Enzyme Regulation (Article) - Khan Academy

8/30/2021 Enzyme regulation (article) | Khan Academy
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Science · AP®︎/College Biology · Cellular energetics

· Environmental impacts on enzyme function
Enzyme regulation
Cofactors and coenzymes. Reversible, irreversible, competitive,
and noncompetitive inhibitors. Allosteric enzymes. Feedback
inhibition.
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Introduction
The cells of your body are capable of making many
different enzymes, and at first you might think:
great, let’s crank all of those enzymes up and
metabolize as fast as possible! As it turns out,
though, you really don’t want to produce and
activate all of those enzymes at the same time, or
in the same cell.
Needs and conditions vary from cell to cell and

change in individual cells over time. For instance,
stomach cells need different enzymes than fat
storage cells, skin cells, blood cells, or nerve cells.
Also, a digestive cell works much harder to process
and break down nutrients during the time that
follows a meal as compared with many hours after
a meal. As these cellular demands and conditions
https://www.khanacademy.org/science/ap-biology/cellular-energetics/environmental-impacts-on-enzyme-function/a/enzyme-regulation#:~:text=The co… 1/19

changes, so do the amounts and functionality of

different enzymes.
Because enzymes guide and regulate the

metabolism of a cell, they tend to be carefully
controlled. In this article, we’ll take a look at factors
that can affect or control enzyme activity. These
include pH and temperature (discussed in the
active site article), as well as:
Regulatory molecules. Enzyme activity may be

turned "up" or "down" by activator and
inhibitor molecules that bind specifically to the
enzyme.
Cofactors. Many enzymes are only active when

bound to non-protein helper molecules known
as cofactors.
Compartmentalization. Storing enzymes in

specific compartments can keep them from
doing damage or provide the right conditions
for activity.
Feedback inhibition. Key metabolic enzymes

are often inhibited by the end product of the
pathway they control (feedback inhibition).
In the rest of this article, we'll examine these

factors one at a time, seeing how each can affect
enzyme activity.

Regulatory molecules
Enzymes can be regulated by other molecules that
either increase or reduce their activity. Molecules
that increase the activity of an enzyme are called
activators, while molecules that decrease the
activity of an enzyme are called inhibitors.
There are many kinds of molecules that block or

promote enzyme function, and that affect enzyme
function by different routes.
Competitive vs. noncompetitive

In many well-studied cases, an activator or
inhibitor's binding is reversible, meaning that the
molecule doesn't permanently attach to the
enzyme. Some important types of drugs act as
reversible inhibitors. For example, the drug
tipranivir, which is used to treat HIV, is a reversible
inhibitor.1 It blocks activity of a viral enzyme that
helps the virus make more copies of itself.
Reversible inhibitors are divided into groups based

on their binding behavior. We won't discuss all of
the types here, but we will look at two important
groups: competitive and noncompetitive inhibitors.
An inhibitor may bind to an enzyme and block

binding of the substrate, for example, by
attaching to the active site. This is called
competitive inhibition, because the inhibitor
“competes” with the substrate for the enzyme.

That is, only the inhibitor or the substrate can

be bound at a given moment.
In noncompetitive inhibition, the inhibitor

doesn't block the substrate from binding to the
active site. Instead, it attaches at another site
and blocks the enzyme from doing its job. This
inhibition is said to be "noncompetitive"
because the inhibitor and substrate can both be
bound at the same time.
Competitive and non-competitive inhibitors can be

told apart by how they affect an enzyme's activity
at different substrate concentrations.
If an inhibitor is competitive, it will decrease

reaction rate when there's not much substrate,
but can be "out-competed" by lots of substrate.
That is, the enzyme can still reach its maximum
reaction rate given enough substrate. In that
case, almost all the active sites of almost all the
enzyme molecules will be occupied by the

substrate rather than the inhibitor.

[See a sports analogy]
If an inhibitor is noncompetitive, the enzyme-

catalyzed reaction will never reach its normal
maximum rate even with a lot of substrate. This
is because the enzyme molecules with the
noncompetitive inhibitor bound are "poisoned"
and can't do their job, regardless of how much
substrate is available.
On a graph of reaction velocity (y-axis) at different

substrate concentrations (x-axis), you can tell these
two types of inhibitors apart by the shape of the
curves:
_Image credit: "Enzymes: Figure 3," by OpenStax College, Biology, CC BY

3.0._
Not familiar with this type of graph? No worries!

The basics of enzyme kinetics graphs article has a
step-by-step walkthrough.
Allosteric regulation
Allosteric regulation, broadly speaking, is just any
form of regulation where the regulatory molecule
(an activator or inhibitor) binds to an enzyme
someplace other than the active site. The place
where the regulator binds is called the allosteric
site.
_Image modified from "Enzymes: Figure 4," by OpenStax College, Biology,

CC BY 3.0._
Pretty much all cases of noncompetitive inhibition

(along with some unique cases of competitive
inhibition) are forms of allosteric regulation.
However, some enzymes that are allosterically

regulated have a set of unique properties that set
them apart. These enzymes, which include some of
our key metabolic regulators, are often given the

name of allosteric enzymes2 . Allosteric enzymes
typically have multiple active sites located on
different protein subunits. When an allosteric
inhibitor binds to an enzyme, all active sites on the
protein subunits are changed slightly so that they
work less well.
There are also allosteric activators. Some allosteric

activators bind to locations on an enzyme other
than the active site, causing an increase in the
function of the active site. Also, in a process called
cooperativity, the substrate itself can serve as an
allosteric activator: when it binds to one active site,
the activity of the other active sites goes up.3 This
is considered allosteric regulation because the
substrate affects active sites far from its binding
site.
[Is hemoglobin an allosteric enzyme?]
Cofactors and coenzymes

Many enzymes don’t work optimally, or even at all,
unless bound to other non-protein helper
molecules called cofactors. These may be attached
temporarily to the enzyme through ionic or
hydrogen bonds, or permanently through stronger
covalent bonds.Common cofactors include
inorganic ions such as iron (Fe2+ ) and magnesium
(Mg2+ ). For example, the enzyme that builds DNA
molecules, DNA polymerase, requires magnesium

ions to function.4
Coenzymes are a subset of cofactors that are

organic (carbon-based) molecules. The most
common sources of coenzymes are dietary
vitamins. Some vitamins are precursors to
coenzymes and others act directly as coenzymes.
For example, vitamin C is a coenzyme for several
enzymes that take part in building the protein
collagen, a key part of connective tissue.
Image modified from OpenStax Biology.
Enzyme compartmentalization
Enzymes are often compartmentalized (stored in a
specific part of the cell where they do their job) --
for instance, in a particular organelle.
Compartmentalization means that enzymes needed
for specific processes can be kept in the places
where they act, ensuring they can find their

substrates readily, don't damage the cell, and have

the right microenvironment to work well.
For instance, digestive enzymes of the lysosome

work best at a pH around 5.0, which is found in the
acidic interior of the lysosome (but not in the
cytosol, which has a pH of about 7.2). Lysosomal
enzymes have low activity at the pH of the cytosol,
which may serve as "insurance" for the cell: even if
a lysosome bursts and spills its enzymes, the
enzymes will not begin digesting the cell, because
they will no longer have the right pH to function.5
Feedback inhibition of metabolic

pathways
In the process of feedback inhibition, the end
product of a metabolic pathway acts on the key
enzyme regulating entry to that pathway, keeping
more of the end product from being produced.
This may seem odd – why would a molecule want

to turn off its own pathway? But it’s actually a
clever way for the cell to make just the right
amount of the product. When there’s little of the
product, the enzyme will not be inhibited, and the
Science AP®︎/College
Biology Cellular pathway will go full steam ahead to replenish the
energetics Environmental supply. When there’s lots of the product sitting
impacts on enzyme function
around, it will block the enzyme, preventing the
Environmental impacts on
enzyme function production of new product until the existing supply
has been used up.
Competitive inhibition
Noncompetitive inhibition
Enzyme regulation
Basics of enzyme kinetics Image credit: OpenStax Biology.

graphs
Practice: Environmental Typically, feedback inhibition acts at the first

impacts on enzyme
function
committed step of the pathway, meaning the first
step that’s effectively irreversible. However,
Next lesson
feedback inhibition can sometimes hit multiple
points along a pathway as well, particularly if the
pathway has lots of branch points. The pathway
steps regulated by feedback inhibition are often
catalyzed by allosteric enzymes.6
For example, the energy carrier molecule ATP is an

allosteric inhibitor of some of the enzymes involved
in cellular respiration, a process that makes ATP to
power cellular reactions. When there is lots of ATP,
this feedback inhibition keeps more ATP from
being made. This is useful because ATP is an
unstable molecule. If too much ATP were made,
much of it might go to waste, spontaneously
breaking back down into its components (ADP and
Pi ).
ADP, on the other hand, serves as a positive

allosteric regulator (an allosteric activator) for some
of the same enzymes that are inhibited by ATP. For
instance, ADP may act by binding to an enzyme
https://www.khanacademy.org/science/ap-biology/cellular-energetics/environmental-impacts-on-enzyme-function/a/enzyme-regulation#:~:text=The c… 10/19
and changing its shape so that it becomes more

active.7
Thanks to this pattern of regulation, when ADP

levels are high compared to ATP levels, cellular
respiration enzymes become very active and will
make more ATP through cellular respiration.
[Attribution and references]
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Zhang, Luyan 3 years ago

more
Allosteric regulation confuses me a lot. I don't

really get it even after I watched the video on
Khan Academy (MCAT) . Can anyone explain it
to me briefly?
(7
Reply • Comment votes)
Upvote m
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tyersome 3 years ago

more
I'll try an analogy — let me know if this

helps.
Imagine that an enzyme is like tiny

sculpture made from a wire twisted
into a very complicated, but somewhat
loose structure.
The substrate is another much smaller
sculpture that fits into a gap in the first
sculpture — let's say it fits perfectly.
Now think of hanging a weight off

another part of the sculpture — the
whole structure shifts a bit under the
strain and now the substrate sculpture
doesn't fit! In this situation the weight
would be analogous to an allosteric
inhibitor.
You could also imagine a similar

scenario, but with the substrate fitting
poorly until you added a weight — in
this case the weight would be
analogous to an allosteric activator.
2 (31
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Sidra 5 years ago

more
whats the difference between non competitive

inhibition and allosteric regulation(involving
inhibitor)? .its all so confusing
(3
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kalid.dalu14 5 years ago

more
Allosteric regulation and

noncompetitive inhibitor bind to site
other than active site but allosteric
regulation change the conformation of
enzyme and making the reaction less

effective while the noncompetitive
inhibitor, like mention in the reading
just poison the enzyme so reaction
does not take place at all.
1 (10
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Hannah Stadelmann 5 years ago

more
What is an allosteric activator?

(2
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Travis Fisher 5 years ago

more
A molecule that attaches to the

enzyme at a site (not the active site),
changing the configuration of the
enzyme, which allows the substrate to
attach to the active site easier.
(11
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Nithya Shenoy 5 years ago

more
In school, we conducted an experiment where a

small piece of paper dipped in a liver solution
was dropped into a test tube filled with
hydrogen peroxide. After a few seconds, the
liver juice coated paper rose to the the top. Why
did it act in that way?
(3
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austin.langley 5 years ago

more
This is because the liver cells contain
enzymes called catalase which speed

up the breaking down of hydrogen
peroxide into water and oxygen. When
the reaction happens, oxygen is
released and it pushes the piece of
paper up to the surface along with it.
This reaction happens faster or slower
depending on the concentration of the
liver juice you soaked the paper with.
(7
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Edna Villapando 3 years ago

more
Life is a process regulated by enzymes. What

might be the sources of these enzymes? If
particular enzyme is not available in person's
cells, what sequence of events might result to
produce it?
(2
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3 years ago
tyersome more
Enzymes are encoded as genes in the

DNA — these genes are then
transcribed to produce RNA and (for
most enzymes§) then translated† to
make a protein that has a catalytic
activity (i.e. is an enzyme).
Typically enzymes found within a cell

are encoded by the DNA of that cell.
However, multicellular organisms are
colonized by many different microbes
(these may be prokaryotes or other
eukaryotes such as fungi) — these
microbes often supply enzyme
activities that aid the organism. The
digestive system of animals is one
example of this. In fact, most
multicellular organisms depend on
their microbiota for survival!
The process of regulating gene

expression is highly complex, but there
is KhanAcademy material on this is
several places — I recommend starting
here:
https://www.khanacademy.org/science
/biology/gene-regulation
§Note: Some enzymes are make of

RNA and these RNAs do not get
translated.
†Note: Many proteins undergo post-

translational modifications that are
essential for them to function.
(5
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seungjumoon2003 a year ago

more
if a allosteric inhibitor casues distortion of the

enzyme's shape so that it cannot function, is it
non-competitive or competitive?
(i looked up many resources ,such as princeton

review AP bio,Barrons, internet, but they all
weren't clear)
(3
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BButler522 a year ago

more
Being allosteric lets you know that the

inhibitor binds somewhere other than
the active site, where the substrate
binds. The location that the allosteric
inhibitor binds is called the allosteric
site.
Because it isn't "competing" for the

same binding site as the substrate, you
can tell that it is non-competitive as
the substrate will still be able to bond

at the active(2site.
Comment votes) Upvote m
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Anh Le 2 years ago

more
what would happen if our bodies do not have

inhibitors?
(3
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2
m…
years
Ivana - Science trainee ago
Good question
Than uncontrolled cell divisions,

uncontrolled sugar breakdowns,
uncontrolled and unlimited
phosphorylations (until the moment of
using up all resources) and ultimately
leading to energy depletion and death.
It may cause a ruckus in the body and

high dysregulation which would end
up fatal.
(2
Comment votes)
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3000785 3 years ago

more
How do you determine whether an enzyme's

activity is enhanced or inhibited by regulatory
cell binding?
(2
Upvote m
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3
m…
years
You follow up with the next steps. If

reactions proceeds - it was activation,

otherwise it must be some kind of
inhibitor. :D
(1
Comment vote)
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aviel seltzer 2 years ago

more
I have a question about the graph. If the

substrate concentration is decreasing, why does
the graph show that substrate concentration
increases as enzyme-reaction increases?
Doesn't the enzyme reaction rate represent the

amount of product being created from the
substrate? So as enemy reactions increase,
product concentration increase, but substrate
concentration decrease?
(2
Upvote m
Downvote
2
m…
years
Good question
Yes, it is true. The more the substrate,

the more the enzymatic activity. Why?
because an enzyme cannot exhibit
catalytic activity on the thin air. It
needs something to act on.
As for product. Why do you think that

a substrate has to decrease in order of
product to increase?
It is true that if you convert let's say

one molecule into another, you
removed that first molecule and initial
concentration of substrate is
decreased, but you can keep adding
substrate and rate of reaction will
increase as well.
2 (1
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comments vote)
Guillaume 3 years ago

more
Once the competitive inhibitor is on the

enzyme, doesn't it stay there? So it can't be
outmuscled by substrate molecules since
substrate molecules themselves have to move,
but the inhibitor can just sit there.
(1
Reply • Comment vote)
Upvote m
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Christian Krach 3 years ago

more
Both the inhibitor and the substrate do

not stick on the enzyme for ever. The
higher its affinity is the longer it stays.
The inhibitor can be replaced by a
higher substrate concentration. You
need more substrate to get the 0,5-
Vmax --> Km (substarte) increases.
Vmax itself stays the same, once the
substrate concentration is high enough
to suppress all inhibitors, Vmax is
determined by the enzyme
concentration alone.
1 (2
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comment votes)
Show more...
Noncompetitive inhibition
Basics of enzyme kinetics graphs
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8/30/2021 Enzyme regulation (article) | Khan Academy

Science · AP®︎/College Biology · Cellular energetics

Google Classroom Facebook

Needs and conditions vary from cell to cell and

https://www.khanacademy.org/science/ap-biology/cellular-energetics/environmental-impacts-on-enzyme-function/a/enzyme-regulation#:~:text=The co… 1/19

changes, so do the amounts and functionality of

Because enzymes guide and regulate the

Regulatory molecules. Enzyme activity may be

Cofactors. Many enzymes are only active when

Compartmentalization. Storing enzymes in

Feedback inhibition. Key metabolic enzymes

In the rest of this article, we'll examine these

https://www.khanacademy.org/science/ap-biology/cellular-energetics/environmental-impacts-on-enzyme-function/a/enzyme-regulation#:~:text=The co… 2/19

There are many kinds of molecules that block or

Competitive vs. noncompetitive

Reversible inhibitors are divided into groups based

An inhibitor may bind to an enzyme and block

https://www.khanacademy.org/science/ap-biology/cellular-energetics/environmental-impacts-on-enzyme-function/a/enzyme-regulation#:~:text=The co… 3/19

That is, only the inhibitor or the substrate can

In noncompetitive inhibition, the inhibitor

Competitive and non-competitive inhibitors can be

If an inhibitor is competitive, it will decrease

https://www.khanacademy.org/science/ap-biology/cellular-energetics/environmental-impacts-on-enzyme-function/a/enzyme-regulation#:~:text=The co… 4/19

substrate rather than the inhibitor.

If an inhibitor is noncompetitive, the enzyme-

On a graph of reaction velocity (y-axis) at different

_Image credit: "Enzymes: Figure 3," by OpenStax College, Biology, CC BY

Not familiar with this type of graph? No worries!

_Image modified from "Enzymes: Figure 4," by OpenStax College, Biology,

Pretty much all cases of noncompetitive inhibition

However, some enzymes that are allosterically

our key metabolic regulators, are often given the

There are also allosteric activators. Some allosteric

[Is hemoglobin an allosteric enzyme?]

Cofactors and coenzymes

molecules, DNA polymerase, requires magnesium

Coenzymes are a subset of cofactors that are

Image modified from OpenStax Biology.

https://www.khanacademy.org/science/ap-biology/cellular-energetics/environmental-impacts-on-enzyme-function/a/enzyme-regulation#:~:text=The co… 8/19

substrates readily, don't damage the cell, and have

For instance, digestive enzymes of the lysosome

Feedback inhibition of metabolic

This may seem odd – why would a molecule want

Basics of enzyme kinetics Image credit: OpenStax Biology.

Practice: Environmental Typically, feedback inhibition acts at the first

For example, the energy carrier molecule ATP is an

ADP, on the other hand, serves as a positive

and changing its shape so that it becomes more

Thanks to this pattern of regulation, when ADP

[Attribution and references]

Sort by: Top Voted

Questions Tips & Thanks

Want to join the conversation?

Zhang, Luyan 3 years ago

Allosteric regulation confuses me a lot. I don't

tyersome 3 years ago

I'll try an analogy — let me know if this

Imagine that an enzyme is like tiny

Now think of hanging a weight off