Biochemistry
Topic 6: Modulation and regulation of enzyme activity
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To regulate the metabolism it is sufficient to change the activity of the enzyme which
catalyzes the slowest step. Most metabolic pathways have key enzymes on which the
regulatory mechanisms operate. The activity of key enzymes is regulated independently at
three different stages:
1. Transcriptional control. Here, Biosynthesis of the enzyme protein is influenced at the
genetic level. Interventions in enzyme synthesis mainly affect synthesis of the
corresponding mRNA—i. e., transcription of the gene coding for the enzyme. The
term “transcriptional control” is therefore used. This mechanism is mediated by
regulatory proteins (transcription factors) that act directly on DNA. The genes have a
special regulatory segment for this purpose, known as the promoter region, which
contains binding sites (control elements) for regulatory proteins. The activity of these
proteins is, in turn, affected by metabolites or hormones. When synthesis of a
protein is increased by transcriptional control, the process is referred to as
induction; when it is reduced or suppressed, it is referred to as repression. Induction
and repression processes take some time and are therefore not immediately
effective.
2. Interconversion of key enzymes takes effect considerably faster than transcriptional
control. In this case, the enzyme is already present at its site of effect, but it is
initially still inactive. It is only when needed that it is converted into the catalytically
active form, after signaling and mediation from second messengers through an
activating enzyme. If the metabolic pathway is no longer required, an inactivating
enzyme returns the key enzyme to its inactive resting state. Interconversion
processes in most cases involve ATP-dependent phosphorylation of the enzyme
protein by a protein kinase or dephosphorylation of it by a protein Phosphatase. The
phosphorylated form of the key enzyme is usually the more active one, but the
reverse may also occur.
3. Modulation by ligands. An important variable that regulates flow through a
metabolic pathway is precursor availability (metabolite A for example). The
availability of precursor A increases along with the activity of themetabolic pathways
that form A and it decreases with increasing activity of other pathways that also
consume A. Transport from one cell compartment to another can also restrict the
availability of A.
4. Coenzyme availability can also often have a limiting effect. If the coenzyme is
regenerated by a second, independent metabolic pathway, the speed of the second
pathway can limit that of the first one. For example, glycolysis and the tricarboxylic
acid cycle are mainly regulated by the availability of NAD+. Since NAD+ is
regenerated by the respiratory chain, the latter indirectly controls the breakdown of
glucose and fatty acids. Finally, the activity of key enzymes can be regulated by
ligands (substrates, products, coenzymes, or other effectors), which as allosteric
effectors do not bind at the active center itself, but at another site in the enzyme,
thereby modulating enzyme activity. Key enzymes are often inhibited by immediate
reaction products, by end products of the reaction chain concerned (“feedback”
inhibition), or by metabolites from completely different metabolic pathways. The
precursors for a reaction chain can stimulate their own utilization through enzyme
activation.
Modification of proteins
Reversible
Irreversible
Reactions
 Formation of disulfide bonds
 addition of some cofactors
 phosphorilation
 glicosilation
 acetylation
 addition of some cofactors
 ubiquitinization
 hydroxylation
 methylation
Types of regulation of the activity of the enzymes:
Type
Feedback inhibition
What’s done:
One of the products of the reaction, or an intermediate,
inhibits the activity of the enzymes. This will make sure the
amount will remain in proper manner. This control can also
be positive, if one substance (a substrate or else) increases
the rate of the reaction.
Regulatory proteins
Those proteins can bind to the enzyme and activate or
inhibit their function.
Covalent modifications
Some enzymes must have some kind of modification in
order to be functional (phosphorilation, glicosilation,
methylation, acetylation, etc) and some will be deactivated
if they are modified
Proteolytic activation or deactivation
Some enzymes are produced as pro-enzymes – mainly
enzymes in the digestive system. Since they have the
capability to harm the producing cell, they are produced
and secreted as an inactive pro-enzyme (zymogens). This
enzyme will be cleaved by another enzyme (or by the low
pH) and will become active (this is also true for some
hormones). Other enzymes will lose their activity if a small
peptide is cleaved of them
pH
The pH can determine the activity of the enzymes. Most
enzymes have a pH range in which their function is
optimum. If we deviate from this range their function will
be impaired.
Controlling the enzyme quantity
By controlling the synthesis and the degradation of
enzymes it is possible to control their quantity
It is important that the human body is able to control the activity of the enzymes, because the
general activation of all enzymes all the time could impair the body’s functions as for example if
carbonic anhydrase is active while alkalosis, the situation will get worse.
Allosteric regulation:
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this is the regulation of a protein or an enzyme by binding an effector molecule to the
enzyme somewhere else than the active site
Allosteric regulation can be used for activation or inhibition. For example hemoglobin:
Binding 1 O2 molecule increases the affinity of the other hemoglobin binding site for other
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oxygen molecules. The inhibition is the decrease in the affinity after binding the effector
molecule – in hemoglobin this is the 2,3-BPG molecule.
Enzymes are subjected to reversible and irreversible inhibition. There are 3 forms of
reversible inhibition: competitive, uncompetitive and noncompetitive:
1. Competitive inhibition: The inhibiting substance competes for the enzymes active
site – forming an EI complex. Thus reducing the available enzymes for the reaction.
This will affect mainly the Km values.
2. Uncompetitive inhibition: The inhibitor binds to the ES complex and forms an ESI
complex. This will affect the Km and Vmax values.
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3. Noncompetitive inhibition: The inhibitor binds the enzyme and forms an EI complex.
This EI complex does not affect the effector site of the enzyme so that an ESI
complex could still form but the capability of the enzyme will be greatly impaired.
The inhibitor will bin to an area distinct from the substrate binding site. This
inhibition will affect mainly the Vmax values.
4. Mixed inhibition is also possible usually with competitive and uncompetitive
inhibitors.
Irreversible inhibition: some inhibitors either bind permanently to the functional enzyme or
destroy the enzyme or its functional group altogether. They can break covalent bonds inside
the enzyme; change its conformation state etc. These changes will persist even after the
inhibitors have been completely removed from the medium. One special group of inhibitors
are the suicide inhibitors: they act as Trojan horses, since they are inactive until they bind an
active site of a specific enzyme. Instead of becoming normal products, they become active
compounds that bind to the enzyme and inhibit its function. They are also called mechanism
based inactivators.