What is an enzyme? A biological catalyst

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What is an enzyme? A biological
catalyst Increases the rate of a reaction Is
not itself consumed in the reaction Lowers the
activation energy of a reaction Reactions will
not occur at a rate that supports survival
unless enzymes are present
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How do enzymes work? Active site- the part of an
enzyme that actually interacts with the
reactants Another word for reactants
substrates An enzyme reacts only with specific
substrates
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Models for enzyme activity

• Induced-fit model says that substrate binding
  changes the active site
• Both models explain that enzyme activity is
  specific

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Control of enzyme activity Temperature and pH of
solution Concentration of enzyme and substrate
molecules Presence of substances that stimulate
or inhibit reactions Availability of cofactors
or coenzyme
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How environmental factors affect enzyme activity
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Cofactors and coenzymes

• Cofactors are metal ions
• Required to shape enzyme properly
• Coenzymes are organic, often vitamins
• Act as electron carriers

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Metabolic pathways Series of reactions, each
catalyzed by a different enzyme The product of
one reaction is the substrate for the next
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Most pathways are branched
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Feedback inhibition End product of a pathway
accumulates It can then inhibit and enzyme that
functions earlier in the pathway When the end
product is depleted, it is not available to
inhibit the enzyme, so the pathway resumes
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Inborn error of metabolism Each enzyme in a
pathway is coded by a different gene Genetic
defect may lead to loss of function of that
enzyme. End product of that pathway cannot
be synthesized may lead to disease Intermediate
will accumulate this may lead to disease
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Errors in the metabolism of phenylalanine

• Enzyme 1 defect- may lead to PKU
  (phenylketonuria)
• Enzyme 5 defect- alkaptonuria
• Enzyme 6 defect- albinism

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Enzyme 1 defect- may lead to PKU
(phenyl- ketonuria Enzyme 5 defect-
alkaptonuria Enzyme 6 defect- albinism Many
such defects have been identified
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Bioenergetics Flow of energy in living
systems Energy flow obeys the laws of
thermodynamics 1. Energy is not created or
destroyed, but can be transformed from one form
to another 2. Entropy increases in the
universe when chemical bonds are broken,
energy is released energy must be added to form
chemical bonds
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Exergonic reactions release energy Endergonic
reactions require input of energy In cells,
reactions are coupled so that energy released
from one is used to drive the other If energy is
released gradually, in a series of reactions, it
can be used more efficiently than if released
all at once.
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ATP is the principal energy carrier of the cell
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Oxidation-reduction reactions Reduction- when an
atom or molecule gains electrons Oxidation-
when it loses electrons These reactions are
always coupled Hydrogen atoms are often
transferred along with the electrons
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NAD and FAD are two important molecules that
transfer hydrogens/electrons in the cell.
Reactions are reversible. Both are derived from
vitamins NAD- niacin FAD- riboflavin Eat your
vitamins!
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Summary Enzymes catalyze reactions Most enzymes
are proteins, sensitive to temperature and
pH Reaction rate affected by substrate and
enzyme concentrations Cofactors and coenzymes
are often required Enzymes participate in
complex metabolic pathways.
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Defective enzymes can lead to disease. Exergonic
reactions release energy. Endergonic reactions
require input of energy. These reactions are
coupled in biologic systems. Redox reactions
are also coupled in one reaction (oxidation)
electrons are lost, and in the other (reduction)
electrons are gained. Electrons transport chain
is itself coupled with ATP synthesis.