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Introduction to Metabolism and its Processes

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					Introduction to Metabolism
CHAPTER 8

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catabolism = metabolic pathways that release energy by breaking down compounds anabolism = metabolic pathways that consume energy to build compounds

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Metabolism

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kinetic energy – energy of motion potential energy – stored energy chemical energy – the potential energy available for release in a chemical reaction

Energy

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first law of thermodynamics – energy can be transferred & transformed, but it cannot be created or destroyed second law of thermodynamics – energy transfer or transformation increases the entropy (disorder or randomness) of the universe

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Laws of Energy Transformation (Thermodynamics)

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free energy

◦ portion of a system’s energy than can perform work when temperature and pressure are uniform throughout the system ◦ measure of a system’s instability (tendency to change to a more stable state)

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chemical reactions that…

◦ lose free energy (G  0) are spontaneous or exergonic ◦ absorb free energy (G  0) are endergonic

Free Energy Change (G)

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equilibrium = state of maximum stability metabolism as a whole is never at equilibrium because of the constant flow of materials in & out of the cell

Equilibrium

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3 kinds:
◦ mechanical
 ex: beating of cilia, muscle contraction, movement of chromosomes during cell division

◦ transport ◦ chemical

 ex: active transport

 ex: endergonic reactions

Cellular Work

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immediate source of cellular energy common to ALL living things responsible for mediating most energy coupling reactions (use of exergonic reaction to drive an endergonic reaction) 10 million consumed & regenerated per second per cell

ATP (Adenosine Triphosphate)

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the hydrolysis of ATP powers cellular work
◦ the bond between the 2nd & 3rd phosphate groups breaks ◦ the phosphate group is transferred to another molecule (phosphorylation)

hydrolysis

phosphorylation

ATP synthesis requires energy

ATP hydrolysis yields energy

ATP Cycle

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many chemical reactions in the cell are slow (even spontaneous reactions) cells use enzymes (catalytic proteins) to speed up reactions enzymes lower the energy required to start a reaction (activation energy – EA)

Enzymes

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the active site of an enzyme has a specific shape that is specific to the shape of the substrate that binds to it induced fit hypothesis – substrate induces a change in the shape of the active site to create a snug fit

Enzymes Show Specificity

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enzymes emerge from reactions in their original form enzymes can catalyze both the forward & reverse reactions

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How Enzymes Work

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active site can help substrates come together in the proper orientation for a reaction to occur enzyme may stretch substrates toward their transition-state conformation active site may provide a microenvironment that is more conducive to a particular type of reaction active site may participate directly in the chemical reaction

How Enzymes Lowering EA

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as substrate concentration increases, reaction rate will increase to a point when enzyme becomes saturated (all enzymes have their active sites engaged), the rate of the reaction will be determined by the rate at which the active site can convert substrate to product

Substrate Concentration

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enzyme reaction rate increases with an increase in temperature to a point
◦ initially, an increase in temperature makes substrates move faster and they are more likely to collide with the active sites of enzymes when temperatures get too high, the enzyme denatures and the reaction stops

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most human enzymes have optimal temperatures between 35-40C

Temperature

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the optimal pH for most enzymes is between 6-8 when the pH deviates from the optimum, the enzyme denatures and the reaction stops 2 exceptions: pepsin & trypsin

pH

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many enzymes require nonprotein helpers called cofactors to be active
◦ may be permanent fixtures or bind reversibly along with the substrate
◦ may be inorganic metal ions: zinc, iron, copper

◦ may be organic (called coenzymes): vitamins

Cofactors

competitive – mimic the substrate; bind to & block the active site  noncompetitive – bind away from the active site; cause the enzyme to change shape which changes the shape of the active site  inhibitors can play a regulatory role
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Enzyme Inhibitors

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allosteric regulation – binding of an activator or inhibitor molecule to a regulator site on an enzyme which stabilizes the functional or inactive form of the enzyme, respectively  ex: ADP acts as an activator & ATP acts as an inhibitor for several catabolic enzymes cooperativity – one substrate binds to an enzyme and primes the enzyme to accept additional substrates feedback inhibition – product of a metabolic pathway binds to & inhibits an enzyme that acts early in the pathway

Regulation of Enzyme Activity


				
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posted:4/16/2008
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