Chapter 8 Notes
An Introduction to Metabolism
Objectives
• Distinguish between the following pairs of terms:
catabolic and anabolic pathways; kinetic and potential energy; open and
closed systems; exergonic and endergonic reactions.
• Explain the laws of thermodynamics
• Explain how cells obtain the energy to do cellular work.
• Explain how ATP performs cellular work.
• Describe how the regulation of enzymes activity helps control
metabolism.
• Explain why activation energy is necessary to initiate a spontaneous
reaction.
• Describe how enzymes lower activation energy.
• Describe how allosteric regulators may inhibit or stimulate the activity
of an enzyme.
The Energy of Life
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The living cell is a chemical factory in miniature, where
thousands of reactions occur within a microscopic space.
The totality of an organisms chemical reactions is called
metabolism.
An organism’s metabolism transforms matter and energy, subject
to the laws of thermodynamics
• arises from interactions between molecules
• A metabolic pathway has many steps that begin with a specific
molecule and end with a product
– each step is catalyzed by a specific enzyme
Metabolic Pathway
Metabolic Pathways
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Catabolic pathways break down complex
molecules into simpler compounds
– release energy
• Anabolic pathways build complicated
molecules from simpler ones
– consume energy
Forms of Energy
• Energy is the capacity to cause change.
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In everyday life, energy is important because
some forms of energy can be used to do work.
– Two types of energy:
• kinetic energy-energy of motion
• potential energy-stored capacity to do work
– energy matter possesses due to location or
structure
The Laws of Energy Transformation
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The study of the energy transformations that occur in a collection of
matter is called thermodynamics.
Two laws of Thermodynamics:
– First law of thermodynamics (energy conservation)
• total amount of energy in universe is constant
– can be transferred or transformed but cannot be created
or destroyed
– Second law of Thermodynamics
• every energy transformation increases entropy
– energy available for doing useful work decreases with
every transformation
Free Energy
Organisms use free energy
– free energy is the energy available to do work
Free energy is the portion of a system's energy that can
perform work when temperature and pressure are
uniform throughout the system, as in a living cell.
Organisms live at the expense of free energy.
During a spontaneous change, free energy decreases
and the stability of a system increases.
At maximum stability, the system is at equilibrium and
can do no work.
•
Free Energy and Metabolism
Based on their free-energy changes, chemical reactions can
be classified as either exergonic or endergonic.
– endergonic reactions require input of energy
• energy input equals difference in potential energy
between reactants and products
– exergonic reactions release energy
• energy released equals difference in potential energy
between reactants and products
– cellular metabolism is sum total of all endergonic
and exergonic reactions in cells
ATP and Cellular Work
• Energy coupling is a key feature in the way
cells manage their energy resources to do
this work
●
A cell does three main kinds of work
– mechanical
– transport
– chemical
ATP (adenosine triphosphate)
– is the cell’s energy shuttle
– provides energy for cellular functions
– most cell reactions require small amounts of
energy
– food storage molecules contain large amounts of
energy
– energy in food molecules is converted to energy
in ATP
• one food molecule=many ATP (1 glucose=36 ATP)
• Hydrolysis of ATP releases energy
– terminal covalent bonds between outer
phosphate
groups are energy rich and easily
hydrolyzed
– forms ADP and phosphate group
How ATP drives Cellular Work
•
ATP drives endergonic reactions
– By phosphorylation, transferring a phosphate to other molecules
• Phosphorylation of a protein usually results in
the protein changing shape
– dephosphorylation (removal of the phosphate)
allows protein to return to original shape
• phosphorylation-dephosphorylation cycle of proteins can
be used to perform tasks in cells
ATP can serve as the energy currency of
cells because ATP can be regenerated from
ADP and Pi (inorganic phosphate)
– catabolic pathways drive the regeneration of ATP
from ADP and phosphate
• endergonic reactions of cellular respiration linked to the
phosphorylation of ADP
– reforms ATP
Enzymes
• Enzymes are macromolecules that acts
as a catalyst
– a catalyst is a chemical agent that speeds up a
reaction without being consumed by the reaction
• an enzyme is a catalytic protein
Activation Energy
• Energy
of activation (EA) is “energy barrier”,
amount of energy needed to initiate a chemical
reaction
– it is often supplied in the form of heat from the
surroundings in a system
Enzymes lower the activation energy barrier
Substrate Specificity of Enzymes
Specific enzymes catalyze each cell
reaction
– reactant=substrate
– binds to enzyme active site
• forms an enzyme-substrate complex
– substrate converted to product
– enzyme unchanged
Induced fit of a Substrate
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Brings chemical groups of the active site into
positions that enhance their ability to catalyze the
chemical reaction
Catalysis in the Enzyme’s Active
Site
• Active site is central to enzyme activity
– binding of substrate forms enzyme-substrate complex
• several mechanisms used by enzyme to lower EA
– orienting substrates correctly
– straining substrate bonds
– providing a favorable microenvironment
– covalently bonding to the substrate
Effects of Local Conditions on Enzyme
Activity
• Factors that affect enzyme activity
– temperature, pH, salt concentration, and
presence of cofactors and coenzymes
• remember that an enzyme is a protein so
anything that denatures a protein affects
an enzyme
– cofactors are non-protein enzyme helpers
– coenzymes are organic cofactors
Enzyme Inhibitors
– Inhibitors block enzyme action
• competitive inhibitors-bind to active
site
• noncompetitive inhibitors-bind to
second site (allostericsite) on enzyme
Enzyme Regulation
Regulation of enzyme activity
helps control metabolism
– a cell’s metabolic pathways
must be tightlyregulated
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Allosteric Regulation
• Allosteric regulation is the term used to describe any case
in which a protein’s function at one site is affected by
binding of a regulatory molecule at another site
– many enzymes are allosterically regulated
• they change shape when regulatory molecules bind to
specific sites, affecting function
• some enzymes are activated by an allosteric activator
– converts inactive enzyme to active enzyme
Cooperativity is a form of allosteric regulation that can
amplify enzyme activity
– binding of substrate can increase binding of more
substrate in enzymes that have multiple active sites
• binding of first substrate molecule causes change in
enzyme shape that favors binding of further substrate
molecules.
•
Within the cell, many enzymes are
associated with specific regions of the cell
– may be grouped into complexes
– may be incorporated into membranes
• Some pesticides and antibiotics function
by
inhibiting enzymes