Concept 8.4: Enzymes speed up metabolic reactions by lowering energy barriers Flashcards
is a macromolecule that acts as a catalyst
enzyme
a chemical agent that speeds up a reaction without being consumed by the reaction.
catalyst
Every chemical reaction between molecules involves both bond breaking and .
bond forming
To reach the contorted state where bonds can change, reactant molecules must absorb
energy from their surroundings.
When the new bonds of the product molecules form, energy is released as heat, and the molecules return to stable shapes with lower
energy than the contorted state.
The initial investment of energy for starting a reaction—the energy required to contort the reactant molecules so the bonds can break—is known as the
free energy of activation, or activation energy abbreviated Eᴀ
We can think of activation energy as the amount of energy needed to push the reactants to the top of an energy barrier, or uphill, so that the
“downhill” part of the reaction can begin.
Activation energy is often supplied by heat in the form of
thermal energy that the reactant molecules absorb from the surroundings
The absorption of thermal energy accelerates the reactant molecules, so they
collide more often and more forcefully.
When the molecules have absorbed enough energy for the bonds to break, the reactants are in an unstable condition known as the
transition state
learn Figure 8.13 Energy profile of an exergonic reaction.
Proteins, DNA, and other complex cellular molecules are rich in free energy and have the potential to
decompose spontaneously; that is, the laws of thermodynamics favor their breakdown
high temperature denatures proteins and
kills cells.
a process by which a catalyst (for example, an enzyme) selectively speeds up a reaction without itself being consumed.
catalysis
an enzyme cannot change the ∆G for a reaction; it cannot make an endergonic reaction
exergonic.
for any reaction to occur, even a downhill reaction, some energy must be
added to get the reaction going
this energy is needed to break bonds in the
reactant molecules
the energy needed to start a chemical reaction is called the
energy of activation (E sub A)
this required energy input represents a barrier that prevents even energy-releasing exergonic reactions from occurring without
some added energy
serves as a biological catalyst, increasing the rate of a reaction without being changed into a different molecule
an enzyme
an enzyme does not add energy to a reaction: it
speeds up a reaction by lowering the energy barrier
an enzyme is very selective, its three-dimensional shape allows it to act on specific molecules, referred to as the
enzyme’s substrates
as the substrates bind to the enzyme’s active site they are held in a position that facilitates the
reaction
this takes less activation energy than the
unaided reaction
product form and are
released
the enzyme emerges
unchanged from the reaction
because of the specific fit between enzyme and substrate, each enzyme can catalyze
one kind of reaction involving specific substrates
thousands of different enzymes may be required to carry out all of a
cells metabolic processes
The reactant an enzyme acts on is referred to as the
enzyme’s substrate
The enzyme binds to its substrate (or substrates, when there are two or more reactants), forming an
enzyme-substrate complex
While enzyme and substrate are joined, the catalytic action of the enzyme converts the substrate to the
product (or products) of the reaction
Most enzyme names end in
-ase.
most enzymes are proteins, and proteins are macromolecules with unique
three-dimensional configurations
The specificity of an enzyme results from its shape, which is a consequence of its
amino acid sequence.
is typically a pocket or groove on the surface of the enzyme where catalysis occurs
active site
Usually, the active site is formed by only a few of the enzyme’s amino acids, with the rest of the protein molecule providing a framework that determines the
shape of the active site.
The specificity of an enzyme is attributed to a complementary fit between the shape of its active site and the shape of the
substrate.
enzymes (and other proteins) seem to “dance” between subtly different shapes in a
dynamic equilibrium, with slight differences in free energy for each “pose.
As the substrate enters the active site, the enzyme changes shape slightly due to interactions between the substrate’s chemical groups and
chemical groups on the side chains of the amino acids that form the active site.
The tightening of the binding after initial contact—called
induced fit
Induced fit brings chemical groups of the active site into positions that enhance their ability to catalyze the
chemical reaction.
In most enzymatic reactions, the substrate is held in the active site by so-called weak interactions, such as
hydrogen bonds and ionic bonds.
The R groups of a few of the amino acids that make up the active site catalyze the conversion of substrate to product, and the
product departs from the active site.
The entire cycle happens so fast that a single enzyme molecule typically acts on about
1,000 substrate molecules per second
although every enzyme performs a different reaction on different substrates they all go through the same general steps in accomplishing their reaction. these general steps include
substrate binding, formation of the enzyme-substrate complex, product formation and dissociation, and enzyme recovery
like all catalysts, enzymes are unchanged by the
overall reaction
first step in any enzymatic reaction is the
binding of the substrates by the enzyme
the enzyme includes an active site that can accommodate the
size, shape, and electron configuration of its substrate or substrates
according to the induced-fit model of enzyme-substrate interaction, a perfect fit does not form until the
enzymes and substrates bind to form an enzyme-substrate complex
after the substrate has bound in the active site, the
enzyme-substrate complex is formed
the shape of the enzyme may change to allow for an efficient chemical reaction, and amino acids may accept or donate protons or electrons to facilitate the
chemical reaction
once the enzyme-substrate complex has formed and any reactive groups have moved to the right position, the reaction proceeds and products are
formed
the product dissociate rapidly, leaving an empty
active site
after the products leave the active site, the enzyme
returns to its original configuration
any groups that donated or accepted protons or electrons are also
returned to their original state
like all catalysts, the enzyme is unchanged by the
overall reaction
Most metabolic reactions are reversible, and an enzyme can catalyze either the forward or the reverse reaction, depending on which direction has a negative
∆G
This in turn depends mainly on the
relative concentrations of reactants and products
The net effect is always in the direction of
equilibrium.
Enzymes use a variety of mechanisms that
lower activation energy and speed up a reaction
When there are two or more reactants, the active site provides a template on which the .
substrates can come together in the proper orientation for a reaction to occur between them
The more substrate molecules that are available, the more frequently they access the active sites of the .
enzyme molecules
there is a _______ to how fast the reaction can be pushed by adding more substrate to a fixed concentration of enzyme
limit
As soon as the product exits an active site, another
substrate molecule enters
At this substrate concentration, the enzyme is said to be ____________, and the rate of the reaction is determined by the speed at which the active site converts substrate to product.
saturated
When an enzyme population is saturated, the only way to increase the rate of product formation is to
add more enzyme.
Cells often increase the rate of a reaction by producing more
enzyme molecules.
learn scientific skills exercise
The activity of an enzyme—how efficiently the enzyme functions—is affected by general environmental factors, such as
temperature and pH.
the rate of an enzymatic reaction increases with increasing temperature, partly because substrates collide with active sites more frequently when the
molecules move rapidly
Above that temperature, however, the speed of the enzymatic reaction
drops sharply.
Most human enzymes have optimal temperatures of about
35–40°C
The thermophilic bacteria that live in hot springs contain enzymes with optimal temperatures of
70°C or higher
The optimal pH values for most enzymes fall in the range of
pH 6–8, but there are exceptions.
Many enzymes require nonprotein helpers for catalytic activity, often for chemical processes like electron transfers that cannot easily be carried out by the amino acids in proteins. These adjuncts, called _________ may be bound tightly to the enzyme as permanent residents, or they may bind loosely and reversibly along with the substrate.
cofactors
The cofactors of some enzymes are inorganic, such as the metal
atoms zinc, iron, and copper in ionic form.
If the cofactor is an organic molecule, it is referred to, more specifically, as a
coenzyme
reduce the productivity of enzymes by blocking substrates from entering active sites.
competitive inhibitors
watch and learn animation enzymes: competitive inhibition and animation enzymes: noncompetitive inhibition
do not directly compete with the substrate to bind to the enzyme at the active site
noncompetitive inhibitors
they impede enzymatic reactions by binding to another part of the
enzyme
