1.3 – Introduction to Metabolism
Energy – the ability to do work
 Living organisms must continually capture, store, and convert energy to carry out
the functions of life
 Organisms do all their work at a molecular level
Metabolism – the sum of all anabolic and catabolic processes
 Catabolic Reactions result in the breakdown of complex substances into simple
subunits
 Anabolic Reactions result in the formation of a complex substances from simpler
subunits
 Cells manage the materials and energy they use through these highly controlled
reactions
Kinetic Energy – energy possessed by moving objects
 Energy comes in many forms: sound, light, heat, electricity, etc.
Potential Energy – energy stored by virtue of an objects position within an attractive or
repulsive force field; at rest
 Gravitational Potential Energy – the attraction between two objects
 Chemical Potential Energy - the attraction of electrons to protons in a chemical
bond
Free Energy – energy that can do useful work
Work – the transfer of energy from on body or place to another
The First Law of Thermodynamics: The total amount of energy in the universe is
constant. Energy cannot be created or destroyed but only converted from one form into
another. If an object or process gains an amount of energy it does so at the expense of a
loss in energy somewhere else in the universe.
 Nature does not always provide energy in a readily usable from. Organisms
obtain energy in one form and convert it into another before it can be used.
 Molecules stable because of the chemical bonds between their atoms
 When atoms form covalent bonds, they become more stable; however, some
chemical bonds are more stable than others.
 The energy needed to break a bond is equivalent to the relative stability of that
bond
 Energy is absorbed when reactant bonds break and energy is released when
product bonds form
Potential Energy Diagram – shows the changes in potential energy that take place
during a chemical reaction
 Reactants are placed at a potential energy level corresponding to the relative
stability of the bonds between their atoms
 Activation energy is the amount of energy needed to strain and break the
reactant’s bonds
 Transition state is the temporary state where the bonds within the reactants are
breaking and the bonds between the products are forming
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If the activation energy is given, then the reactants will reach the transition stage
In an Exothermic Reaction, the bonds in the products are more stable then the
bonds in the reactants.
 More energy released during bond breaking then absorbed during
bond formation; net release of energy
 In an Endothermic Reaction, the bonds in the reactants are more stable then the
bonds in products.
 Amount of energy absorbed in bond breaking is more than amount
released during bond formation; net intake of energy
 Enthalpy of Reaction (ΔH) is the overall change in energy that occurs in a
chemical reaction.
 The most common form of energy absorbed and released by chemical reactions
in living things is thermal energy as heat
Entropy - a measure of the randomness or disorder in a collection of objects or energy
 Entropy increase when disorder increases; universe favors increase in entropy
 This is achieved when arrangement of a collection of objects become
more randomly assorted
 Heat is the main form entropy takes
Free Energy - energy that can do useful work
 During the 1870’s Josiah Gibbs discovered a relationship between change in
entropy, change in energy, and temperature of reaction
 This will predict whether a reaction will be spontaneous or not
 ΔG = ΔG final – ΔG initial
 ΔG = change in Gibb’s free energy
 When ΔG is positive, change is spontaneous and when ΔG is negative, change is
negative
 A reaction that is spontaneous in one direction will be non-spontaneous
backwards
The Second Law of Thermodynamics: Every energy transfer of transformation increase
the amount of entropy in the universe; ΔS > 0
 In living organisms, anabolic processes within cells build ordered structures
 E.g. (proteins and DNA from a random assortment of amino acids and
nucleotides)
 The order created by biological processes is at the expense of creating
disorder in the universe
 A spontaneous process in a system (an environment) occurs when the stability of
the system increases.
Exergonic Reactions – a chemical reaction in which the energy of the products is less
than the energy of the reactants
 “Energy outward” – proceeds with a net release of free energy
 Occurs spontaneously; ΔG = negative
Endergonic Reactions – a chemical in which the energy of the products is more than the
enregy of the reactants
 “Energy inward” – proceeds with a net increase of free energy
 Not spontaneous; ΔG = positive
Metabolic Reactions
 The reactions of metabolism are enzyme catalyzed and are all reversible
 When a reversible reaction reaches equilibrium, ΔG = 0 and its free energy
constant is zero
 A cell whose reversible reactions reached equilibrium is a dead cell
 This can be avoided by preventing the build up of solutes in a solution
Metabolic Disequilibrium – constant flow of material out of the cell - some reactions
are “pulled” in direction because the reactants for the next reaction
E.g. A + B C + D E + F and so on
 The products don’t increase in concentration – so we need more reactants of
A+B
 E.g. Photosynthesis – constant input of CO2, H2O, and light energy
 Energy Coupling is the use of exergonic reactions to drive endergonic reactions
 Cells do three types of
work:
 Mechanical:
Mitosis
 Chemical: Across
cell membrane
 Transport: anabolic
reactions – those
that build larger
molecules
Adenosine Triphosphate (ATP) – is a primary source of free energy in living cells
 The molecule that allows the transfer of energy ATP
 Uses ATP to perform work
 The phosphate from ATP is attracted to another molecule causing that molecule
to become unstable
 When the cell requires free energy to drive an endergonic reaction an enzyme
called ATPase catalyzes the hydrolysis of the terminal phosphate
 This results in an ADP (Adenosine Diphosphate), a molecule of inorganic
phosphate, and the
release of 31kJ/mol of
energy
 ATP + H2O ADP + Pi; ΔG = 31kJ/mol
 This is an exergonic process, but
its usually coupled with an
endergonic process through
phosphorylation
 A living cell needs large amounts of ATP to drive all endergonic reactions
Redox Reactions
 Oxidation: a chemical reaction in which an atom loses one or more electrons
 Reduction: a chemical reaction in when an atom gains one or more electrons
 Redox Reaction: a reaction in which oxidation and reduction occur
 Reducing Agent: the substance that causes the reduced atom to become
reduced
 Oxidizing Agent: the substance that causes the oxidized atom to become
oxidized
 Many chemical reactions involve the transfer of one or more electrons from one
reactant to another
 Sometimes a chain of redox reactions occur in which the product of one redox
reactant is the reactant in another
 Reduction and Oxidation are achieved by partial transfer of electrons from one
atom to another
 Electrons are in a covalent bond move closer to a more electronegative
atom
1.4 Enzymes
Enzymes – are protein catalysts (substances that speed up a chemical reaction without
being consumed in the reaction)
 The reactants are converted into products faster with enzymes; enzyme is
regenerated at the end of the reaction
 Protein’s function relies on proper structure; unique 3D structure
 Every chemical reaction involves bond breaking and bond formation
 Reactant molecules collide with a lot of force and a correct geometric
orientation, bonds between reactants break, transition state is
reached, new bond form between products
 All reactions posses an activation energy (EA) barrier that must be
overcome in order for the reaction to occur
 Heat provides the EA for many reactions
 Although an increase in temperature increases the rate of reaction, proteins
denature at high temperatures, losing their function and devastating the cell
 Enzymes allow reactions to proceed at normal temperature by reducing the
activation energy (EA); enzymes cannot change ΔG
Substrate – the reactant that an enzyme acts on when it catalyzes a chemical reaction
Active Site – the location where the substrate binds to an enzyme
 Groove or pocket on the surface of the enzymes
 The substrate binds to a particular site on the enzymes to which it is attracted
 Enzymes are very specific for the substrate to which they bind; recognize
substrate
Enzyme-Substrate Complex - an enzyme with its substrate attached to the active site
Induced-Fit Model – enzyme as a dynamic protein molecule that changes shape to
better accommodate the substrate
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Enzymes can catalyze thousands of reactions per second
Most metabolic reactions are reversible; enzymes catalyze both directions
Enzyme + Substrate → Enzyme-Substrate Complex → Enzyme + Products
Every enzyme has an optimal temperature at which it works best (human
enzymes → 37oC)
 Digestive enzyme pepsin works best in acidic environment of the
stomach; pH 2
Cofactors – non-protein components, such as dissolved ions (zinc, iron, copper), that are
needed for some enzymes to function
Coenzymes – organic non-protein cofactors that are needed for some enzymes to
function
 Some enzymes require cofactors or coenzymes in order to work properly
 Many coenzymes shuttle molecules from one enzyme to another
 Cofactors (e.g. Mg2+) and coenzymes (e.g. NAD+) may bind to the active site with
covalent bonds or they may bind weakly from one enzyme to another
Enzyme Inhibition
 A variety of substances inhibit enzyme activity
 Competitive Inhibitors are so similar to the enzyme
substrate that they are able to enter the enzyme’s active
sit and block the normal substrate from binding
 It’s reversible by increasing the concentration of
the enzyme substrate, this allows for the
substrate to compete favorably with the
inhibitor
 Noncompetitive Inhibitors do not compete for the active
site, but attach to another site on the enzyme, causing a
change in the enzymes shape
 Enzyme loses affinity for the substrate or may
affect the parts of the active site that perform the
work of catalysis, resulting in the enzymes loss of
activity
Allosteric Regulation
 Cells control enzyme activity through allosteric regulation
 Cells do this in two ways, by restricting the production of a particular enzyme, or
by inhibiting the action of an enzyme that has already been produced
 Some enzymes have receptor sites, called allosteric sites, that are some distance
away from the active site
 Allosterically controlled enzymes are usually composed of proteins in quaternary
structure, having several subunits each with an active site
 Binding an activator to the allosteric site, stabilizes the protein conformation
keeping all active sites available to their substance
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Binding an inhibitor stabilizes the inactive form of the enzyme making the active
sites unusable
Noncompetitive Inhibitors attach to the allosteric sties of certain enzymes
Feedback Inhibition
 Feedback inhibition is a metabolic pathway is switched off by the inhibitory
binding of its end product to an enzyme that acts earlier in the pathway
 This reduces the production of the inhibitor, which is at the same time,
the product of the process
 As the product is used up over time, its concentration decreases causing the
enzyme to be in active form more often and the production of the inhibitor
product increases
 As the inhibitor concentration increases, production of the inhibitor is
reduced once again
 The cell restricts the location of enzymes because enzymes are where they’re
needed to speed up reactions.
Commercial and Industrial Uses of Enzymes
 Enzymes are used to make chesses, cleaners, and paper
Additional References:
1) How Enzymes Work, Enzyme Inhibition, Feedback Inhibition, Allosteric Regulation
http://www.northland.cc.mn.us/biology/biology1111/animations/enzyme.swf
2)Introduction to Metabolism
http://www.youtube.com/watch?v=NzxgpsW_rJI