Enzymes are protein catalysts that ↑se
the rate of reaction 33 without being
changed in the overall process. Thus
enzymes direct all metabolic events.
Enzymes are neither consumed nor
produced during the course of a
Enzymes only expedite the reaction &
do not cause reactions to take place.
Enzymes are invariably proteins
Enzymes function within a moderate Ph
& Temp range
E + S → ES → E + P
The names of enzymes in many cases end in
“ ase” which is preceded by the name of its
subtrate e.g. sucrase, lipase, etc. in other
cases their name describes the action of an
enzyme e.g. transmethylase. In other case
their names do not all point out their
substrate or action e.g. pepsin, trypsin.
International union of biochemistry &
molecular biology drafted specific rules for
the classification & nomenclature of
In this system each enzyme has been
assigned as 4-digit classification number
along with a systemic name which
indicates the catalysed reaction. The
digits represent the “class” “sub class” &
An i.u of an enzyme is defined as the qty.
of enzyme needed to transform 1.0
micromole of its substrate to the
product/min at 30 ⁰ & optium pH.
Measure of enzyme activity are the
specific activity and the Katal. The
specific activity is the no. of units of
enzyme activity/mg of enzyme protein.
The Katal is the amount of enzyme
activity that transform one mole of its
substrate to the product/second.
There are six main classes of enzymes,
each one of these is further sub ÷ed x
subclasses & sub-subclasses. The main
classes are the following:
These enzymes catalytyse oxidationreduction reactions by transfer of electrons.
This group is further sub ÷ed x 6 subgroups
In reactions catalyzed by these enzymes,
oxygen is added to H-atoms removed
from the substrate for e.g. ascorbic acid
HO – C
HO – C
H - C I
HO – C – H
Ascorbic acid ( vit C)
½ O 2 H2 o
H - C
HO – C – H
These enzymes catalyze the removal of H 2 from a
substrate & can use either oxygen or substances
like methylene blue as H2 acceptors. e.g. Glucose
oxidase which catalyses the oxidation of glucose
C - OH
H – C - OH
HO – C - H
H - C - OH
H – C
C - OH
H2 o –
HO – C – H
H - C - OH
HO – C
These enzymes catalyse the removal of
hydrogen from the substrate & are unable
to use oxygen as hydrogen acceptor the Hacceptors take the H-atoms e.g. NAD+ ,
NADP+ and FAD all these three substances
act as co-enzymes for their respective
HO - C - H + NAD+
C = O + NADH + H+
There are 2 enzymes in this class called
peroxidase + catalase. Catalyze the
decomposition of H2 O2. Catalase.
Catalyze the following reaction:
2H2O + O2
These enzymes catalyze the incorporation
of molecular O2 x the substrate. An e.g. is
the conversion of phenylalanine to tyrosine
by the enzyme phenylalanine hydroxylase.
CH2 - C – COOH
C – COOH
H2 o CH2 I
+ NADPH + H+ -----------→
NH2 + NADP+
These enzymes catalyze the reduction of
their substrate by adding H-atoms. e.g.
glutathione reductase which catalyze the
conversion of oxidised glutathione to
- Glu - Cys - Gly
- Glu - Cys - Gly
- Glu - Cys – Gly + NADP+
these being about an exchange of
functional groups such as phosphates,
amino, acyl & methyl b/w 2 compounds
diff. types of transferases are:
Transaminases: These catalyze the
exchange Of –NH2 gp b/w an amino & a
ketoacid. The ketoacid becomes amino
acid & the amino acid becomes
Glutamic – Oxaloocetic
H2N C H
H2 N C H
catalyse the transfer of PO4 gps. & are
called kinases. e.g. hexokinase which
catalyze the reaction, Glucose + ATP →
glucose 6 PO4+ ADP
Transmethylases: These catalyze the
transfer of methyl gps. e.g. conversion of
noradrenaline to adrenaline
CHOH – CH2 –
CHOH – CH2 –
Transpeptidases: These catalyze the
transfer of α. α or peptides e.g.
formation of hippuric acid from benzyl
CoA + glycine
O = C – S – CoA
+ H2N – CH2 – COOH
O = C – N - COOH
+ CoA - SH
Benzyl glycine or Hippuri acid
Transacylases: These catalyze the
transfer of acyl gps. e.g Choline
acetyltransferase which catalyzes the
synthesis of acetylcholine
Acetyl – CoA + Choline → acetylcholine +
Catalyze hydrolosis i.e add H2o molecule
to the substrate + Simultaneously
decompose it. Various sub-groups are:
1. Protein hydrolyzing enzymes i.e
proteinases or proteases or proteolytic
A. Expopeptidases: Catalyze the hydrolysis
of terminal peptide bonds
B. Endopepridases: Attack the centrally
located peptide bonds
Exopeptidases: further sub÷ed x:Polypeptidases : 2 types
Aminopolypeptidases: Occurs in the intestinal
juice & attacks the protein molecule from the
side containing a free amino gp. Yielding an
amino acid + peptide smaller in size by one α .α
b. Carboxypolypeptidases: acts in the same way
as amino polypeptidase + attacks from the side
containing a free carboxyl gp + is present in
Act on tripeptide liberating 3 α .α
act on dipeptides liberating 2 α .α
e.g. pepsin, trypsin, chymotrypin +
elastase. All these effect hydrolysis at
particular α .α residues
catalyzxe the hydrolysis of the glucosidic
bonds e.g. enzyme amylase converts
starch to maltose. Maltose is further
hydrolysed by the enzyme maltase to
glucose. Sucrase, lactase & cellulase
also are e.g. of this gp. of enzyme.
Lipid hydrolyzing enzymes: e.g.
Lipases: Act on triglycerides or neutral
fats to liberate glycerol, F.A +
monoglycerides + diglycerides.
Pancreatic lipase is very impt. In the
digestion of fats.
Cholesteryl esterase: Hydrolyses
Phospholipase: Also known as
lecithinases + splits lecithins as well as
Deaminases or aminohydrolases:
Include adenases + guanase which
catalyze the following reactions:Adernine +H2O → Hypoxanthine + NH3
Guanine +H2O → xanthine + NH3
Deamidases or amidohydrolases:
Catalyze the hydrolysis of amides +
include urease, arginase, glutaminase +
asparginase which catalyze the
following reactions respectively:Urea + H2O → Co2 + 2NH3
Arginine + H2O → orinithine + urea
Glutamine + H2O → Glutamine acid +
Aspargine + H2O → Aspartic acid + NH3
Other ester hydrolyzing enzyme:grouped x 2 types
Phosphomonoesterases: occur in blood,
plasma, bone, prostate, kidney, RBC,
milk + intestinal mucosa, e.g. the
hepatic enzyme glucose 6-PO4 – PO4 ase
are which catalyze the reaction glucose
6PO4 + H2O → glu. + Phosphoric acid
Phosphodiesterase: it splits off one PO4
gp of diesters.
Phosphorylase: Adds inorganic PO4 to split
a bond e.g. glycogen Phosphorylase
present in hepatocytes + SK. Muscle fibres
which catalyze the reaction.
Glycogen + H3PO4 → Glucose 1-PO4
Pyrophosphatase:- this enzyme hydrolyze
pyrophosphates to orthophosphate
PPi + H2O → 2 pi
Nucleases or polynucleatidases: Present
in the intestinal juice + tissues these
decompose nucleic acid i.e. DNA + RNA
Nucleotidases: Occur in intestinal juice +
tissues + hydrolyses mononucleotides to
nucleosides + H3PO4
Nucleosidases: Catalyze the reaction
Nucleoside + H3 PO4→ free nitrogenous
base + pentose PO 4
Miscellaneous ester hydrolyzing
enzymes:Cholinesterase: 2 types
True type : Hydrolyzes only acetylcholine to
ecatic acid + choline.
Pseudo Type : variety is not specific +
hydrolyzes other related substrates as well.
Sulfatase: hydrolyses sulfate esters e.g.
phenol sulfatase which is present in
kidney + brain catalyzes the hydrolysis of
phenol sulfate to phenol + H2 SO4
Lyases: Catalyze the addition of NH3,
H2O or CO2 to double bonds or the
removal of these from double bonds
e.g. conversion of fumaric acid to Lmalic acid
CH + H2O
COOH Fumaric acid
L- Malic acid
Carbonic an hyderase present ion red blood cell
gastric mucosa renal tubules catalyzes the
H2 O + Co2
Isomerases: Catalyze the transfer of
gps. With in molecules to yield isomeric
forms of the substrate . eg.
Glucose 6 - PO4
fructose 6- PO4
other e.g are reductases, oxidases
Ligases: Catalys reactions joining 2 mol
by forming C- O, C – S, C - N & C - C
C = O + CO2 + ATP
Acetyl – CoA carboxylase
CH2 + ADP +P1
S - CoA
Protein nature: Enzymes are protein
catalysts ↑es the velocity of a chemical
reaction + are not consumed during the
chemical reaction their catalytic activity
depends upon the integrity of their
structure as proteins e.g. when boiled
with acid or incubated with trypsin that
will cleave the polypeptide chain & their
catalytic activity is lost.
Showing primary backbone structure is
reqd. Disruption of characteristics folding
of polypeptide chain of a native enzyme
protein by heat & by exposure to
extreme pH or temp. with other
denaturing agents the catalytic activity
is lost . Enzymes like other protein have
mol. wt. ranging from 12,000 to over a
Chemical nature: Some enzymes consist
only of polypeptides & contain no
chemical gp. Other than α. α resides e.g.
“ pancreatic ribonuclease”. Other
enzymes require an additional chemical
component for their activity called a “ co
factor” which may be inorganic like Fe+2,
Mn +2 or Zn 2+ ions.
Organic:- Called co enzymes. Some
enzymes require both coenzymes & one
or more metal ions which may be loosely
or tightly bond to the protein
Prosthetic group:- The co-enzymes or
metal ions when tightly & permanently
bond that do not dissociate from the
enzyme is known as prosthetic group e.g.
Holo Enzymes:- Refer to the active
enzyme with its non protein component
whereas the enzyme without its
nonprotein moiety is termed as “
apoenzyme” & is inactive if the non
protein moiety is metal ion such as Zn++
is called a “ co factor” if it is a small
organic mol. It is termed a “co enzyme”.
Active sites:- enzyme mol. Contain a
special pocket or clefty called the
active site. Which contains α. α side
chains that participate in substrate
binding & catalysis. The substrate binds
the enzyme forming an ES complex. ES is
converted to an enzyme product (EP)
that subsequently dissociates to enzyme
E + S → ES → EP → E+P
Catalytic efficiency : Enzyme catalysed
reaction are highly efficient. The
number of molecules of substrate
converted to product per enzyme
molecule per second is called the turn
over number or K cat.
Specificity: Enzymes are highly specific
interacting with one or a few substrates
& catalysing only one type of chemical
Regulation: Enzyme activity can be
regulated i.e. ↑ed or ↓ed.
Location within the cell:- Many enzymes
are localized in specific organelles within
the cell which serves to isolate the
reaction substrate or product from other
competing reaction , providing
favorable environment for the reaction
& organizes the no. of enzymes present
in the cell x useful pathways.
Energy changes occurring during a
All chemical reaction have an energy
barrier separating the reactants & the
products which is called “the free energy
of activation”. This barrier is the energy
difference between that of the reactants &
a high energy intermediate that occurs
during the formation of product.
Reactants imitate state
i. Free energy of reaction
ii. Rate of reaction
iii. Alternate reaction pathway
Progress of reaction
Free energy of reaction: The peak of
energy is the diff. in free energy
between the reactant & T where high
energy intermediate is formed during
the conversion of reactant to product
which of the high free energy of
activation the rates of uncatalyzed
chemical reactions are often slow.
Rate of reaction : It is determined by the
no. of energized molecules. In general
the lower the free energy of activation,
the more molecules have sufficient
energy to pass through the transition
state. & thus the faster the rate of the
Alternate reaction pathway: An enzyme
allows a reaction to proceed rapidly under
conditions prevailing in the cell by
providing an alternate reaction pathway
with a lower free energy of activation.
Enzymes do not change the activation.
Enzymes do not change the equilibrium of
the reaction but accelerate the reaction
which with equilibrium is reached.
Specificity : Enzyme exclusively binds to
& react with particular molecules or
classes of molecules that are substrates
for the reactions they catalyse . They
specificity of enzyme action has been
explained by two theories:a. Lock & key theory
b. Induced fit theory
a. Lock & key theory:- The active site of
enzyme is complementary in confirmation to
the substrate, so that the enzyme +
substrate recognize each other.
Enzyme substrate complex
b. Induced fit theory:- The enzyme changes
shape upon binding the substrate, sop that
the confirmation of substrate & enzyme
protein are only complementary after the
the important feature of this model is the
flexibility of the region of active site.
According to this the active site doesn't
process a rigid performed structure on
enzyme to fit the substrate. On the other
hand the substrate during its binding
induces conformational changes in the
active site to attain the final catalytic
shape & form.
This explains several matters related to enzyme
action such as :
enzyme become in active on denaturation
• Saturation kinetics
• All osteric modulation
Enzyme with substrate
Product fit b/w
enzyme & substrate
Functional groups of enzyme
The active site of the enzyme may furnish
R groups of the Sp α.α resides that are:
Good proto….. Or acceptors. Such
general acid or base gps. are powerful
catalysis for many organic reactions in
aqueous system” proton donor” gps.
may be – COOH, +NH3 – SH + “ proton
acceptors” may be - COO- , - NH2 – S
Nucleoplinlic gps or enzyme may
participate in reaction e.g proteolytic
enzymes i.e trypsin chymotrypsin,
Some enzymes bond convalatlly which
substrate to form ES complex & form
products more rapidly
Coenzymes are defined as heat stable,
low molecular weight organic
compounds required for the activity of
Most coenzymes are linked by now
covalent forces. Those which form
covalent bonds are prosthetics gps.
Enzyme that require co enzymes
catalyze following reactions:Oxidoraduction
Group transfer reactions
Reactions resulting in formation of
Many enzymes are derived from …..
These compounds are recycled and are
needed only in catalytic amount.
Coenzyme function as substrate in twosubstrate reactions being bond
momentarily to the enzyme during
catalysis. They are chemically altered
during the course of reaction & are
recovered to their original forms by same
or another enzyme.
a. Nicotinamide adenine dineralotide –(NAD+)
b. Nicotinamide adenine Phosphate (NADP+)
c. Flavin mononucleotide (FMN)
d. Flavin adernine dinucleotide (FAD)
a.b. Drived form Niocin & require it for their
synthesis, small amount of Niocin is ….
From tryptophane – essentail α. α
c.d. Drived from riboflavin
Lipoic acid – also involved in acl gp
Biopterinm – a pteridine containing
compound & participate in certain
hydroxylase .eg. Phynylalamine
Coenzymes Q- is a gp of closely related
compounds differing only in length of
side chain. They can be synthesized in
humans from ……. Pyrophosphate – an
intermediate in cholesterol biosynthesis.
Thiamine pyrophosphate (TPP) is used for
oxidatine decarboxylation of
Ketoacids & in the trans ketolase
catalyzed steps of the pentose
Pyridoxal phosphate:- involved in
variety of reactions on amino acids e.g.
transamination, elimination of H2o or
hydrogen sulfide . It is derived from vit.
Tera hydrofolic acid (FH4): is a carrier of
one carbon fragments. It is derived from
Coenzyme A(COA, COASH): takes part
in acetyl + other acyl gp. Transfer +
require vit. Pantothenic acid for its
Biotin is vit. Tightly bond to apoenz…. In
an amide likage δ – amino gp. Of lysyl
resid.. & involved in carboxylation
Cobamide coenzyme: contains cobalt
bond in a porphyrin like ring system. It is
involved in methyl transfer reactions. It is
derived from cyanocarbalamin ( vit. B 12)
Adenosine triphosphate (ATP) can be a
donor of phosphate, adenosine +
adenosine monophosphate (AMP) for
Cytidine dipphosphate (CDP) is a carrior
of phosphoeyl choline, dioxyl glycerols &
other molecules during synthesis of
Uridine diphosphate (UDP) ia a carrior of
monoisacehorides + their derivation in
Phospho adenosine phospho sulfate is a
sulfate donor in the synthesis of sulfurs
containing monocopolysacharides & in
detoxification of sterol steroid & other
Isienzymes are the physically distinct
forms of the same enzyme but catalyze
the same chemical reaction or reactions
& differ from each other structurally,
electrophoretically & immunologically.
Isoenzymes are different molecular forms
of enzymes that may be isolated from
the same or different tissues. Their
physical proportion are different
because of genetically determined
differences in amino acid sequence.
Different organs contain certain proportion
of different isoenzymes. The pattern of
isoenzymes found in plasma may there for
same as a mean of identifying the site of
tissue damage e.g.
Creatine kinase – CK
lactate dehydroganase – LDH
Alkaline phosphatase - ALP
Many isoenzymes contain different subunits in various combinations e.g
LDH. Various types are
LDH – 2,
LDH – 3,
LDH – 4, LDH
ii. Inhibition with urea
iii. Reaction with changed substrate
In normal serum H3 M isoenzyme is
present in highest conc. In an individual
who has suffered a myocardial
infarction, particularly H4 are elevated .
The ↑se in H4 confirms the diagnosis that
the patient suffered a myocardial
Possible isoenzymes of CPK
Ion exchange chronatography
Radio immuno assay (RIA)
Brain , bladder
↑ed level in condition
CK - 1
CNS – shock carcinomas.
Placenta / utrine trauma.
Co-poisoning acute &
chronic renal failure.
CK – MB
MI , angina
CK – 2
CK – MM
Exists as a no. of isoenzymes
Major ALP isoenzyme in normal serum of
healthy adult person is derived from liver.
In growing children bone isoenzyme
Hepatic isoenzymes ↑es in liver diseases.
Bone isoenzyme ↑es due to osteoblastic
activity & is normally elevated in children &
adults over 50 years. chronic haemodialysis.
Placental isoenzyme ↑es in last 6 wks. Of
Intestinal isoenzymes ↑es consumption of
fatty meal. In disorders of GIT, cirrhosis of
liver & in patients undergoing
Sources of plasma enzymes:
1. Plasma derived :- their activity is higher in
plasma than cells. E.g. coagulation
2. Cell derived:- Their activity is higher in
cells overflow in plasma.
a. Secretory – from digestive gland
b. Metabolic – concerned with metabolism
Normal turnover of tissue
lack of substrate,
coenzymes & protenase)
Leakage through cell mem. Uptake by tissues with
subsequent in activation
Removal by RES.
↑se enzyme synthesis
Excretion in urine of low
› Cell necrocesis
› ↑sed cell mem. Permeability without cell
› ↑sed enzyme production
› An ↑se in cell no. producing enzymes
Impaired disposition/ secretion
↓ed formation of enzymes
Enzyme inhibition (poising)
Lake of cofactor.
Diagnosis of different pathological
conditions e.g. MI (CK,LDH, AGT)
Prognosis: serial serum enzyme assay
reqd. & change in serum enzyme level in
Therapeutic use: streptokinase, an
enzyme that facilitates the breakdown
of clot, commonly used to dissolve a clot
that causes MI.
Catalyes the reaction
Creatinine – P + ADP → creatine + ATP
It is present in high  in SK , muscle,
myocardia+ brain. In small it is present
in lung thyroid + kidneys. It is absent in
Normal value: 4- 60 iμ/L at 37 ⁰C
↑es after 6hrs.
Peak level – 24 – 30 hrs.
Normal level – 2 – 4 days
Serum CK level is a very sensitive
indicator in early stages of myocardial
Enzym Start to
4 – 8 hrs.
24 – 48 hrs.
6 - 8 hrs.
24 – 48 hrs.
12 -24 hrs. 48 – 72 hrs.
7 – 12
Hepatocellular damage or ↑ed liver cell
Extrac hepatic or intrahepatic obstruction
(being or malignant)
Trans aminases (SGOT)
Oenithinie carboxyl transferase (OCT)
Serum LDH – unidespread malignancies
Β – Glucouromidase in urine – cancer of
A metabolic pathway involves many enzymes
functioning in a sequential manner. Control of
the pathway is achieved through modulation
of the activity of only one / few key enzymes i.e
regulatory enzymes. A regulatory enzyme
catalyze a rate – limiting chemical reaction
that controls the overall pathway. It may also
catalyze a chemical reaction unique to that
pathway – committed step.
These enzymes which catalyze the rate limiting
step or committed step of a pathway are
When the end product exceeds the
steady – state level concentration, it
inhibits the regulatory enzyme in an
attempt to normalise the overall process.
Enzymes may be altered suppression. This
regulation at the genetic level occurs.
During various phases of reproduction,
growth & development.
Compartmentalization of enzyme system
e.g. fatty acid synthesis occurs in the
soluble fraction of cytoplasm.
7. Non covalent/ allosteric
8. Induction & repression of enzyme
Some covalent chemical modifications are:
i. Phosphorylation & de phosphosylation
ii. Acetylation & de acetylation
iii. Adenylytion & de adenylytion
iv. Uridylylation & de uridylylation
v. Methylation & de methylation
Phosphorylationis catalysed by
proteinkinases & occurs at specific seryl
residues& occasionally at through residues.
These αα residues are not usually part of the
catalytic site of the enzyme
dephosphorylation is accomplished by
The overall process of phosphorylation &
dephosphorylation consists of an
extracellular signal commonly referred to as
first messenger e.g. hormones which
combines with specific receptor on the cell
membrane of target cell which produces
an intracellular signal the 2nd messenger.
Depending on specific enzyme the
phosphorylated form may be more or
Enzymes are regulated by molecules
called effectors modifiers or modulators,
that binds non covalently at a site other
than active site . They alter the affinity of
enzyme for to substrate or modify the
maximal catalytic activity of enzyme or
Negative effecter: inhibit enzyme
Positive effecter: increase enzyme
H-omotropic effecter: substrate – severe
as an effecter
Hetrotropic effecter: effecter may be
different from substrate.
Cells can regulate the amount of
enzyme present by altering the rate of
enzyme degradation or the rate of
Change in Y ``
or V max
Change in V ``
max or KS
Change in V ``
max or Km
the amount. days
metabolite Of enzyme
Quantization of enzyme activity:- the
rate at which the substrate changes to
the product is directly proportional to :
2. Enzyme concentration
In zero order reaction, the rate , or
velosity (V) is constant & is independent
of the reactant concentration
In first order reactions, the rate is
proportioned to the reactant
In second order reactions, the rate is
proportional to the product of the
concentrations of the reactants.
The enzyme sensibly combines with its
substrate to form ES complex that
subsequently breaks down to product,
regenerating free enzyme.
K 1+ K1 + K2 = rate constant
Michaerlis describes how reaction velocity varies
with substrate concentration
Vi = V max [ ]
Km + [s]
initial reaction velocity
Michaerlis constant = ( K-1+K2)
Km is not equilibrium constant, it is ratio of
Following assumptions are made in deriving
Relatime concentrations of E + S
The conc. Of substrate is much greater
than the concentration of enzyme [E] so
that the amount of substrate bond by
the enzyme at any one time is small.
[ES] does not change with time – that is
the rate of formation of ES is equal to
that of the breakdown of ES ( E+ S + to
In general, intermediate in a series of
reaction is said to be in steady state
when the rate of synthesis is equal to its
rate of degradation.
Only initial reaction velocities are used in
the analysis of enzyme reactions i.e the
rate of the reaction is measured as soon
as enzyme & substrates are mixed . At
that time the concentration of product is
very small and therefore the rate of back
reaction from P to S can be ignored.
Characteristic of Km:
The michaelis constant is characteristic of
an enzyme & a particular substrate &
reflect the affinity of the enzyme for that
Km is numerically equal to substrate [ ] at
with the reaction velocity is equal to ½ V
Km does not vary with the concentration
Small Km high affinity of enzyme for
Large Km low affinity of enzyme for
The rate of reaction is directly
proportioned to the enzyme [ ] at all
substrate [ ] e.g. if the enzyme [ ] is halved.
The initial rate of reaction (V) is related to
one half that of original.
Km + Vmax may be influenced by pH,
temp. & other factors.
In a metabolic pathway, Km values for
enzymes that catyalyze the sequential
reactions may indicate the rate –
limiting step for the pathway the highest
Km corresponds roughly to the slowest
When K >> K2
[ES] + [S]
ES dissociating more after to yield E+S than
to yield product
When K2 >> K-1
The rate of dissociation of ES to E+S is small,
so that products are usually formed.
When [S] >>Km
The characteristic property of the turnover
number for an enzyme can be invoked. This
no. provides information regarding how
many times it forms the ES complex & is
regenerated by yielding P.
Order of reaction:
At high [ ] of substrates the velocity of
reaction is zero order. i.e constant &
independent of substrate concentration. At
low [ ] of substrate, the velocity of reaction is
1st order i.e proportional to substrate
When the reaction velocity Vi is plotted
against substrate concentration, [S}, it is
not always possible to determine when V
max has been achieved, because of the
curve at high substrate [ ]. However of
I/Vo plotted Vs I/[S ] a straight line is
obtained. This plot is called line weaver
burke plot & can be used to calculate Km
& Vmax as well as determine mechanism
of action of enzyme inhibitions.
The activity is usually defined as that
quantity of enzymes which catalyzes the
conversion of one micromole of substrate to
product per minute under defined set of
It is repressed in terms of μ/ml of biological
specimen e.g. serum or μ/l.
6. Effect of time
concentration of activity
8. Effect of inhibitors
All major factors that affect the rate of enzyme
catalyzed reactions are of chemical interact.
Good health requires not only that hundreds of
enzyme catalyzed reactions to take place, but
also that they proceed at appropriate rates.
Failure to achieve this disturbs the homeostatic
balance of our tissues with potentially profound
Maximal velocity: The rate or velocity of
a reaction (V) is the number of substrate
molecules converted to product per unit
time & is usually suppressed as μ mole
product formed per minute.
If the concentration of a substrate [s] is
↑ed while all other conclusions are kept
constant the measured initial velocity Vi
↑ed to a maximum value Vmax. The
velocity ↑ed as the substrate  is ↑ed up
to a point where the enzyme is said to
be saturated with substrate.
At point A+B only a portion of enzyme
present to combined with substrate.
At point A or b ↑ing or ↓ing [S] with
therefore ↑ or ↓ the amount of E
associated with S as ES of Vi will depend
on [s] at C all enzyme is combined with
substrate so that further ↑se in [S],
although it ↑es the frequency of
collision b/w enzyme & substrate, cannot
result in ↑ rate of reaction since no free
enzyme is available to react.
The initial rate of reaction is the rate measured
before sufficient product has been formed to
permit the severe reaction to occur. The initial
rate of reaction catalyzed by enzyme is always
proportional to the concentration of enzyme.
↑se in the concentration of enzyme ↑se the
rate of reaction. Becauset here are more
active sites available to change substrate into
The formation of product is essentially
irreversible the severe reaction does not occur
to any appreciable retent.
Thus K-2 is much less than K2. if the product is
removed the reaction will be complete, but if
not removed the reaction will remain
incomplete. Under steady-state conditions, the
net effect of enzyme is to convert substrate
products as rapidly as the products are
pH dependence of enzyme activity is result of
Ionization gps. In the active site of the
enzyme in the substrate or in enzymesubstrate complex can affect catalysis
depending or whether the gps. Are
dissociated or undissociated. Ionization of
these gps. Depends on their pK values, the
chemical properties of surrounding gps. &
the pH of the reaction medium .
Changes in pH effect the binding of the
substrate at the active site of enzyme &
also the rate of breakdown of ES
complex e.g. catalytic activity may
require an amino gps of the enzyme be
in protonated from (-NH3+) at alkaline
pH this gp is deprotonated & the rate of
reaction therefore declines.
The enzymes in living systems function at
nearly constant pH because they are in
an environment that contains buffers.
The pH enzyme activity profile of most
enzymes delineates or bell shaped curve
each exhibiting an optimal pH- ie the pH at
with enzyme activity is maximal. E.g. pepsin,
a digestive enzyme in the stomach is
maximally active at pH2. whereas other
enzymes arte designed to work at neutral
pH. Are denatured by such an acidic ….
Extremes of pH can also lead to
denaturation of the enzyme ΅ the
structure of catalytically active protein
molecule depends on the ionic
character of the amino acid side chain.
Increased velocity with temperature: The
reaction velocity ↑es with temperature
un till a peak velocity is reached. This ↑es
the result of the ↑sed no. of molecules
having sufficient energy to pass over the
energy barrier & form the products of
Further elevation of temperature results
in ↑es in the reaction velocity as a result
of temperature induced denaturation of
The rate at which the substrate changes
to products is directly proportional to
Many enzymes require the presence of
metal ions to function. Those enzymes that
bind the metal ions loosely are called
metal-activated enzymes. Common …
activators include Mg+2, Mn +2, fe2+, Ca2+, Zn2+, K+
aminos may also function as activators.
Magnesium is an obligate activator for
allkinase enzyme i.e PO4 transfer enzymes.
Amylase is a calcium metallo enzyme that
displays full activity in the presence of
variety inorganic ions(Ce`, Br`, NO-+
In activation dependant enzyme reactions
the substrate should be present in …
concentration. Excess activator may also
function to inhibit activity. Therefore in some
cases optimal activator should be used. So
in absence of coenzyme of activator
enzyme may be inactive or sluggish.
Any substance that can dimish the velocity of
an enzyme catalysed reaction is called as
The inhibitors are:A. Reversible inhibitors B. irreversible inhibitors
Competative inhibitors - ↑se Km but has
no effect on V max
Now competetive inhibitors - ↓ both
Vmax + Km.
Providing information about shape of
Types of αα side chains there
Working out enzyme mechanism
Providing information about control of
Design of drugs.
Reversible inhibitors bind to enzyme
through noncovalent bonds. Dilution of
the enzyme inhibitor complex results in
dissociation of reversibility bond inhibitor
& recovery of enzyme activity. Its further
In reversible competetive inhibition,
inhibitors compete with the substrate for
binding to active site & they form
enzyme inhibitor complex.
The effect of a competitive inhibitor is
reversed by ↑ing [S] . At a sufficiently high
[S] the reaction velocity reaches the
Vmax observed in the absence of
In the presence of competitive inhibitor
more substrate is needed to achieve ½
Apparent competitive inhibition occurs in 4
Compete with substrate for binding at
active site e.g. inhibition of succinate
In this reaction FAD, a coenzyme serves
as a hydrogen acceptor. This enzyme is
competeively inhibited by malonate,
oxalate or oxaloocetate, all are
structural analogues of succinate.
Competitive inhibition of a biosynthesis
step in folate synthesis accounts for the
antimicrobial action of sulforaminades
which are structural analogues of para
amino benzoic acid (PABA)
Para amino benzoic acid is used by the
bacteria in the synthesis of folic acid.
Sulforamides inhibit the bacterial enzymes
responsible for incorporation of PABA into
7-8 dihydropteroic acid & lead to inhibition
of growth of a wide range of gram+ve &
gram –ve microorganism.
Microorganism susceptable to
sulfonamides are those with synthesize
their … folic acid derived from host.
Sulfonamides however have no effect
on host cells. That require preformed folic
Uric acid is the end product of purine
catabolism in humans.
allopurinol a structural analogue of
hypoxanthine is a comprtetive inhibitor
as well as substrate for xanthine oxidase.
Allopurine inhibits the formation of xanthine & of uric acid.
In two substrate enzyme-catalysed
reactions, high  of 2nd substrate may
complete with the first substrate for
binding e.g. reaction catalysed by
aspartate aminotransferase. L-aspartate
& ketoglutarate Lglutamate +
Competetive inhibition in reversible reactions
due to accummlation of products. Inhibits e.g.
alkaline phosphatase causes hydrolysis of a
wide variety of organic mono-phosphate
esteers x the corresponding alcohols & in
organic PO4 occurs, the inorganic Po4 acts as
a competetive inhibitor. Both the inhibitor &
the substrate have similar binding affinutis.
Metal ions act as inhibitors:- In reactions
that require metal ions as cofactors.
similar,metal ions can compete for the
same binding site on enzyme e.g. prymate
kinase catalyzees the reaction.
Phosphoand pyrumate & ADP→ ATP +
pyrmate for which K+ is an obligatory
activators whereas Na+ + Li+ are potent
Inhibitor does not usually bear any structural resemblance
to the substrate & it binds to the enzyme at site distinct
from substrate b/w the inhibitor & substrate & inhibition
cannot be overcome by ↑se of substrate concentration.
An inhibitor may bind either to a free enzyme or to an
enzyme-substrate complex in both cases, the complex is
catalytically in active
E I (inactive)
ES + I
ESI ( inactive)
Vmax is reduced as non-competetive
inhibition cannot be overcome by ↑ing
the concentration of substrate.
Km is un affacted because the affinity of
S for E is unchanged.
Lead covalent bounds with sulphydeyl
group side of cyteine in proteins e.g.
ferochelatase an enzyme that catalyze the
insertion of Fe+2 into porphyrin & δ amino
lenulinate dehydrase, both enzymes are
sensitive to inhibition by lead i.e why lead
poisoning cause anemia.
For activity are inhibited by chelating
agent e.g ethylenediamine tetra
acetate that remove the metal ion from
Uncompetitive inhibitions combine
reversibly only with ES to form ESI which
cannot yield product. It is not reversed by ↑
ES + I
Inhibitions bind only to the ES at a site
distiunct from active site.
Uncompetitive inhibition is rarely
observed in single-substrate reaction. It is
more common in 2-substrate reactions
with a double displacements reaction
mechanism e.g. inhibition of intestinal
alkaline PO4 by L-phynylalanine.
Occurs when the inhibition acts at or near the
active site of the enzyme with covalent
modification of the active site or when the
inhibitor binds so tightly that there is no
dissociation of enzyme inhibitor. Thus physical
separative processes are ineffective in
removing the irreversible inhibitor from the
E + I
i. Enzymes that contain free sulphydeyl
groups at the active site e.g.
glyceraldelyde -3-Po4 dehydrogenase.
Enzyme – SH+ iodoaaxetic acid →
inactive covalent derinactive of enzyme
Enzymes with seryl hydroxyl group at active
site. These enzymes can be inactiveated
by organophosphorus compounds. Several
organophosphorous compounds are used
as agricultural insecticides, improper
exposure to which can result in toxic
manifestation & death.
Acetylcholine is a neurotransmitter which is
related on arrival of a nerve impulse at the
ending of neuron & it ↑ses the premeability
of Na+ across the postsyoptic membrane &
result in progation of action potential.
Acetycholine is quickly destroed by
The organophosphorous compounds cause
inactivation of acetylcholine esterase, the
continued presence of acetylcholine
causes extended transmission of nerve
impulses. In muscle fibers continues
depolarization leads to paralysis. The cause
of death is respiratory failure due to
poralysis of respiratory muscles.
Competitive inhibition is the basis for the
treatment of some intoxicants e.g.
nethanol which is widely used in industry
as a solvent. Methanol is metabolised
mainly in liver and kidneys.
Major toxic effects are caused by
formaldehyde causing damage to retinal
cells leading to blindness. Formic acid –
severe acidosis – deaths . Retardation of 1st
step is accomplished by administration of
ethanol, the oxidation products of which
are not as toxic as those of methanol.
Drugs can also inhibit enzymes e.g.
Penicillin which inhibits the reaction with
transpeptidase that is important in the
development of bacterial membranes.
Thus destroying normal growth of
T/m of gout