Transcript Enzymes

Lecture :ch2–sec4
Chemical Reactions & Enzymes
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2–4 Chemical Reactions and Enzymes
Chemical Reactions
Chemical Reactions
A chemical reaction is a process that changes
one set of chemicals into another set of chemicals.
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2–4 Chemical Reactions and Enzymes
Chemical Reactions
Chemical reactions always involve changes
in the chemical bonds that join atoms in
compounds.
–Breaking bonds releases energy
–Forming bonds stores energy
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2–4 Chemical Reactions and Enzymes
Energy in Reactions
•In order to stay alive, organisms need to carry out
reactions that require energy.
•Because matter and energy are conserved in
chemical reactions, every organism must have a
source of energy to carry out chemical reactions.
•Plants get their energy from the sun.
•Animals get their energy from eating plants or other
animals.
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2–4 Chemical Reactions and Enzymes
Energy in Reactions
Activation Energy
●Chemical reactions
that release energy do
not always occur
spontaneously.
●Chemists call the
energy that is needed to
get a reaction started
the activation energy.
Activation Energy
Barrier
Reactants
Products
Jumping bean analogy
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2–4 Chemical Reactions and Enzymes
Energy in Reactions
Activation energy is a factor in whether the overall
chemical reaction releases energy or absorbs
energy.
Ex: Photosynthesis
Ex: Cellular respiration
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2–4 Chemical Reactions and Enzymes
Enzymes
Why are enzymes important to living
things?
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2–4 Chemical Reactions and Enzymes
Enzymes
Enzymes
●Some chemical reactions that make life possible
are too slow or have activation energies that are
too high to make them practical for living tissue.
●These chemical reactions are made possible by
catalysts (biological catalysts are Enzymes).
How: Lower Activation energy
Result: Speed up rate of reaction
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2–4 Chemical Reactions and Enzymes
An enzyme can decrease the energy of activation
which speeds up the how fast the products are
produced
Enzyme
EA
Reactants
Products
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Figure 5.5A
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2–4 Chemical Reactions and Enzymes
Enzymes
Lowering the activation energy has a dramatic effect
on how quickly the reaction is completed.
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2–4 Chemical Reactions and Enzymes
Enzymes
Enzymes are very specific, generally catalyzing only
one chemical reaction.
For this reason, part of an enzyme’s name is usually
derived from the reaction it catalyzes.
• Ex:
Amylase breaks down amylose ( as type of
starch found in flour)
• Amylose (starch)
amylase
lots of glucose
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2–4 Chemical Reactions and Enzymes
Enzymes
• Ex: sucrase breaks down sucrose (table sugar)
Substrate
Enzyme
Products
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2–4 Chemical Reactions and Enzymes
• Exception:
Enzymes
Catalase breaks down Hydrogen peroxide
2 Hydrogen peroxide 
2 water
+
oxygen
+
heat
energy
Catalase
Oxygen gas – O2
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2–4 Chemical Reactions and Enzymes
Enzyme Action
Enzyme Action
●For a chemical
reaction to take place,
the reactants must
collide with enough
energy so that existing
bonds will be broken
and new bonds will be
formed.
●If reactants do not
have enough energy,
no reaction will take
place.
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Activation Energy
Barrier
Reactants
Products
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2–4 Chemical Reactions and Enzymes
Enzyme Action
The Enzyme-Substrate
Complex
Enzymes provide a site
where reactants can be
brought together to react,
reducing the energy
needed for reaction.
The reactants of enzymecatalyzed reactions are
known as substrates.
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2–4 Chemical Reactions and Enzymes
Enzyme Action
The substrates
glucose and ATP bind
to the active site on
the enzyme,
hexokinase, forming
an enzyme-substrate
complex.
The fit is so precise
that the active site
and substrates are
often compared to a
lock and key.
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2–4 Chemical Reactions and Enzymes
Enzyme Action
The enzyme and substrates remain bound together
until the reaction is done and the substrates are
converted to products.
The products of the reaction are released and the
enzyme is free to start the process again.
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2–4 Chemical Reactions and Enzymes
Enzyme Action
An Enzyme-Catalyzed Reaction
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2–4 Chemical Reactions and Enzymes
Enzyme Action
Regulation of Enzyme
Activity
pepsin
trypsin
Enzymes can be affected
by any variable that
Enzymatic
influences a chemical
Activity 
reaction.
●Many enzymes are affected by
changes in temperature.
●Enzymes work best at certain
pH values.
1
2
3
4
5
6 7
pH
8
9
10
●Enzymes work best at certain
ionic (salt) concentrations
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2–4 Chemical Reactions and Enzymes
Enzyme Action
Regulation of Enzyme Activity
Temperature is a measurement of
the average amount of heat/
movement a group of molecules
has
–Too Cold: molecules move too
slow  decrease reaction rate
●Ex: enzyme responsible for
producing key tomato flavor
molecule stops working below
55 deg. F
–Too Hot: weak bonds between
amino acids of the enzyme break
 denatured enzyme!!!
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2–4 Chemical Reactions and Enzymes
Enzyme Action
Regulation of Enzyme Activity
pH measures how acidic or basic the environment is
Basic:
pH greater than 7 -14 [Less H+]
neutral: pH 7 (pure water and most cells) [H+ = OH-]
Acidic:
pH less than 7
[more H+]
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2–4 Chemical Reactions and Enzymes
Enzyme Action
Regulation of Enzyme Activity
Altered concentration of H+ interfere with amino acid
chain folding of the enzyme (remember enzymes are
proteins!)
H+
H+
H+
H+
H+
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2–4 Chemical Reactions and Enzymes
Enzyme Action
Regulation of Enzyme Activity
Enzymes work best at certain ionic (salt) concentrations
Ex: Increased Na+ Cl- (table salt)
ClClNa+
Na+
Cl-
Na+
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2–4 Chemical Reactions and Enzymes
Enzymes
Cells can regulate
the activities of
enzymes.
•Most cells contain
proteins that help
to turn key
enzymes “on” and
“off” at critical
stages in the life of
the cell.
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2–4 Chemical Reactions and Enzymes
Enzymes
Enzymes play essential roles in:
regulating chemical pathways.
making material that cells need.
• Many antibiotics interfere with bacteria running
necessary chemical pathways including stopping the
enzymes that bacteria use to make their protective cell
walls.
releasing energy.
• Carbon monoxide poisoning stops cell’s enzymes from
getting energy out of glucose
transferring information.
• Pesticides stop enzymes that run insects nervous
system
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