An Introduction to Metabolism

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Chapter 8

• An Introduction to Metabolism

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Metabolism

• The total of an organisms chemical reactions.
• Types
• Catabolic (break down)
• Anabolic (building)

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Bioenergetics

• The study of how organisms manage their energy.
• Energy- the capacity to cause change
• Kinetic- energy of motion
• Potential- energy that matter possesses
• Chemical- potential energy available for release
  in a chemical reaction

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Transformations between kinetic and potential
energy
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Thermodynamics

• The study of energy transformations that occur in
  a collection of matter.

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Figure 8.3 The two laws of thermodynamics
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The relationship of free energy to stability,
work capacity, and spontaneous change
.
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Free energy changes (?G) in exergonic and
endergonic reactions
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Complementary Reactions

• Photosynthesis- Endergonic
• Cell Respiration- Exergonic

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Redox Reactions

• Oxidation Reduction

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Oxidation

• Loses electron
• Loses Hydrogen atom
• Adding oxygen
• Cell Respiration
• CH2O O2 ? CO2 H2O Energy

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Reduction

• Gains electron
• Gains hydrogen atom
• Photosynthesis
• CO2 H2O ? CH2O O2

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Redox and Energy

• An electron loses potential energy when it moves
  from a less electronegative atom to a more
  electronegative one (like oxygen)
• This energy can be used for work.

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The structure of adenosine triphosphate (ATP)
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The hydrolysis of ATP
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Figure 8.11 How ATP drives cellular work

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The ATP cycle
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Enzymes

• Protein catalysts
• Reusable
• Lower Activation Energy (energy needed to start a
  reaction)
• The reaction catalysed by orotidine 5'-phosphate
  decarboxylase will consume half of its substrate
  in 78 million years if no enzyme is present.
  With enzyme- just 25 milliseconds.

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Example of an enzyme-catalyzed reaction
hydrolysis of sucrose by sucrase
CH2OH
CH2OH
CH2OH
CH2OH
O
O
O
O
H
H
H
H
H
H
H
Sucrase
H
HO
OH
H
H
OH
H2O
HO
O
H

H
OH
O
HO
CH2OH
CH2OH
HO
OH
H
H
H
H
OH
OH
OH
Fructose
Glucose
Sucrose
C12H22O11
C6H12O6
C6H12O6
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Energy profile of an exergonic reaction
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The effect of enzymes on reaction rate.
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Note

• Amount of energy released is NOT affected. The
  amount of energy needed IS (activation energy)

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Induced Fit Hypothesis

• Enzymes are specific.
• Enzyme slightly changes shape (at the active
  site) when substrates bind.
• Enhances fit, and therefore, the catalytic
  ability.
• In contrast to Lock and Key model

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Induced fit between an enzyme and its substrate
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The active site and catalytic cycle of an enzyme
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Factors affecting Enzymes

• Temperature
• High- denatures enzymes
• Low- molecules move slow
• pH- denatures enzymes
• Amount of substrate
• Presence of inhibitors

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Environmental factors affecting enzyme activity
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Cofactors/Coenzymes

• Enzyme helpers (some enzymes dont work unless
  cofactor is bound to it)
• Coenzymes- organic cofactors i.e. vitamins

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Competitive Inhibition

• Something else binds to the active site

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Noncompetitive Inhibition

• Something else binds to enzyme, causing it to
  change shape, and altering the active site.
• Allosteric Enzyme- changes shape

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Inhibition of enzyme activity
Competitive inhibitor
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.
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Allosteric regulation of enzyme activity
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Feedback inhibition in isoleucine synthesis