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