chapter 10 introduction to metabolism. metabolism the sum of the chemical changes that convert...
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Chapter 10
Introduction to Metabolism
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Metabolism
• The sum of the chemical changes that convert nutrients into energy and the chemically complex products of cells
• Hundreds of enzyme reactions organized into discrete pathways
• Substrates are transformed to products via many specific intermediates
• Metabolic maps portray the reactions
• Intermediary metabolism
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A Common Set of Pathways
• Organisms show a marked similarity in their major metabolic pathways
• Evidence that all life descended from a common ancestral form
• There is also significant diversity
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The Sun is Energy for Life
• Phototrophs use light to drive synthesis of organic molecules
• Heterotrophs use these as building blocks
• CO2, O2, and H2O are recycled
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Metabolism
• Metabolism consists of catabolism and anabolism
• Catabolism: degradative pathways– Usually energy-yielding!
• Anabolism: biosynthetic pathways– energy-requiring!
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Catabolism and Anabolism
• Catabolic pathways converge to a few end products
• Anabolic pathways diverge to synthesize many biomolecules
• Some pathways serve both in catabolism and anabolism
• Such pathways are amphibolic
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Organization in Pathways
• Pathways consist of sequential steps
• The enzymes may be separate• Or may form a multienzyme
complex• Or may be a membrane-bound
system• New research indicates that
multienzyme complexes are more common than once thought
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Mutienzyme complex
Separateenzymes
Membrane Bound System
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Organization of Pathways
Linear(product of rxns are substrates for subsequent
rxns)
Closed Loop(intermediates recycled)
Spiral(same set of
enzymes used repeatedly)
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Metabolism Proceeds in Discrete Steps
•Enzyme specificity defines biosynthetic route
•Controls energy input and output
•Allow for the establishment of control points.
•Allows for interaction between pathways
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Regulation of Metabolic Pathways
• Pathways are regulated to allow the organism to respond to changing conditions.
• Most regulatory response occur in millisecond time frames.
• Most metabolic pathways are irreversible under physiological conditions.
• Regulation ensures unidirectional nature of pathways.
• Flow of material thru a pathway is referred to as flux.
• Flux is regulated by supply of substrates, removal of products, and activity of enzymes
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Common mechanisms• feedback inhibition – product of pathway
down regulates activity of early step in pathway
• Feedforward activation – metabolite produced early in pathway activates down stream enzyme
Enzyme Regulation of Flux
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Metabolic Control Theory
• Pathway flux is regulated by multiple enzymes in a pathway.
• Control coefficient determined for each enzyme. = activity / enzyme concentration.
• Enzymes with large control coefficients impt to overall regulation.
• Recent finding suggest that the control of most pathways is shared by multiple pathwayt enzymes
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Regulating Related Catabolic and Anabolic Pathways
• Anabolic & catabolic pathways involving the same compounds are not the same
• Some steps may be common to both
• Others must be different - to ensure that each pathway is spontaneous
• This also allows regulation mechanisms to turn one pathway and the other off
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Metabolic Pathways are not at Equilibrium
• Metabolic pathways are not at equilibrium A <-> B• Instead pathways are at steady state.A -> B -> CThe rate of formation of B = rate of utilization
of B.Maintains concentration of B at constant level.All pathway intermediates are in steady state.Concentration of intermediates remains
constant even as flux changes.
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Thermodynamics and Metabolism
• Standard free energy A + B <-> C + D
• Go’ =-RT ln[C][D]/[A][B]
• Go’ = -RT ln Keq
• Go’ < 0 (Keq>1.0) Spontaneous forward rxn
• Go’ = 0 (Keq=1.0) Equilibrium
• Go’ > 0 (Keq <1.0) Rxn requires input of energy
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G (not Go’) is impt in vivo
• G = Go’ + RT ln Q
• Q (mass action ratio) = [C]’[D]’/[A]’[B]’
• Actual [reactants] and [products] used to determine Q.
• Because reactions are at steady state not equilibrium, Q does not equal Keq
• When Q is close in value to Keq = near-equilibrium rxn (reversible)
• If Q is far from Keq = metabolically irreversible rxn.
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ATP• ATP is the energy currency of cells
• In phototrophs, light energy is transformed into the light energy of ATP
• In heterotrophs, catabolism produces ATP, which drives activities of cells
• ATP cycle carries energy from photosynthesis or catabolism to the energy-requiring processes of cells
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Phosphoric Acid Anhydrides
• ADP and ATP are examples of phosphoric acid anhydrides
• Large negative free energy change on hydrolysis is due to: – electrostatic
repulsion – stabilization of
products by ionization and resonance
– entropy factors
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Phosphoryl-group Transfer
• Energy produced from a rxn can be coupled to another rxn that requires energy to proceed.
• Transfer of a phosphate group from high energy phosphorylated compounds can activate a substrate or intermediate of an energy requiring rxn.
A-P + ADP -> A + ATP, ATP +C-> ADP + C-P• The ability of a phosphorylated compound
to transfer a phosphoryl group is termed its phosphoryl-group-transfer-potential.
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Phosphoryl-group Transfer