Download Enzyme Review Sheet

Review Questions
Enzymes
1. Define reactant and product.
A reactant is a substance that enters into and is altered in the course of a
chemical reaction.
A product is a substance resulting from a chemical reaction.
Na + Cl =
Reactants
NaCl
Product
2. How does energy flow in chemical reactions? Compare and contrast exergonic
and endergonic reactions.
Potential energy is stored in the chemical bonds between atoms and molecules.
In chemical reactions, energy can be stored or released. Exergonic reactions are
chemical reactions where energy is released. Exergonic is Greek for “energy
out”. The products of an exergonic reaction will always have less energy than the
reactants. Any hydrolytic breakdown is exergonic. Aerobic cellular respiration,
for example, starts with a high energy reactant, glucose, plus a little oxygen, then
dismantles the sugar piece by piece releasing the stored energy, and ending up
with the low energy products, C02 and H2O.
C6H1206 + O2 → CO2 + H2O + energy
Endergonic reactions require energy. Endergonic means “energy in” in Greek.
The products of an endergonic reaction will always have more energy than the
reactants. Any chemical reaction requiring dehydration synthesis is endergonic.
For example, photosynthesis is endergonic. Water mixed with CO2 (low energy
reactants) plus sunlight (provides energy) yields glucose (high energy product)
and O2. The energy from sunlight is stored as chemical bonds in a carbohydrate.
CO2 + H2O+Sunlight → C6H12O6 + O2
3. What is a coupled reaction? What provides the energy for endergonic
reactions?
Exergonic reactions provide the energy for endergonic reactions. Since one is
energy-dependent upon the other, we describe them as coupled reactions.
What drives the energy a plant uses to make sugar (endergonic reaction)? It is
an exergonic reaction in the sun (nuclear fusion) that provides the energy.
Nuclear fusion and photosynthesis are coupled reactions.
What provides the energy for making ATP? It is the exergonic reaction of
breaking down sugars in aerobic cellular respiration that provides the energy.
Aerobic cellular respiration and ATP synthesis are coupled reactions.
4. Describe activation energy
Every chemical reaction, regardless of whether it is exergonic or endergonic,
requires a small investment of energy to make it occur. This energy investment is
called activation energy. For example, in a hydrolysis reaction, a little bit of
energy is added to contort the starting molecule into an unstable state so the
reaction can proceed. In a synthesizing reaction, before the reactants can
combine, a little energy has to be added to overcome the mutual repulsion of the
negatively-charged electron clouds surrounding the atoms.
Activation energy is often supplied in the form of heat that the reactants absorb
from their environment. The chemical bonds will only break if they absorb enough
heat to become unstable.
Each reaction requires a specific amount of activation energy. Sometimes just
the room temperature or body temperature provides enough heat (activation
energy) to initiate a chemical reaction. These reactions are classified as having
“low activation energy”. The heat energy present in body temperature is not
enough for “high activation energy reactions”.
All of the chemical reactions required for life have high activation energies. This
means that none of life’s reactions can occur as is at moderate temperatures. So
how do cells get around this barrier? They can’t increase their temperature, the
heat will destroy their proteins. They have to use a catalyst.
5. What is a catalyst?
Catalysts are substances that reduce the activation energy barrier enabling the
reactants to absorb enough energy to reach the transition state at moderate
temperatures. Here are some general characteristics of a catalyst: a catalyst
speeds up chemical reactions, is independent and does not become part of the
product. Catalysts are recycled over and over and therefore only required in
small amounts.
6. How is an enzyme a biological catalyst?
Since all of life’s chemical reactions have high activation energies, they can’t get
started without some help. Living organisms reduce the activation energy
required for a chemical reaction to occur by using biological catalysts called
enzymes. Enzymes are specialized proteins that speed up a cell’s chemical
reactions. They don’t become part of the product, are constantly recycled, and
only required in small amounts. Enzymes are the resident chemists of cells. They
facilitate every chemical reaction that occurs inside a cell. There are thousands
of different enzymes in a cell. Each type of enzyme is very specific and works on
only one kind of reactant. There are enzymes that build compounds. There are
enzymes that transform compounds, and still others that break things down. We
could not live without enzymes.
7. Describe an enzymatic reaction.
An enzyme is a globular protein with a pocket or groove called an active site. The
substrate (the reactant) fits inside the active site. Each enzyme only holds only
one specific kind of substrate. They fit like a lock and key. Once inside the active
site, the enzyme and its substrate are called an enzyme-substrate complex. The
enzyme will often change shape slightly to hold the substrate more securely; a
process known as induced fit.
How does the enzyme reduce the activation energy? In synthesizing reactions,
the active site holds the reactants close together in the proper orientation for the
reaction to take place. An enzyme may stretch a reactant to make it easier to
break apart on a degradation reaction. Once the reaction is complete, the
enzyme releases the product and begins the process again. One enzyme
molecule often acts on a thousand substrates in a single second. Other enzymes
are much faster.
8. For enzymes, what is meant by the term “optimal conditions”?
Enzyme activity is measured by the reaction rate. The environmental conditions
in which an enzyme hits its highest reaction rate is called the enzymes “optimal
conditions”. For most human enzymes, the optimal conditions are body
temperature (37° C) and a neutral pH.
On the other hand, the enzymes of thermophilic bacteria stop functioning at 37°
C. Their optimal temperature condition is close to boiling. Pepsin, a proteindigesting enzyme in our stomach, works best in acidic conditions. A neutral pH
will shut it down.
9. How do high temperatures, extremes in pH and high salt concentrations affect
enzymes?
As the temperature heats up above optimal conditions, there is a fairly sharp
drop-off in enzyme activity. At high temperatures, the enzyme not only stops
working but also unravels. High temperatures break the chemical bonds holding
the protein’s conformation together. The enzyme unravels and loses it active site.
This is often irreversible.
Strong acids and bases will also unravel enzymes. The abundant charged ions in
strong acids and bases interfere with the chemical bonds in the conformation of a
protein. The protein unravels and the enzyme ceases to function. Likewise, the
sodium and chlorine ions in a highly saline solution disrupt chemical bonding in a
protein and cause it to change shape and lose its function.
10. Describe denaturation.
The technical term denaturation means the unraveling of a protein. Any kind of
protein (not just enzymes) can be denatured. High temperatures, extremes in pH,
and high salt concentrations denature proteins. Below are some examples of
denaturation:
Cooking an egg: Egg white is an amino acid storage protein called
ovalbumin. When heated, the egg white transforms from a clear viscous liquid
into an opaque solid. The ovalbumin was denatured.
High fevers: Body temps above 104° F for prolonged periods kill brain cells
by denaturing the proteins.
Marinating: You can tenderize meat by marinating it before cooking. All
marinades contain an acid (usually vinegar). The acid will denature the meat
proteins.
Ceviche: In this dish, seafood is cooked with an acid (lime juice) instead of
heat. The acid denatures the meat proteins.
Cheese: Milk is inoculated with lactic acid bacteria. The bacteria make lactic
acid that denatures casein (the protein in milk) making it curdle.
Permanent Wave: Hair is straightened or curled using an acid or a base. The
hair protein is denatured.
11. Why do our bodies need vitamins and trace minerals?
Most trace minerals and vitamins are required by the body as cofactors.
Cofactors are non-protein helpers that are attached onto an enzyme. Cofactors
activate enzymes. Without them many enzymes will not function. For example,
we have over 300 enzymes that require magnesium as a cofactor. Inorganic
cofactors are called “activators”. Organic cofactors (vitamins) are called
coenzymes.
12. What is an inhibitor?
Many of our drugs, poisons, and pesticides are enzyme inhibitors. An inhibitor is
any substance that slows or stops enzyme activity.
13. Describe the two kinds of enzyme inhibition.
Competitive inhibition occurs when an inhibitor (with a similar shape as the
normal substrate) competes with the normal substrate for the active site.
Noncompetitive inhibition occurs when an inhibitor binds to a place on the
enzyme away from the active site (called an allosteric site). The bonding of the
inhibitor causes the active site to change shape so it won’t accommodate the
normal substrate.
14. Give two examples of inhibitors.
The insecticide malathion is an inhibitor of acetylcholinesterase, the enzyme that
breaks down the neurotransmitter acetylcholine after a nerve impulse is passed
to a muscle. The neurotransmitter keeps working and so the muscle keeps
contracting. Critical muscles can’t relax. The insect dies from convulsions.
Penicillin, the first antibiotic, kills bacteria by inhibiting the enzyme bacteria use to
build cell walls.