Ch 6 Enzymes- The Catalysts of Life

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In general, enzymes are what kinds of molecules?
nucleic acids


Enzymes work by _____.
increasing the potential energy difference between reactant and product
decreasing the potential energy difference between reactant and product
adding energy to a reaction
reducing EA
adding a phosphate group to a reactant

Enzymes work by reducing the energy of activation.

An enzyme _____.
is a source of energy for endergonic reactions
can bind to nearly any molecule
is an organic catalyst
is a inorganic catalyst
increases the EA of a reaction

-is an organic catalyst -Enzymes are proteins that behave as catalysts.

What name is given to the reactants in an enzymatically catalyzed reaction?
active sites


As a result of its involvement in a reaction, an enzyme _____.
is used up
loses energy
loses a phosphate group
permanently alters its shape.
is unchanged

is unchanged

In a graph of energy vs reactants and products, what part of the parabola is found the energy of activation?

it is found between the reactant and the top or cuspid of the curve

what must be overcome in order for a reaction to proceed?

The energy of activation must be overcome in order for a reaction to proceed.

exergonic reaction

A spontaneous chemical reaction, in which there is a net release of free energy.

If exergonic reactions occur spontaneously, what keeps molecules from breaking apart and cell chemistry from racing out of control? (include what happens within the reaction to make it happen)

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 (EA). This required energy input represents a barrier that prevents even energy-releasing exergonic reactions from occurring without some added energy.

How does a living cell overcome the energy barrier so that its metabolic reactions can occur quickly and precisely?

A special kind of protein called an enzyme is the answer.

An enzyme serves as a _(role), _(doing what) without _(what happens to it during reaction). An enzyme does not _(what does not do to make a reaction to happen); instead, it _(what it does to make a reaction to happen)

An enzyme serves as a biological catalyst, increasing the rate of a reaction without being changed into a different molecule. An enzyme does not add energy to a reaction; instead, it speeds up a reaction by lowering the energy barrier.

an enzyme is_(selective vs nonselective)

very selective

why an enzyme is so specific?

Its three-dimensional shape allows it to act only on specific molecules,


The reactant on which an enzyme works

active site

The specific portion of an enzyme that attaches to the substrate by means of weak chemical bonds.

A plot of enzyme velocity against temperature for an enzyme indicates little activity at 0 degrees Celsius and 40 degrees Celsius, with peak activity at 35 degrees Celsius. The most reasonable explanation for the low velocity at 0 degrees Celsius is that __________.
the enzyme was denatured at this temperature
substrate binding at the active site is thermodynamically unfavorable at low temperature
there is too little activation energy available from the environment
the hydrogen bonds that define the enzyme’s active site are broken as temperature increases

there is too little activation energy available from the environment

how many active sites are found in an enzyme? what happens here?

In general, an enzyme has one active site at which catalysis can occur. When the substrates are bound to the active site, the enzyme will catalyze the reaction.

what happens to enzymes as the substrate concentration increases?

As the concentration of substrate increases, the reaction rate increases, until the point where the active site is saturated with substrate.

what happens to the rate of a reaction involving an enzyme when saturation is reached?

When the enzyme is saturated, the rate of the reaction will not increase with the concentration of substrates.

In a graph of reaction rate vs substrate concentration, at what point does the reaction rate remains constant?

-when saturation point happens, the point where there is a high substrate concentration -when the hyperbola is a horizontal line

when saturation is reached, the reaction rate is independent of what?

he reaction rate is independent of substrate concentration.

In a graph of reaction rate vs substrate concentration, In which region is the enzyme saturated with substrate?

-in the region where there is a high substrate concentration and the hyperobola begins to be a horizonal line

Consider a situation in which the enzyme is operating at optimum temperature and pH, and has been saturated with substrate. What is your best option for increasing the rate of the reaction?
Increase the pH.
Increase the temperature.
Increase the enzyme concentration.
Increase the substrate concentration.

Increase the enzyme concentration.

enzyme inhibitors.

Molecules other than substrates bind to enzymes. Some of these other molecules slow down the rate of the enzymatic reaction.

what are the 2 classifications of enzyme inhibitors?

-irreversible -reversible

what are the two types of reversible enzyme inhibitors?

-competitive -non-competitive

A _inhibitor has a structure that is so similar to the substrate that it can bond to the enzyme just like the substrate

competitive inhibitor

a_inhibitor binds to a site on the enzyme that is not the active site


usually, an _inhibitor forms a covalent bond with an amino acid side group within the active site, which prevents the substrate from entering the active site or prevents catalytic activity


the competitive inhibitor competes with the substrate for the_on an enzyme

active site

when the noncompetitive inhibitor is bonded to the enzyme, the shape of the _is distorted


enzyme inhibitors disrupt normal interactions between an enzyme and its_


how can you decrease the effect of the competitive inhibitors?

they can be outcompeted by adding extra substrate

what noncompetitive inhibitors do?

Noncompetitive inhibitors do not compete for the active site, but inhibit the enzyme by binding elsewhere and changing the enzyme’s shape

what irreversible inhibitors do?

Irreversible inhibitors bind directly to the active site by covalent bonds, which change the structure of the enzyme and inactivate it permanently

what is an example of competitive inhibitors?

-Most medications are enzyme inhibitors of one kind or another. -drugs like antibiotic penicillin (which inhibits an enzyme involved in bacterial cell-wall synthesis) and aspirin (which inhibits cyclooxygenase-2, the enzyme involved in the inflammatory reaction)

You have an enzymatic reaction proceeding at the optimum pH and optimum temperature. You add a competitive inhibitor to the reaction and notice that the reaction slows down.
What can you do to speed the reaction up again?

Add more substrate; it will outcompete the inhibitor and increase the reaction rate.

The phosphorylation of glucose to generate glucose-6-phosphate is catalyzed by the enzyme hexokinase. This enzyme, however, is allosterically inhibited by its own product, glucose-6-phosphate. This is an example of __________.

feedback regulation

feedback inhibition is also known as_

end-product inhibition.

Which of the following best describes a metastable state?

The metastable state is a state of the substrate in which the reaction can proceed but typically requires a catalyst.

The site on an enzyme that will bind the substrate is called the
activation site.
active site.
metastable site.
prosthetic group.

active site.

Which of the following is an enzyme?
N-acetylmuramic acid
carboxypeptidase A

carboxypeptidase A

As new enzymes are discovered, the EC system for naming enzymes is to be used. The names are to be based on which of the following criteria?
the size of the enzyme
a description of substrate function
the six major classes of enzyme function
an indication of the size of the substrate
the name of the substrate

the six major classes of enzyme function

Substrate activation may involve (4)

donation of protons to the enzyme. a change in enzyme conformation induced by substrate binding. formation of temporary covalent bonds. accepting protons from the enzyme.

The induced-fit model
involves a conformational change in the shape of the enzyme.
was proposed by Hans Buchner.
proposes that very strong covalent bonds are formed upon substrate binding.
states that enzyme-substrate interactions are rigid.
is also called the lock-and-key model.

involves a conformational change in the shape of the enzyme.

Enzyme regulation may occur by several methods. Which of the following is not a means of enzyme regulation?
allosteric regulation
substrate-level phosphorylation
covalent modification
feedback inhibition


Covalent modification
affects the activity of an enzyme by adding or removing a chemical group.
produces modifications that can sometimes be reversed.
can involve the addition of phosphate groups.
can activate an enzyme.
all of the above

all of the above

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