2. DEFINITION
Enzymes are proteins that speeds up the rate of a chemical reaction in a
living organism. An enzyme acts as catalyst for specific chemical
reactions, converting a specific set of reactants (called substrates) into
specific products.
Catalysts increase the rate at which chemical reactions occur without
being consumed or permanently altered themselves. A chemical
reaction is a process that converts one or more substances (known as
reagents, reactants, or substrates) to another type of substance (the
product). As a catalyst, an enzyme can facilitate the same chemical
reaction over and over again.
3. PROPERTIES OF ENZYMES
Enzymes are proteins. Like all proteins, enzymes are composed of one
or more long chains of interconnected amino acids. Each enzyme
possesses a unique sequence of amino acids that causes it to fold into a
characteristic shape. An enzyme's amino acid sequence is determined
by a specific gene in the cell's nucleus. This ensures that each copy of
the enzyme is the same as all others.
4. On the surface of each enzyme is a special cleft called the active site,
which provides a place where reagents can 'meet' and interact. Much
like a lock and its key, an enzyme's active site will only accommodate
certain reagents, and only one type of chemical reaction can be
catalyzed by a given enzyme.
5.
6. When reagents enter an enzyme's active site, the
enzyme undergoes a temporary change in shape
that encourages interaction between the
reagents. Upon completion of the chemical
reaction, a specific product is released from the
active site, the enzyme resumes its original
conformation, and the reaction can begin again
with new reagents.
7. Many enzymes are incorporated into metabolic
pathways. A metabolic pathway is a series of
chemical reactions that transform one or more
reagents into an end-product that's needed by
the cell. A metabolic pathway can be quite short,
or it can have many steps and multiple enzymes.
The metabolic pathway that converts tryptophan
(an amino acid found in dietary protein) to
serotonin (a chemical that's necessary for
normal brain function) is only two steps long.
8. Some enzymes needs non protein components to carry out biological
reactions these non protein components are called cofactors. An
enzyme along with its cofactor is called holoenzyme. If such enzyme
looses its cofactor then its called apoenzyme. The cofactors can either
be inorganic or organic molecules. The inorganic cofactors are usually
metal ions that are requried for its activity. The organic cofactors can
either be tightly bound with the enzyme or released free after the
reaction is complete. The tightly bound organic cofactor is called
prosthetic group where as the organic cofactor that is released after
catalysis is called coenzyme.
9. The organic cofactors can either be tightly bound with the enzyme or
released free after the reaction is complete. The tightly bound organic
cofactor is called prosthetic group where as the organic cofactor that is
released after catalysis is called coenzyme.
10. In order to function, many enzymes require the help of cofactors or
coenzymes. Cofactors are often metal ions, such as zinc, copper, iron,
or magnesium. Magnesium, one of the most common cofactors,
activates hundreds of enzymes, including those that manufacture DNA
and many that help metabolize carbohydrates.
12. 1. RELATIVE, LOW OR
BOND SPECIFICITY
e.g.
a. Amylase, which acts on α 1-4 glycosidic,
bonds in starch, dextrin and glycogen.
b. Lipase that hydrolyzes ester bonds in
different triglycerides
In this type the enzyme acts on
substrates that are similar in structure
and contain the same type of bonds
13. 2.MODERATE,
STRUCTURAL OR
GROUP SPECIFICITY
For example:
Pepsin is an endopeptidase that hydrolyzes
central peptide bonds in which the amino group
belongs to aromatic amino acids e.g. phenyl
alanine, tyrosine and tryptophan.
Trypsin is an endopeptidase that hydrolyzes
central peptide bonds in which the amino group
belongs to basic amino acids e.g. arginine, lysine
and histidine.
Chymotrypsin is an endopeptidase that
hydrolyzes central peptide bonds in which the
carboxyl group belongs to aromatic amino acids.
Aminopeptidase is an exopeptidase that
hydrolyzes peripheral peptide bond at the amino
terminal (end) of polypeptide chain.
Carboxypeptidase is an exopeptidase that
hydrolyzes peripheral peptide bond at the
carboxyl terminal of polypeptide chain.
In this type of specificity, the enzyme
is specific not only to the type of
bond but also to the structure
surrounding it.
14. 3. ABSOLUTE, HIGH OR
SUBSTRATE SPECIFICITY
e.g.
a) Uricase, which acts only on uric acid.
b) Arginase, which acts only on arginine.
c) Carbonic anhydrase, which acts only on
carbonic acid.
d) Lactase, which acts on lactose.
e) Sucrase, which acts on sucrose.
f) Maltase, which acts on maltose.
In this type of specificity, the enzyme
acts only on one substrate
15. 4. OPTICAL OR STEREO-
SPECIFICITY
L amino acid oxidase acts only on L amino
acids.
D amino acid oxidase acts only on D amino
acids.
α- glycosidase acts only on α- glycosidic
bonds, which are present in starch, dextrin
and glycogen.
β- glycosidase acts only on β- glycosidic
bonds that are present in cellulose.
N.B. We can digest glycogen and starch due to
presence of α- glycosidase, but we can not
digest cellulose due to the absence of β-
glycosidase
In this type of specificity, the enzyme
is specific not only to the substrate but
also to its optical configuration
16. 5. DUAL SPECIFICITY
A- The enzyme may act on two substrates by one
reaction type.
e.g. xanthine oxidase enzyme acts on xanthine and
hypoxanthine (two substrates) by oxidation (one
reaction type).
Xanthine oxidase Xanthine oxidase
Hypoxanthine ------Xanthine--- Uric acid
B- The enzyme may act on one substrate by two
different reaction types e.g. isocitrate dehydrogenase
enzyme acts on isocitrate (one substrate) by oxidation
followed by decarboxylation (two different reaction
types).
Isocitrate dehydrogenase
Isocitrate ---------------α- ketoglutarate
NAD -NADH+H+ CO2
There are two types of dual
specificity:
17. ENZYME UNITS
The enzyme unit, or international unit for enzyme (symbol U,
sometimes also IU) is a unit of enzyme's catalytic activity. 1 U
(μmol/min) is defined as the amount of the enzyme that catalyzes the
conversion of one micromole of substrate per minute under the
specified conditions of the assay method.
The SI unit is the katal, 1 katal = 1 mol s−1, but this is an excessively
large unit.
1 Katal [kat] = 59 999 999.88 Enzyme unit [U]
18. SPECIFIC ENZYME
ACTIVITY
Specific enzyme activity (usually stated simply
as ‘specific activity’) is the number of enzyme
units per ml divided by the concentration of
protein in mg/ml. Specific activity values are
therefore quoted as units/mg or nmol/min/mg
(if unit definition B is applied).
Specific activity is an important
measure of enzyme purity and values
for different batches of a pure
enzyme should be the same, within
normal experimental error.
19. NC-IUBMB
NC-IUBMB Enzyme List, or, to give it its full title, “Recommendations of
the Nomenclature Committee of the International Union of
Biochemistry and Molecular Biology on the Nomenclature and
Classification of Enzymes by the Reactions they Catalyse,is a functional
system, based solely on the substrates transformed and products
formed by an enzyme.
20. EC
EC numbers
Enzymes are identified by EC (Enzyme Commission) numbers. These are
also valuable for relating the information to other databases. They were
divided into 6 major classes according to the type of reaction catalysed
and a seventh, the translocases, was added in 2018.3 These are shown
in Table 1.
22. ENZYME CLASSES
Name Reaction catalysed
1 Oxidoreductases *AH2 +B=A+BH2
2 Transferases AX + B = BX + A
3 Hydrolases A-B + H2O = AH + BOH
4 Lyases
A=B + X-Y = A-B
XY
5 Isomerases A=B
6 Ligases †A + B + NTP = A-B + NDP + P
(or NMP + PP)
7 Translocases AX+B=A+X+ B
(side 1) (side 2)
*Where nicotinamide-adenine
dinucleotides are the acceptors, NAD+
and NADH + H+ are used, by
convention. †NTP = nucleoside
triphosphate.
23. The EC number is made up of four components separated by full stops. The
first identifies the class of reaction catalysed. The second number (the
subclass) generally contains information about the type of compound or
group involved. For the oxidoreductases, the subclass indicates the type of
group in the donor that undergoes oxidation or reduction (e.g., 1.1. acts on
the CH- OH group of donors whereas 1.4. acts on the CH-NH2 group of
donors).
The third number, the sub-subclass, further specifies the type of reaction
involved. For instance, EC 1.x.1.- indicates that NAD+ or NADP+ is the
acceptor, while 1.x.2.- has a cytochrome as the acceptor, etc. The fourth is a
serial number that is used to identify the individual enzyme within a sub-
subclass.
24.
25.
26. Isomerases[edit]
EC 5.1 includes enzymes that catalyze racemization (racemases) and
epimerization (epimerases)
EC 5.2 includes enzymes that catalyze the isomerization of geometric
isomers (cis-trans isomerases)
EC 5.3 includes intramolecular oxidoreductases
EC 5.4 includes intramolecular transferases (mutases)
EC 5.5 includes intramolecular lyases
EC 5.99 includes other isomerases (including topoisomerases)
27. Ligases
EC 6.1 includes ligases used to form carbon-oxygen bonds
EC 6.2 includes ligases used to form carbon-sulfur bonds
EC 6.3 includes ligases used to form carbon-nitrogen bonds (including
argininosuccinate synthetase)
EC 6.4 includes ligases used to form carbon-carbon bonds
EC 6.5 includes ligases used to form phosphoric ester bonds
EC 6.6 includes ligases used to form nitrogen-metal bonds
