Enzymes Flashcards
1 molecule of an enzyme can act upon
About 1000 molecules of substrate per min
Lack of enzymes will lead to
Block in metabolic pathways causing inborn errors of metabolism
Characteristics of enzymes (5)
Almost all enzymes are proteins. Enzymes follow the physical and chemical reactions of proteins.
They are heat labile
Water soluble
Pptd by protein ppting reagents (ammonium sulfate or trichloroacetic acid)
Contain 16% weight as nitrogen
IUBMB classification of enzymes in order
Oxidireductases
Transferases
Hydrolases
Lyases
Isomerases
Ligases
Translocases
Enzymes are grouped into
7 major classes
Enyme classification.
NAME
EXPLANATION
ENZYME EXAMPLE
REACTION
2 EXAMPLES
OXIDIREDUCTASES
This group of enzymes will catalyse oxidation of
one substrate with simultaneous reduction of
another substrate or co-enzyme.
This may be
represented as
AH2 + B ————-→ A + BH2
for example,
Alcohol + NAD+ —-→Aldehyde+NADH + H+
The enzyme is Alcohol dehydrogenase;
IUB
name is Alcohol-NAD-oxidoreductase; Code
number is EC.1.1.1.1.
Oxidoreductases may also
oxidise substrates by adding oxygen, e.g.
oxidases, oxygenases and dehydrogenases
TRANSFERASES
This class of enzymes transfers one group
(other than hydrogen) from the substrate to
another substrate.
This may be represented as
A-R + B → A + B-R
,
For example,
Hexose + ATP → Hexose-6-phosphate + ADP
The name of enzyme is Hexokinase and
systematic name is ATP-Hexose–6-phosphate-
transferase.
HYDROLASES
This class of enzymes can hydrolyse ester, ether,
peptide or glycosidic bonds by adding water and
then breaking the bond.
Acetyl choline + H2O ——–→ Choline + acetate
The enzyme is Acetyl choline esterase or
Acetyl choline hydrolase (systematic).
All
digestive enzymes are hydrolases.
LYASES
These enzymes can remove groups from
substrates or break bonds by mechanisms other
than hydrolysis.
For example,
Fructose-1,6-bisphosphate ——-→
Glyceraldehyde-3-phosphate
+dihydroxy acetone phosphate
The enzyme is aldolase
Subclass : hydratase, hydration of a double bond or adding water to a double bond
ISOMERASES
These enzymes produce optical, geometric
or positional isomers of substrates.
Racemases,
epimerases, cis-trans isomerases are examples.
Glyceraldehyde-3-phosphate —–→
Di-hydroxy-acetone-phosphate
Enzyme is Triose phosphate isomerase.
LIGASES
These enzymes link two substrates together,
usually with the simultaneous hydrolysis of ATP
For example,
Acetyl CoA + CO2 + ATP → Malonyl CoA + ADP +
Pi
Enzyme is Acetyl CoA carboxylase.
TRANSLOCASES (3)
Catalyse the translocation of ions and other molecules across membranes (from one side of the membrane to other side)
Some of em use E of hydrolysis of ATP for the translocation and were previously included with Atpases(class3)
There are specific transl for transferring H ions inorganic anions cations aas carbohydrates and other compounds
Enzyme inhibition and clinical significance (6). TYPES
Competitive
Non competitive
Uncompetitive
Suicide inhibition
Allosteric regulation/ inhibition
Feedback inhibition
COMPETITIVE INHIBITION
- Explain
- Reaction velocity is decreased
- Structural analog
- Reversible
- From the graphs
1.Here inhibitor molecules are competing with
the normal substrate molecules for binding to
the active site of the enzyme,because the
inhibitor is a structural analog of the substrate.
E + S E-S ——-→ E + P
E + I E-I
- Since E-I (enzyme–inhibitor complex) can react
only to reform the enzyme and inhibitor, the
number of enzyme molecules available for E-S
formation is reduced. Suppose 100 molecules
of substrate and 100 molecules of inhibitor are
competing for 100 molecules of the enzyme.
So, half of enzyme molecules are trapped by
the inhibitor and only half the molecules are
available for catalysis to form the product.
Since effective concentration of enzyme is
reduced, the reaction velocity is decreased. - competitive inhibition, the inhibitor will be a
structural analog of the substrate. There will
be similarity in three dimensional structure
between substrate (S) and inhibitor (I). For
example, the succinate dehydrogenase
reaction is inhibited by malonate - Competitive inhibition is usually reversible.
Or, excess substrate abolishes the inhibition.
In the previous example of 100 moles of E and
100 moles of I, if 900 moles of S are added,
only 1/10th of enzyme molecules are attached
to inhibitor and 90% are working with substrate.
Thus 50% inhibition in the first example is now
decreased to 10% inhibition - From the graphs, it is obvious that in the case
of competitive inhibition, the Km is increased
in presence of competitive inhibitor. Thus
competitive inhibitor apparently increases the
Km. In other words, the affinity of the enzyme
towards substrate is apparently decreased in
presence of the inhibitor.
But Vmaxis not changed.
NONCOMPETITIVE INHIBITION
Definition (5)
Examples
Irreversible
Graph(2)
A variety of poisons, such as iodoacetate,
heavy metal ions (lead, mercury) and oxidising
agents act as irreversible non-competitive inhibitors.
There is no competition between S and I.
Also known as mixed inhibition
The inhibitor usually binds to a different domain on the enzyme other than the substrate binding site.
Since these inhibitors have no structural resemblance to the substrate an increase in S conc generally does not relieve this inhibition.
Examples :
CN inhibits cytochrome oxidase
Fluoride will remove Mg and Mn ions and so will inhibit the enzyme enolase and consequently glycolysis
Iodoacetate inhibits E having SH groups in their active centres
BAL - British Anti Lewisite ; dimercaprol is used as an antidote for heavy metal poisoning. The heavy metals acts as enzyme poisons by reacting with the SH group. Bal has several SH groups with which the heavy metals ions can react and thereby their posionous effects are reduced.
The inhibitor combines with the enzymes by
forming a covalent bond and then the reaction
becomes irreversible. The velocity (Vmax) is
reduced. But Km value is not changed,
because the remaining enzyme molecules
have the same affinity for the substrate.
UNCOMPETITIVE INHIBITION
Definition (3)
Examples (2)
Inhibitor does not have any affinity for free enzyme
Inhibitor binds to E-S complex but not to free E
Both Vmax and Km are reduced
Examples:
Inhibition of placental alkaline phosphatase(Regan isoenzyme) by phenylalanine
Anticonvulsant valproate and anticancer drug camptothecin are un inhibitors
Valp inhibits it’s own mechanism
Campto blocks cell division by inhibiting topoisomerase1
SUICIDE INHIBITION
Definition (3)
Process(3)
It is a special type of irreversible inhibition of enzyme activity
Also known as mechanism based inactivation
I makes use of Es own reaction mechanism to inactivate it.
Structural analog is converted to a more effective inhibitor with the help of E to be inhibited
S like compound initially bind with the E and the first few steps of pathway are catalyzed
This new product irreversiblly binds to the E and inhibits further reactions
Examples of suicide inhibition (2)
Allopurinol which is oxidised by xanthine oxidase to alloxanthine that is a strong I of xanthine oxidase
Anti inflammatory action of aspirin
Arachidonic acid is converted to prostaglandin by E cyclooxygenase. Aspirin acetylates a serine residue in the active center of cyclo thus prostaglandin synthesis is inhibited and so inflammation subsides
