Enzymes and Metabolism
Chapter 4
Chemical Reactions
• First Law of Thermodynamics
– Matter and energy cannot be created or
destroyed, but can be converted from one form
to another
• Second Law of Thermodynamics
– As energy is transformed from one form to
another, the universe becomes increasingly
disorganized
Chemical Reactions
• Order = Free Energy
• All chemical reactions must involve a loss
in free energy
• Change in free energy during a chemical
reaction depends upon…
1. Particular energetic properties of the
reactant(s) vs. that of the product(s)
2. The concentration of the reactants(s) relative
to that of the product(s)
Reversible vs. Irreversible Reactions
• Reversible reaction
– Intrinsic energetic properties of
the reaction differ little between
converting reactants into
products and converting
products into reactants
– Relative ratio of products vs.
reactants determines net
tendency for the reaction to go
one way vs. another.
– Equilibrium – point where rate
of reaction proceeding in one
direction is equal to the rate the
reaction
[Reactant]
[Product]
Reversible vs. Irreversible Reactions
• Irreversible reaction
– Intrinsic energetic
properties of the
reaction greatly favor
conversion of reactants
into products
– Reactants converted to
product even if there is
very little reactant and
lots of product.
Enzymes Protein Catalysts
Enzymes = Protein Catalysts
• speed up the rate of chemical reactions
• are not permanently altered in the reactions
• do not change the nature of the reaction
– Change in free energy and equilibrium point
will be the same
How do enzymes speed up
reactions?
• Lower the activation
energy for a reaction
– amt of energy needed to get
the reaction going
• Enzymes bind reagents
(substrates)
– orient them so that less
energy is needed to get the
reaction going
Enzymes catalyze specific
reactions
• Have Complex 3-D structures
• Pockets act as active sites
– bind specific substrates
– catalyze specific chemical reaction
– produce specific products
E + S  E-S Complex  E + P
Enzyme Names
• Names of enzymes typically indicate
function:
– lactate dehydrogenase – removes hydrogen
from lactate
– phosphatase – removes phosphate groups from
different organic compounds
• Some do not have descriptive names
– chymotrypsin - hydrolyzes peptide bonds
– ptyalin – hydrolyzes glycosidic bonds
• Concentration of Enzyme
–  Enzyme,  Rate
• Concentration of Substrate
–  Substrate,  Rate
–  in rate limited by amount
of enzyme
• may saturate the enzyme
Reaction Rate
Factors Affecting Enzyme
Activity
[E]
Factors Affecting Enzyme
Activity
– Cofactors - additional
substances needed to
catalyze reactions
• Inorganic ions
• Coenzymes - organic
cofactors (vitamins)
– Inhibitors – substances
that bind to enzyme and
reduce its catalytic activity
Reaction rate
• Concentration of
Cofactors/Inhibitors
[inhibitor]
[cofactor]
Factors Affecting Enzyme
Activity
• Temperature
–  Temp,  kinetic energy,
 reaction rate
– high Temp changes
structure of enzymes
• ’s enzyme function
Factors Affecting Enzyme
Activity
• pH
– 3D structure of enzymes
changes at different pH
– optimal enzyme function at
specific pH
–  function at higher or
lower pH’s
Metabolism
• Metabolism
– energetic sum of all chemical reactions
occurring in a living organism
– break down of some substances (catabolism)
– build up of other substances (anabolism)
• Enzymes facilitate and control metabolism
Metabolism
• Metabolic Pathways
– long series of enzyme-catalyzed reactions
– allow high degree of control + regulation
– Unfavorable – would gain free
energy as a result of the reaction
• To drive an energetically
unfavorable reaction, it must be
coupled to an energetically
favorable one
• Cells require a supply of free
energy (fuel) to drive biochemical
reactions
Reaction B:
Lose free energy
• Enzymes facilitate reactions, but
cannot drive energetically
unfavorable reactions
Reaction A:
Gain free energy
How Does a Cell Maintain Order?
Reaction A + Reaction B =
OVERALL LOSS OF FREE ENERGY!
Adenosine Triphosphate
• Energy carrier molecule
ATP  ADP + Pi + energy
• Energy used to drive
biochemical reactions