enzyme inhibition & factors affecting the velocity of enzyme action
DESCRIPTION
ENZYME INHIBITIONTRANSCRIPT
Factors affecting enzyme activity
Gandham. Rajeev
Factors affecting enzyme activity
• The contact between the enzyme and substrate is the most essential pre-requisite for enzyme activity
1. Enzyme concentration2. Substrate concentration3. Temperature4. Hydrogen ion concentration (pH)5. Product concentration6. Presence of activators7. Time 8. Light & radiation
Enzyme concentration
• Enzyme Concentration:
• Rate of a reaction or velocity (V) is directly proportional to
the enzyme concentration, when sufficient substrate is
present.
• Velocity of reaction is increased proportionately with the
concentration of enzyme, provided substrate concentration
is unlimited
• Substrate is a molecule on which enzyme acts.
• Velocity (Reaction rate) refers to change in the concentration
of substrate or reaction product (s) per unit time.
• It is expressed as moles/liter/sec.
• Maximum velocity (Vmax):
• It refers to maximum change in the product or substrate
concentration at a given enzyme concentration.
• Vmax = Kcat (e)
• e-enzyme concentration & Kcat is catalytic rate
constant.
• Kcat (catalytic rate constant) – defined as the number of
substrates molecules formed by each enzyme molecule
in unit time.
• Expressed as moles produced/mol enzyme/time.
Effect of enzyme concentration
Effect of Substrate Concentration
• Increase in the substrate concentration gradually
increases the velocity of enzyme reaction within the
limited range of substrate levels.
• A rectangular hyperbola is obtained when velocity is
plotted against the substrate concentration
• Three distinct phases of the reaction are observed in
the graph (A-linear; B-curve; C-almost unchanged.
Effect of Substrate Concentration
Explanation
• At lower concentrations of substrate (point A in the curve),
some enzyme molecules are remaining idle.
• As substrate is increased, more and more enzyme molecules
are working.
• At half-maximal velocity, 50% enzymes are attached with
substrate (point B in the curve).
• As more substrate is added, all enzyme molecules are
saturated (point C).
• Further increase in substrate cannot make any effect in the
reaction velocity (point D).
• The maximum velocity obtained is called Vmax.
• It represents the maximum reaction rate attainable in
presence of excess substrate (at substrate saturation level).
Michaelis-Mention Equation
• Michaelis-Mention equation is a rate equation for
reaction kinetics in enzyme catalysed reaction
• Written as
V max (S)Km + SV =
Michaelis-mention Plot
• The velocity of enzyme catalysed reactions is altered as
the substrate concentration is increased.
• First order reaction:
• At low substrate concentration, velocity increases
proportionally as the concentration of the substrate is
increased.
• Mixed order reaction:
• When the concentration of the substrate is further
increased (at mid substrate concentration), the velocity
increases, but not proportionally to substrate
concentration.
• Zero order reaction:
• At high substrate concentration, the velocity is
maximum & is independent of substrate concentration.
Enzyme kinetics & Km value
• The enzyme (E) reacts with substrate (S) to form
unstable enzyme-substrate (ES) complex.
• The ES complex is either converted to product (P) or can
dissociate back to enzyme (E) & substrate (S).
Substrate (S) + Enzyme (E) Enzyme substrate (ES) Product (P) + Enzyme (E)
• K1,K2 & K3 are velocity constants.
• Km, Michaelis-mention constant is given by the
formula…
E + S ES E + PK2
K1 K3
Km = K2 + K3
K1
• Michaelis-mention set up mathematical expressions for the rate of
all the three reactions in the equation.
• V as the initial rate of reaction (velocity)
• S as the initial concentration of the substrate
• V max as the maximum velocity attained with high substrate
concentration when all the enzyme molecules are occupied.
• Km as Michaelis-mention constant
V = V max (S)Km + (S)
• Measured velocity (V) is equal to ½ Vmax.
• So,
½ V max = V max (S)Km + (S)
Km + (S) = 2V max (S)V max
Km + (S) = 2 (S)
Km = (S)
K stands for constant & M stands for Michaelis
Michaelis constant
• The formation of enzyme - substrate complex is a reversible reaction,
while the breakdown of the complex to enzyme + product is
irreversible.
• 50% velocity in Y axis is extrapolated to the corresponding point on
X-axis, which gives the numerical value of Km.
• The lesser the numerical value of Km, the affinity of the enzyme for
the substrate is more.
• E.g: Km of glucokinase is 10 mmol/L and hexokinase is 0.05 mmol/L.
• 50% molecules of hexokinase are saturated even at a lower
concentration of glucose.
• Hexokinase has more affinity for glucose than glucokinase.
Effect of enzyme concentration on Km
Salient features of Km
• Km value is substrate concentration (expressed in moles/L) at half-
maximal velocity.
• It denotes that 50% of enzyme molecules are bound with substrate
molecules at that particular substrate concentration.
• Km is independent of enzyme concentration.
• If enzyme concentration is doubled, the Vmax will be double.
• But the Km will remain exactly same.
• In other words, irrespective of enzyme concentration, 50% molecules
are bound to substrate at that particular substrate concentration.
• Km is the Signature of the Enzyme.
• Km value is thus a constant for an enzyme.
• It is the characteristic feature of a particular enzyme for a
specific substrate.
• The affinity of an enzyme towards its substrate is
inversely related to the dissociation constant, Kd for the
ES complex.
• Km denotes the affinity of enzyme for substrate.
• The lesser the numerical value of Km, the affinity of the
enzyme for the substrate is more.
Double reciprocal plot
• Sometimes it is impractical to achieve high substrate
concentrations to reach the maximal velocity conditions.
• So, ½Vmax or Km may be difficult to determine.
• The experimental data at lower concentrations is plotted as
reciprocals.
• The straight line thus obtained is extrapolated to get the
reciprocal of Km.
• Called as Lineweaver–Burk Plot or Double Reciprocal Plot
which can be derived from the Michaelis-Menten equation
Lineweaver-Burk plot
Effect of Temperature
• The velocity of enzyme reaction increases when temperature of the medium is increased; reaches a maximum and then falls (Bell shaped curve).• The temperature at which maximum amount of the substrate
is converted to the product per unit time is called the optimum temperature.• Temperature is increased, more molecules get activation
energy, or molecules are at increased rate of motion. • Their collision probabilities are increased and so the reaction
velocity is enhanced.
Temperature coefficient Q10
• The temperature coefficient (Q10) is the factor by which
the rate of catalysis is increased by a rise in 10°C.
• Generally, the rate of reaction of most enzymes will
double by a rise in 10°C.
• When temperature is more than 50°C, heat denaturation
and consequent loss of tertiary structure of protein occurs.
• Activity of the enzyme is decreased.
• Most human enzymes have the optimum temperature
around 37°C.
• Certain bacteria living in hot springs will have enzymes
with optimum temperature near 100°C.
Effect of Temperature
Effect of pH
• Each enzyme has an optimum pH (usually pH between 6 and 8).• On both sides of which the velocity will be drastically reduced. • The graph will show a bell shaped curve • The pH decides the charge on the amino acid residues at the
active site. • The net charge on the enzyme protein would influence
substrate binding and catalytic activity.• Optimum pH may vary depending on the temperature,
concentration of substrate, presence of ions etc. • Pepsin (optimum pH 1-2); ALP (optimum pH 9-10) & acid
phosphatase (4-5)
Effect of pH
Effect of product concentration
• The accumulation of reaction products generally decreases
the enzyme velocity.
• For certain enzymes, the products combine with the active
site of enzyme and form a loose complex and, thus, inhibit
the enzyme activity.
• In the living system, this type of inhibition is generally
prevented by a quick removal of products formed
Effect of activators
• Some of the enzymes require certain inorganic metallic cations
like Mg2+, Mn2+, Zn2+, Ca2+, Co2+, Cu2+, Na+, K+, for their
optimum activity
• Anions are also needed for enzyme activity e.g. chloride ion for
amylase
• Metals function as activators of enzyme velocity through
various mechanisms combining with the substrate, formation
of ES-metal complex, direct participation in the reaction and
bringing a conformational change in the enzyme.
• Two categories of enzymes requiring metals for their
activity
• Metal-activated enzymes
• Metalloenzyme
• Metal-activated enzymes:
• The metal is not tightly held by the enzyme and can be
exchanged easily with other ions.
• e.g. ATPase (Mg2+ and Ca2+) & Enolase (Mg2+)
• Metalloenzyme:
• These enzymes hold the metals rather tightly which are not
readily exchanged.
• e.g. Alcohol dehydrogenase, carbonic anhydrase, alkaline
phosphatase, carboxypeptidase and aldolase contain zinc.
• Phenol oxidase (copper)
• Pyruvate oxidase (manganese)
• Xanthine oxidase (molybdenum)
• Cytochrome oxidase (iron and copper)
Effect of time
• Under ideal and optimal conditions (like pH, temperature
etc.), the time required for an enzyme reaction is less.
• Variations in the time of the reaction are generally related
to the alterations in pH and temperature.
Effect of light and radiation
• Exposure of enzymes to ultraviolet, beta, gamma &
X-rays inactivates certain enzymes due to the
formation of peroxides. e.g. UV rays inhibit salivary
amylase activity
Enzyme inhibition
Enzyme inhibitor
• Enzyme inhibitor is defined as a substance, which binds
with the enzyme and brings about a decrease in catalytic
activity of that enzyme.
• They are usually specific and they work at low
concentrations
• They block the enzyme but they do not usually destroy it
• Many drugs and poisons are inhibitors of enzymes in the
nervous system
Type of Enzyme Inhibitors
Reversible
Irreversible
Type of Inhibitors
Competitive
Uncompetitive
Non- Competitive
Active Site Directed
Suicide / kcat
Inhibitors
Reversible inhibition
• The inhibitor binds non-covalently with enzyme and the
enzyme inhibition can be reversed if the inhibitor is removed.
• Binding is weak and thus, inhibition is reversible.
• Do not cause any permanent changes in the enzyme
• Subtypes:
• Competitive & Non-competitive Inhibition
Competitive inhibition
• The inhibitor (I) molecules resembles the real substrate (S)
• Also called as substrate analogue inhibition
• Binds to active site – forms EI complex.
• EI complex cannot rive rise to product formation.
• As long as the competitive inhibitor holds the active site, the enzyme
is not available for the substrate to bind.
• Relative concentrations of S, I determine inhibition.
E
ES
EI
E + P+S
+INo product formation
Binding of S & I in different Situations
• Classical Competitive Inhibition (S & I compete for the same binding site)
S I
Enzyme
• Binding of I to a distinct inhibitor site causes a conformational change in the enzyme that distorts or masks the S binding site or vice versa.
Enzyme
I S
Enzyme
I
S
Enzyme
I
S
• A competitive inhibitor diminishes the rate of catalysis by
reducing the proportion of enzyme molecules bound to a
substrate.
• Competitive inhibition can be relieved by increasing the
substrate concentration & maximum velocity is regained.
• A higher substrate concentration is therefore needed to achieve
a halfmaximum rate, Km increases
• High concentrations of the substrate displace the inhibitor
again.
• The V max, not influenced by this type of inhibition.
• E. g. Malonate – structural analog of succinate-inhibits succinate dehydrogenase.
The effect of enzyme inhibition
Succinate Fumarate + 2H++ 2e-
Succinate dehydrogenase
CH2COOH
CH2COOH CHCOOH
CHCOOH
COOH
COOH
CH2
Malonate
• The compounds malonic acid, glutaric acid and oxalic acid,
have structural similarity with succinic acid and compete
with the substrate for binding at the active site of SDH.
• Antimetabolites:
• These chemical compounds that block the metabolic
reactions by their inhibitory action on enzymes.
• Antimetabolites are usually structural analogues of
substrates and thus are competitive inhibitors.
• They are in use for cancer therapy, gout etc.
Examples of competitive inhibition
Enzyme Substrate Competitive inhibitor
Succinate Dehydrogenase Succinate Malonate
Dihydrofolate Reductase 7,8-dihydrofolate Aminopterin
Xanthine Oxidase Hypoxanthine Allopurinol
Acetyl cholinesterase Acetylcholine Succinylcholine
Lactate Dehydrogenase Lactate Oxamate
HMG CoA Reductase HMG Co A HMG
Reversible, Competitive Inhibitors
In the presence of a competitive inhibitor Km increases
V max unchanged
No inhibitor
+ C Inhibitor
Vmax
½ Vmax
Km Kmapp[s]
v
Lineweaver Burk plot
[I]2
[I]1
1
Kmapp
1
Km
Slope = Km
Vmax ( 1+
)[I]Ki
• In the presence of a
competitive inhibitor Km
increases
• V max unchanged
Non-Competitive Inhibition
• The inhibitor binds at a site other than the active site on
the enzyme & causes conformational changes on
enzymes or some times it may react with functional group
at the active site & inactivates the enzyme.
• This binding impairs the enzyme function.
• Inhibitor has no structural resemblance with the substrate.
• There is no competition for the active site of the enzyme
molecule.
• There usually exists a strong affinity for the inhibitor to
bind at the second site.
• The inhibitor does not interfere with the enzyme-substrate
binding.
• But the catalysis is prevented, possibly due to a distortion
in the enzyme conformation
• The inhibitor generally binds with the enzyme as well as
the ES complex.
• Km value is unchanged & V max is lowered.
• Heavy metal ions (Ag+, Pb2+, Hg2+ etc.) can non-competitively inhibit
the enzymes by binding with cysteinyl sulfhydryl groups & inactivates
the enzymes.
• Heavy metals also form covalent bonds with carboxyl groups &
histidine, results in irreversible inhibition.
• Non-competitive inhibition is also called as enzyme poisons
E + S ES+I
EI + S
E + P
+I
EIS
Non-Competitive Inhibition
Enzyme Enzyme
Enzyme Enzyme
S
IS
I
Non-Competitive Inhibition
No inhibitor
+ NC Inhibitor
Vmax
½ Vmax
Km [s]
v
½ Vmax i
Vmax i
Vmax = Decreases.
Km = Unchanged
Lineweaver – Burk Plot
[I]2
[I]1
No Inhibitor
1
Vmax
1
Vmaxi
1
Km
1/[s]
1/v
• Km value is unchanged
• V max is lowered
Comparison between competitive & Non-competitive inhibition
Competitive Inhibition Non-competitive Inhibition
Acting on Active site May or may not
Structure of inhibitor Substrate analogue Unrelated molecule
Inhibition is Reversible Generally Irreversible
Excess Substrate Inhibition Relieved No effect
Km Increased No Change
V max No Change Decreased
Significance Drug Action Toxicological
Uncompetitive Inhibition
• Here inhibitor does not have any affinity for the active
site of enzyme.
• Inhibitor binds only with enzyme-substrate complex; but
not with free enzyme.
• Both V max and Km are decreased
• UC Inhibition is rare in single-substrate reactions.
• E.g. Phenylalanine inhibits alkaline phosphatase in intestinal
cells
• It is common in multi-substrate reactions
E + S E S E + P
+
I
ESI
Uncompetitive Inhibition
EnzymeEnzyme
S
Enzyme
IS
Uncompetitive Inhibition
No inhibitor
+ UC Inhibitor
Vmax
½ Vmax
Km [s]
½ Vmax i
Vmax i
Vmax = Decreases
Km = Decreases
Kmapp
v
• In this type, Inhibitor binds at or near the active site of the enzyme
irreversibly, usually by covalent bonds, so that it can’t
subsequently dissociate from the enzyme
• The I destroys as essential functional group on the enzyme that
participates in normal S binding or catalytic action.
• As a result the enzyme is permanently inactive
• Compounds which irreversibly denature the enzyme protein or
cause non-specific inactivation of the active site are not usually
regarded as irreversible inhibitors.
Irreversible inhibition
• These inhibitors are toxic poisonous substances.
• Iodoacetate:
• It is an irreversible inhibitor of the enzymes like papain and
glyceraldehyde 3-phosphate dehydrogenase
• Iodoacetate combines with sulfhydryl (-SH) groups at the active site
of these enzymes and makes them inactive.
• Diisopropyl fluorophosphafe (DFP) is a nerve gas developed by the
Germans during Second World War.
• DFP irreversibly binds with enzymes containing serine at the active
site, e.g. serine proteases, acetylcholine esterase.
Examples
Examples
• DFP (Diisopropylphosphofluoridate) is a nerve poison.
• It inactivates acetylcholinesterase that plays an
important role in the transmission of nerve impulses.
E CH2-OH + F—P=O
E CH2-O- F—P=O + HF
OCH(CH3)2
OCH(CH3)2
OCH(CH3)2
OCH(CH3)2
DFP Catalytically inactive enzyme
• Disulfiram (Antabuse)s a drug used in the treatment of
alcoholism.
• lt irreversibly inhibits the enzyme aldehyde dehydrogenase.
• Alcohol addicts, when treated with disulfiram become sick
due to the accumulation of acetaldehyde, leading to alcohol
avoidance
Suicidal inhibition
• This is a special type of irreversible inhibition.
• Also called as mechanism based inactivation.
• In this case, the original inhibitor (the structural
analogue/competitive inhibitor) is converted to a more
effective inhibitor with the help of same enzyme that ought to
be inhibited.
• The formed inhibitor binds irreversibly with the enzyme.
• Allopurinol, an inhibitor of xanthine oxidase, gets converted to
alloxanthine, a more effective inhibitor of this enzyme.
• A suicide inhibitor is a relatively inert molecule that is transformed
by an enzyme at its active site into a reactive compound that
irreversibly inactivates the enzyme
• They are substrate analogs designed so that via normal catalytic
action of the enzyme, a very reactive group is generated.
• The latter forms a covalent bond with a nearby functional group
within the active site of the enzyme causing irreversible inhibition.
• Such inhibitors are called suicide inhibitors because the enzyme
appears to commit suicide.
• e.g. FdUMP is a suicide inhibitor of thymidylate synthase.
Suicidal inhibition
• The use of certain purine and pyrimidine analogues in
cancer therapy is also explained on the basis suicide
inhibition.
• 5-fluorouracil gets converted to fluorodeoxyuridylate
which inhibits the enzyme thymidylate synthase, and thus
nucleotides synthesis
During thymidylate synthesis, N5,N10- methyleneTHF is converted to 7,8-dihydrofolate; methyleneTHF is regenerated in two steps
Allosteric regulation
• The catalytic activity of certain regulatory enzymes is modified by
certain low molecular weight substances or molecules known as
allosteric effectors.
• Allosteric enzyme has one catalytic site where the substrate binds
and another separate allosteric site where the modifier binds (allo =
other)
• Allosteric and substrate binding sites may or may not be physically
adjacent.
• The binding of the regulatory molecule can either enhance the
activity of the enzyme (allosteric activation), or inhibit the activity of
the enzyme (allosteric inhibition).
• The binding of substrate to one of the subunits of the
enzyme may enhance substrate binding by other subunits.
• This effect is said to be positive co-operativity
• If the binding of substrate to one of the subunits decreases
the activity of substrate binding by other sites, the effect is
called negative co-operativity.
• In most cases, a combination is observed, resulting in a
sigmoid shaped curve
The switch: Allosteric inhibition
Allosteric means “other site”
E
Active site
Allosteric site
Switching off
• These enzymes have two
receptor sites
• One site fits the substrate
like other enzymes
• The other site fits an
inhibitor molecule
Inhibitor fits into allosteric
site
Substratecannot fit into the active site
Inhibitor molecule
Allosteric inhibition
Salient Features, Allosteric Inhibition
• The inhibitor is not a substrate analogue
• It is partially reversible, when excess substrate is added.
• Km is usually increased & V max is reduced.
• The effect of allosteric modifier is maximum at or near
substrate concentration equivalent to Km.
• When an inhibitor binds to the allosteric site, the
configuration of catalytic site is modified such that
substrate cannot bind properly.
• Most allosteric enzymes possess quaternary structure.
• They are made up of subunits, e.g. Aspartate
transcarbamoylase has 6 subunits and pyruvate kinase
has 4 subunits
Allosteric enzymes
Enzyme Allosteric Inhibitor Allosteric Activator
ALA synthase Heme
Aspartate transcarbamoylase CTP ATP
HMGCoA-reductase Cholesterol
Phosphofructokinase ATP, citrate AMP, F-2,6-P
Pyruvate carboxylase ADP AcetylCoA
Acetyl CoA carboxylase AcylCoA Citrate
Citrate synthase ATP
Carbamoyl phosphate synthetase I NAG
Carbamoyl phosphate synthetase II UTP
• For understanding the regulation of enzyme activity within
the living cells
• To elucidate the kinetic mechanism of an enzyme catalyzing
a multi-substrate reaction
• Useful in elucidating the cellular metabolic pathways by
causing accumulation of intermediates
• Identification of the catalytic groups at the active site
• Provide information about substrate specificity of the
enzyme
Importance of Enzyme Inhibition
Regulation of enzyme activity
• Allosteric regulation
• Activation of latent enzymes
• Compartmentation of metabolic pathways
• Control of enzyme synthesis
• Enzyme degradation
• lsoenzymes
Allosteric Regulation or Allosteric Inhibition
• Enzymes possess additional sites, known as allosteric sites
besides the active site.
• Such enzymes are known as allosteric enzymes.
• The allosteric sites are unique places on the enzyme
molecule
• Allosteric effectors:
• The catalytic activity of certain regulatory enzymes is
modified by certain low molecular weight substances or
molecules known as allosteric effectors or modifiers bind at
the allosteric site and regulate the enzyme activity.
• The allosteric effectors may be positive or negative effectors
• The enzyme activity is increased when a positive (+) allosteric
effector binds at the allosteric site known as activator site.
• A negative (-) allosteric effector binds at the allosteric site called
inhibitor site and inhibits the enzyme activity.
• Classes of allosteric enzyme:
• They are divided into two classes based on the influence of
allosteric effector on Km and V max
• K-class of allosteric enzymes:
• The allosteric inhibitor increases the Km and not the V max.
• Double reciprocal plots, similar to competitive inhibition are
obtained e.g. phosphofructokinase.
• V-class of allosteric enzymes:
• The allosteric inhibitor decreases the V max and not the
Km.
• Double reciprocal plots resemble that of non-competitive
inhibition e.g. acetyl CoA carboxylase
Feedback regulation
• The process of inhibiting the first step by the final product, in a
series of enzyme catalysed reactions of a metabolic pathway is
referred to as feedback regulation.
• The very first step (A to B) by the enzyme is the most effective for
regulating the pathway, by the final end product D.
• This type of control is often called negative feedback regulation
A B C D
Feedback regulation
Carbamoyl phosphate + Aspartate
Carbamoyl Aspartate + Pi
Cytidine triphosphate (CTP)
Aspartate transcarbamylase
Feedback control
Activation of latent enzymes
• Some enzymes are synthesized as Proenzymes or zymogens which
undergo irreversible covalent activation by the breakdown of one or
more peptide bonds
• Chymotrypsinogen pepsinogen and plasminogen, are respectively-
converted to the active enzymes chymotrypsin, pepsin and plasmin.
• Certain enzymes exist in the active and inactive forms which are
interconvertible
• The inter-conversion is brought about by the reversible covalent
modifications, namely phosphorylation and dephosphorylation, and
oxidation and reduction of disulfide bonds
Examples
• There are some enzymes which are active in
dephosphorylated state and become inactive when
phosphorylated e.g. glycogen synthase, acetyl CoA
carboxylase.
• A few enzymes are active only with sulfhydryl (-SH) groups
• E.g. succinate dehydrogenase, urease.
• Glutathione bring about the stability of these enzymes.
Compartmentation
• Generally, the synthetic (anabolic) and breakdown
(catabolic) pathways are operative in different cellular
organelle.
• E.g. Enzymes for fatty acid synthesis are found in the
cytosol whereas enzymes for fatty acid oxidation are
present in the mitochondria
Control of enzyme synthesis
• Most of the enzymes, the rate limiting ones, are present in very low
concentration.
• Many rate limiting enzymes have short half-lives
• This helps in the efficient regulation of the enzyme levels.
• Constitutive enzymes (house-keeping enzymes)-The levels of which
are not controlled and remain fairly constant.
• Adaptive enzymes-Their concentrations increase or decrease as per
body needs and are well-regulated.
• The synthesis of enzymes (proteins) is regulated by the genes.
Induction and repression
• Induction is used to represent increased synthesis of enzyme while
repression indicates its decreased synthesis.
• Induction or repression which ultimately determines the enzyme
concentration at the gene level through the mediation of hormones or
other substance.
• E.g of Induction: The hormone insulin induces the synthesis of glycogen
synthetase, glucokinase, phosphofructokinase and pyruvate kinase.
• All these enzymes are involved in the utilization of glucose.
• The hormone cortisol induces the synthesis of many enzymes e.g. pyruvate
carboxylase, tryptophan oxygenase and tyrosine aminotransferase
• Examples of repression:
• In many instances, substrate can repress the synthesis of
enzyme.
• Pyruvate carboxylase is a key enzyme in the synthesis of
glucose from non-carbohydrate sources like pyruvate and
amino acids.
• lf there is sufficient glucose available, there is no necessity
for its synthesis.
• This is achieved through repression of pyruvate carboxylase
by glucose.
Enzyme degradation
• Every enzyme has half-life.
• It is in days while for others in hours or in minutes,
• e.g. LDH4 - 5 to 6 days;
• LDH1 - 8 to 12 hours;
• Amylase -3 to 5 hours
• The key and regulatory enzymes are most rapidly degraded.
• lf not needed, they immediately disappear and, when
required, they are quickly synthesized
Units of enzyme activity
• Katal:
• One kat denotes the conversion of one mole substrate per second
(mol/sec).
• Activity may also be expressed as millikatals (mkat), microkatals (µkat)
• International Units (lU):
• One Sl unit or International Unit (lU) is defined as the amount of enzyme
activity that catalyses the conversion of one micromol of substrate per
minute.
• Sl units and katal are interconvertible
Non-protein enzymes
• Ribozymes are a group of ribonucleic acids that function
as biological catalysts, and they are regarded as non-
protein enzymes.
• RNA molecules are known to adapt a tertiary structure
just as in the case of proteins
• The specific conformation of RNA may be responsible
for its function as biocatalyst.
THANK YOU