Enzymes are biological catalysts that increase the rate of biochemical reactions by lowering the energy of activation. They are typically proteins that bind specifically to substrates and induce a shape change that facilitates the chemical reaction. This induced fit allows the enzyme to reduce the energy needed for the reaction to occur and be used repeatedly without being consumed in the process. Changing conditions like temperature, pH, substrate concentration, or adding inhibitors can impact the reaction rate by altering how well the enzyme and substrate interact.
2. Energy of Activation
• If you mix two moles of hydrogen gas H2 with one
mole of oxygen gas-nothing happens.
• If you add a spark to the container, the following
reaction occurs. KABOOM
• 2H2 + O2 2 H2O G= -58 kcal/mole
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In order for water to be
produced H2 must become 2H
and the O2 must become 2O
as this frees up the electrons
tied up in covalent bonds, to
form chemical bonds forming
water, H2O.
3. Energy of Activation
• The energy used
to break the
bonds in the
reactants so they
can be reformed
in the products is
called the energy
of activation.
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4. Enzymes
• Enzymes are biological catalysts that increase
the reaction rate of biochemical reactions.
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Characteristics of
enzymes
A. Made of proteins (or
RNA).
B.They are very specific
and only work with a
certain set of reactants
or substrates that fit on
their active site.
The enzyme shown is lysozyme
5. Induced Fit
C. Enzymes can be used over and over again.
D. When an enzyme binds with the substrate, the
substrate interacts with the enzyme causing it to
change shape. This change in shape facilitates
the chemical reaction to occur. This is called
the induced fit.
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6. Enzyme Example Ribonuclease
• Ribonuclease
decomposes RNA,
and the nucleotides
can be recycled.
• The purple part is the
enzyme; the green
part is the substrate
(RNA).
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7. Enzymes Work by Lowering the Energy
of Activation
E. Enzymes increase the reaction rate by
lowering the energy of activation. They
do NOT change Gibbs free energy or G.
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10. Initial Velocity
• The reaction rate of an enzymatic reaction is always
fastest at the beginning of the reaction when there is
the greatest concentration of substrate. Why?
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As the hydrogen gas and oxygen gas bonds are broken and new ones are formed the system has a net loss of free energy. In a system, such as the Hindenburg, that energy is released as heat and light. However, unlike the Hindenburg, where a spark provided the energy of activation, the combining of hydrogen and oxygen to make water during cellular respiration cannot rely on heat for the energy of activation. Heat energy would cause molecules such as proteins to decompose.
Cells must rely on catalysts, which are molecules that speed up a chemical reaction without being consumed in the chemical reaction. Most inorganic catalysts provide a surface on which the chemical reaction can take place and thereby lower the amount of activation energy required to a more cell friendly quantity. Organic catalysts come in the form of proteins (or RNA molecules).
Most enzymes are proteins, but recently it had been discovered that certain molecules of RNA can also have enzymatic properties.
Terms
Substrate- These are reactants that interact with the enzyme during a biochemical reaction.
Active site- This is the part of the enzyme actually involved in the chemical reaction.
Note that names of molecules ending in the suffix –ase are enzymes.
The graph represents the amount of product formed. At first, the amount of product formed increases, then the rate slows down as the concentration of substrate decreases. The fastest rate of product formation is at the beginning and is called the initial velocity. Ask students to explain why, logically, this has to be true. (i.e. more likely to get an effective collision between enzyme and substrate molecules if there are more substrate molecules present; as in the beginning of the reaction.) If the enzyme is kept constant and there is an increase in the amount of substrate, there will be an increase in the initial velocity until a saturation point is reached.
The previous experiment is repeated with the same amount of enzyme but increasing amount of substrate. The initial velocities increase because there is more substrate to attach the enzyme ,but eventually there is no increase in initial velocity because all of the enzymes have become saturated with substrate.
If the enzyme is kept constant and there is an increase in the amount of substrate, there will be an increase in the initial velocity until a saturation point is reached.
There are many factors that can affect the reaction rates of enzymes.
Temperature- at first an increase in temperature will increase the reaction rate because of the kinetics of the reaction, but after a certain temperature is reached, the hydrogen bonds fall apart and the enzyme will denature. Notice that with thermophiles natural selection has favored enzymes that can tolerate higher temperatures. For these bacteria, the optimum temperature is 70o C for most of their enzymatic reactions.
2. pH can also affect the reaction rates. Most enzymes work best at a range of 6 to 8, but there are some exceptions, such as pepsin. If the environment changes much from the optimum pH, again hydrogen bonds are affected, denaturing the enzyme. Notice that this is more of a bell shaped curve because both an increase and a decrease in pH from the optimum can denature the enzyme. Sometimes the hydrogen bonds that are affected are not at the active site and therefore the enzyme will still work. As the pH moves further from the optimum pH, it increases the likelihood that the active site is disrupted.
Enzyme inhibitors- Some chemicals inhibit the action of an enzyme.
A competitive inhibitor is a molecule that resembles the substrate enough that it can bind to the active site in place of the substrate. This will slow down the reaction rate as a certain percentage of the enzyme will combine with the inhibitor.
A noncompetitive inhibitor is one that does not bind to the receptor site but to some other place on the molecule causing a conformational change in the enzyme (protein). This causes the active site to change shape so that substrate cannot bind. This also slows down the reaction rate.
4. Allosteric Regulation-= regulation by changing the structure of a molecule (allosteric = Greek for “different structure”). Such enzymes have two or more polypeptide chains each with its own active site. This enzyme also has two conformations-one with a functional active site and the other with a nonfunctional active site. This enzyme also has a place for the binding of an activator and an inhibitor. The activator will stabilize the conformation with the functional active site and the inhibitor will stabilize the inactive form of the enzyme.
5. Feedback inhibition- Usually enzymes work in biochemical pathways in which there are a series of intermediate chemical reactions that occur in order to get from point A (reactants) to point B (products). It is not unusual for an end product to act as an inhibitor to shut down the pathway when there is sufficient product present.
6. Cooperativity- This occurs when there are two or more sub-units to an enzyme, each with its own active site. When one substrate binds with one active site, the enzyme changes conformation so that the binding of other substrates are easier to achieve.
