has more potential energy. Diving converts potential energy to kinetic energy. Climbing up converts kinetic energy of muscle movement to potential energy. In the water, the diver has less potential energy. Concept 6.2: The free-energy change of a reaction tells us whether the reaction occurs spontaneously • Biologists want to know which reactions occur spontaneously and which require input of energy • To do so, they need to determine energy changes that occur in chemical reactions Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • The change in free energy (∆G) during a process is related to the change in enthalpy, or change in total energy (∆H), and change in entropy (T∆S): ∆G = ∆H - T∆S • Only processes with a negative ∆G are spontaneous • Spontaneous processes can be harnessed to perform work Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Free Energy, Stability, and Equilibrium • Free energy is a measure of a system’s instability, its tendency to change to a more stable state • During a spontaneous change, free energy decreases and the stability of a system increases • Equilibrium is a state of maximum stability • A process is spontaneous and can perform work only when it is moving toward equilibrium Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Gravitational motion Diffusion Chemical reaction Concept 6.3: ATP powers cellular work by coupling exergonic reactions to endergonic reactions • A cell does three main kinds of work: – Mechanical – Transport – Chemical • To do work, cells manage energy resources by energy coupling, the use of an exergonic process to drive an endergonic one Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Adenine Phosphate groups Ribose P P P Adenosine triphosphate (ATP) H2O Pi + Inorganic phosphate P P Adenosine diphosphate (ADP) + Energy Pi P Motor protein Protein moved Mechanical work: ATP phosphorylates motor proteins Membrane protein ADP + Pi ATP Pi P Solute transported Solute Transport work: ATP phosphorylates transport proteins P NH2 Glu + NH3 + Pi Glu Reactants: Glutamic acid and ammonia Product (glutamine) made Chemical work: ATP phosphorylates key reactants ATP Energy for cellular work (endergonic, energyconsuming processes) Energy from catabolism (energonic, energyyielding processes) ADP + P i Concept 6.4: Enzymes speed up metabolic reactions by lowering energy barriers • A catalyst is a chemical agent that speeds up a reaction without being consumed by the reaction • An enzyme is a catalytic protein • Hydrolysis of sucrose by the enzyme sucrase is an example of an enzyme-catalyzed reaction Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Sucrose C12H22O11 Glucose C6H12O6 Fructose C6H12O6 The Activation Energy Barrier • Every chemical reaction between molecules involves bond breaking and bond forming • The initial energy needed to start a chemical reaction is called the free energy of activation, or activation energy (EA) • Activation energy is often supplied in the form of heat from the surroundings Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings A B C D Free energy Transition state A B C D EA Reactants A B DG < O C D Products Progress of the reaction Free energy Course of reaction without enzyme EA without enzyme EA with enzyme is lower Reactants Course of reaction with enzyme DG is unaffected by enzyme Products Progress of the reaction Substrate Specificity of Enzymes • The reactant that an enzyme acts on is called the enzyme’s substrate • The enzyme binds to its substrate, forming an enzyme-substrate complex • The active site is the region on the enzyme where the substrate binds • Induced fit of a substrate brings chemical groups of the active site into positions that enhance their ability to catalyze the reaction Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Substrate Active site Enzyme Enzyme-substrate complex Substrates enter active site; enzyme changes shape so its active site embraces the substrates (induced fit). Substrates held in active site by weak interactions, such as hydrogen bonds and ionic bonds. Substrates Enzyme-substrate complex Active site is available for two new substrate molecules. Enzyme Products are released. Substrates are converted into products. Products Active site (and R groups of its amino acids) can lower EA and speed up a reaction by • acting as a template for substrate orientation, • stressing the substrates and stabilizing the transition state, • providing a favorable microenvironment, • participating directly in the catalytic reaction. Effects of Local Conditions on Enzyme Activity • An enzyme’s activity can be affected by: – General environmental factors, such as temperature and pH – Chemicals that specifically influence the enzyme Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Effects of Temperature and pH • Each enzyme has an optimal temperature in which it can function • Each enzyme has an optimal pH in which it can function Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Optimal temperature for typical human enzyme 0 Optimal temperature for enzyme of thermophilic (heat-tolerant bacteria) 40 60 Temperature (°C) 20 80 100 Optimal temperature for two enzymes Optimal pH for pepsin (stomach enzyme) 0 1 2 3 4 Optimal pH for trypsin (intestinal enzyme) 5 pH Optimal pH for two enzymes 6 7 8 9 10 Cofactors • Cofactors are nonprotein enzyme helpers • Coenzymes are organic cofactors Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Enzyme Inhibitors • Competitive inhibitors bind to the active site of an enzyme, competing with the substrate • Noncompetitive inhibitors bind to another part of an enzyme, causing the enzyme to change shape and making the active site less effective Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings A substrate can bind normally to the active site of an enzyme. Substrate Active site Enzyme Normal binding A competitive inhibitor mimics the substrate, competing for the active site. Competitive inhibitor Competitive inhibition A noncompetitive inhibitor binds to the enzyme away from the active site, altering the conformation of the enzyme so that its active site no longer functions. Noncompetitive inhibitor Noncompetitive inhibition Concept 6.5: Regulation of enzyme activity helps control metabolism • Chemical chaos would result if a cell’s metabolic pathways were not tightly regulated • To regulate metabolic pathways, the cell switches on or off the genes that encode specific enzymes Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Initial substrate (threonine) Active site available Isoleucine used up by cell Threonine in active site Enzyme 1 (threonine deaminase) Intermediate A Feedback inhibition Enzyme 2 Active site of enzyme 1 can’t bind Intermediate B theonine pathway off Enzyme 3 Isoleucine binds to allosteric site Intermediate C Enzyme 4 Intermediate D Enzyme 5 End product (isoleucine)