Bio 211 Intro Molecular Cell Biology
Lecture 7 and Lecture 8 “Enzymes”
Reading: Campbell Chap. 6 pp. 91-97
In the last lecture, we looked at the role of energy in the cell. Cells follow
physical and chemical laws in their behavior: energy is conserved through biological
processes although the products of metabolism produce altered forms of energy such as
heat. The energy of cells is mostly in the form of chemical energy especially as ATP. In
today’s lecture, we will look at the role of specialized proteins called enzymes in the cell.
Outline:
1.
2.
3.
4.
Role of enzymes
Substrate binding
Effects on enzyme activity
Enzyme regulation
Enzymes are catalysts, substances that speed up the rate of a reaction. They are
nearly always proteins, although there are some enzymes such as the one that makes
proteins (peptidyl transferase) which are catalytic RNAs. Enzymes like other catalysts are
not themselves used up by the reaction.
How do enzymes speed up the rate of the reaction?
Reactions involve the breaking and reforming of chemical bonds. The breaking of
bonds needs energy to get started even if the overall process is energetically favorable.
The energy needed to get started is called the “activation energy”.
Activation energy = EA
Let’s look at the energy changes that take place in a given chemical reaction. (Fig.
6.9) Given a hypothetical reaction
AB
+
CD
--->
AC
+
BD
In this reaction, the reactants (substrates) swap pieces with each other to give the
products. The bonds of the reactants break only when the molecules have absorbed
enough energy to be unstable.
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The activation energy represents the uphill portion of the graph.
As the reactants absorb energy, they become unstable. This is the “transition
state”. As new bonds are formed, energy is released into the surroundings. This is the
downhill portion of the curve which indicates a loss of free energy by the products. The
difference in free energy of the products vs. the reactants is G for the reaction which is
negative for an exergonic reaction.
To overcome the activation energy requires energy input. For some chemical
reactions, this can be accomplished by heating. Cells can be damaged by heat, therefore
they use enzyme catalysts to lower the activation energy.
An enzyme cannot change the G for a reaction. They speed up reactions that
would have occurred anyway, but perhaps at an extremely slow rate.
Fig. 6.10 shows how enzymes lower the EA without affecting G for the reaction.
2. Substrate binding
Enzyme catalyzed reactions can be summarized as follows:
enzyme
substrate
---->
products
substrate = reactant(s) acted on by an enzyme
Example
sucrose
+
H2O
sucrase
---->
substrates
glucose
+
products
2
fructose
Enzymes usually are highly specific for their substrates. The region of the enzyme
responsible for binding to substrate is called the “active site”.
The active site is typically a pocket or groove created by some of the amino acids
of the enzyme.
Fig. 6-11 (or equivalent) shows binding of the substrate (glucose) to the groove
that makes up the active site of hexokinase.
Models that explain different aspects of substrate binding
1) lock and key
--good for explaining specificity
--doesn’t explain why substrates that resemble the transition state are the best fit
2) induced fit
--Substrate induces a conformational change that creates better fit of enzyme to
substrate and places bonds in key positions to be acted on by the enzyme
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.
Key features of enzyme/substrate interations:
--Substrate is bound to the enzyme by many weak interactions (hydrogen bonds,
ionic bonds, hydrophobic interactions).
--Catalysis is begun by amino acids making up the active site of the enzyme; R
groups are usually crucial in catalysis.
--Enzymes can catalyze thousands of reactions per second.
--The more substrate available, the more rapid the reaction, until the enzyme is
saturated by substrate.
How do we measure the rate of an enzyme catalyzed reaction?
Mix
--enzyme
--substrate (vary)
--buffer (maintain correct pH for enzyme activity)
Measure amount of product produced.
3. Factors that affect enzyme activity
a. Temperature: Most enzymes have optimal activity at body temperature (37C).
Enzymes from thermophilic organisms may have optimal activity at 70C. Fig.
6.13 (a)
b. pH: Most enzymes have optimal activity around pH 7.
Pepsin, digestive enzyme of stomach works best in the acid environment of the
stomach, pH 2.0. Fig. 6.13 (b)
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c. cofactors: Ions or coenzymes that are required by the enzyme for catalytic
activity.
d. inhibitors: Chemicals that slow or stop the action of enzymes. Fig. 6.14
compares two types of inhibitors.
competitive inhibitors: Bind to active site, prevent access of substrate to active site.
noncompetitive inhibitors: Bind to enzyme somewhere other than the active site.
4. Enzyme regulation
Allows cell to respond to environmental signals and slow down or speed up steps
in metabolism.
allosteric regulation: Binding of a small molecule to site other that the active site which
changes the enzyme conformation.
Fig. 6.15 gives example
feedback inhibition: When end product in reaction signals to enzyme at beginning of
pathway to slow down.
A ---> B ---> C ---> D
D is a feed back inhibitor of enzyme that converts A to B
Summary: Today our topic has been enzymes. Enzymes are mostly protein catalysts that
speed up the rate of a chemical reaction in cells. Enzymes must be in their proper
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conformation to bind substrate and catalyze reactions. Changes in pH or temperature or
presence of chemical inhibitors can inactivate enzymes. In the next group of lectures, we
will observe the critical roles of enzymes in carrying out metabolism.
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