Enzymes, Chapter Notes, Class 11, Biology PDF Download

ENZYMES

Enzymes are proteinaceous, biocatalysts.
First enzyme discovered by Buchner
Term enzyme was given by Kuhne.
Zymase (from yeast) was the first discovered enzyme. (Buchner)
The first purified and crystalized enzyme was urease (by J.B. Sumner) from Canavalia/Jack Bean (Lobia plant)
Proteinaceous nature of enzyme was established by Northrop and Sumner.

DEFINITION

Enzymes are biocatalysts made up of proteins (except ribozyme), which increases the rate of biochemical reactions by lowering down the activation energy.
First discovered ribozyme was L19 RNAase by T.Cech from rRNA of a protozoan Tetrahymena thermophila and RNAase P or Ribionuclease P by Altman in prokaryotic cell (Nobel prize).

GENERAL PROPERTIES OF ENZYMES

Large sized biomolecules, colloid nature with high molecular weight -
Large size (equal to colloid particles) provide, more surface area so passes large no. of active site. Large number of substrate converted into product by one molecule of enzyme at a time.
Highest molecular weight is of enzyme pyruvate dehydrogenase complex (46 lakh) participate in link reaction of respiration.
 

Proteinous nature

Monomer unit of a enzyme is Amino acid

Amino acids linked togather to form polypeptide chain
Enzymes are polypeptide chains
Most of enzymes arrange in tertiary structure of protein or globular proteins except isoenzyme (Quaternary st.)
Tertiary structure of protein provides stability and water soluble nature to enzymes.
synthesis of enzymes occurs on ribosomes under the control of genes.
According to one gene one polypeptide hypothesis, if a enzyme is made up of same kind of polypeptide chains then synthesize under control of same gene and if made up of different kinds of polypeptide chains then synthesized under the control fo different genes. Ex. Rubisco, cytochrome oxidase, Nitrogenase.
 

Specificity

Enzymes are specific for pH, temperature and substrate
pH - The common pH range of enzymes activity is 6 - 8.
Every enzyme works on specific pH, Pepsin-2.5 pH, Hydrolase-4-5,
Rubisco, Pepcase-8.5 pH, Trypsin - 8.5 pH

Temperature

Common range of temperature for enzyme activity is 20 – 40°C
Enzymes works on body temperature of organism not on environmental temperature.
Enzymes of plants are affected by evironmental temperature change as plants does not show homeostasis.
At low temperature enzymes become functionally inactive, at high temperature denatured.
 

Substrate

Every enzyme works on specific substrate
Substrate binds at active site of enzyme which is made of specific sequence of amino acids and recognise
it’s substrate.
Ex. succinic dehydrogenase acts on succinic acid while pyruvate dehydrogenase acts on pyruvic acid.
 

Enzymes increase the rate of reaction by decreasing activation energy -

Activation Energy - Minimum amount of energy more than the free energy of reactents required to reach
the transiation state of chemical reaction or to undergo the chemical reaction.

Turn Over Nubmer (T.O.N) -

The number of reactent moleules converted into product by one molecule of enzyme in unit time
Highest T.O.N is of carbonic anhydrase (360 lakh / minute)
CO2 + H2O anhydrase

KM constant -

Enymes follows the Michaelis-Menten reaction kinetics
It represents the substrate concentration at which rate of enzymetic reaction becomes the half of maximum
velocity or rate.
If a enzyme passes high km constant then it’s affinity towards substrate is low and rate reaction is
also low.

Ki constant (Enzyme inhibitor complex dissociation constant)

The substrate concentration at which enzyme inhibitor complex dissociate and reaction becomes normal
It is applicable only for competitive reversible inhibitions.

STRUCTURE OF ENZYME

Simple enzymes
They are made up of only protein. eg. pepsin, trypsin.

Conjugated enzymes
They are made up of protein & non protein part.
TChoe-eyn azryem geesn -e rCaoll-ye nmzaydmee su pa rteo nvoitna-mpriontse.in, orgainc groups, which are loosely attached to apoenzymes.
Prosthetic group - When non-protein part is tightly or firmly attached to apoenzymes.
CMae,t aMl ag,c tCivuators/co-factros/metallic factor :- Lossely attached inorganic co-factor eg. Mn, Fe, Co, Zn,

Active site :- The part of polypeptide chain made up of specific sequence of amino acids at which specific
saut bascttriavtee sisit eto i sb ed ebtienrdmedin eadn db yc agtaelnyeseticd , ckondoewsn. as active site. Very specific sequence of amino acids,
 

Allosteric site : - Besides the active site's some enzymes posess additional sites, at which chemical
eonthzeymr eth wanith s aulblossttreartice s(aitellso satreer icca mlleodd ausl aatlolorss)t earriec beinnzdy. mTehse.s ee .gs.it ehse xaorek inknaoswe,n p ahso sapllohsotferruicc tsoitkeisn aasned.

TERMINOLOGY

Endoenzymes - Enzymes which are functional only inside the cells. Ex. Enzymes of metabolism.

Enxzoyemnzeysm oefs i n- sEencztiyvmoeros ucsa tpallaynstesd, Zthyem raesaec ticoonms poleuxts iodfe f ethrme ecnetlal tEiogn:.- enzymes of digestion, some
TPrryopesninzoygmeen/ Zeytcm. ogen- These are precursor of enzymes or inactive forms of enzymes.eg. Pepsinogen,
Icsaolleend ziysmoeensz-y Emnezsy.m 1e6s fhoarmvisn go fs imilar action, but little difference in their molecular configuration are 3

Forms of Pepcase are known. T-haemsyel aaslle i soofe wnzhyemate &fo 5rm fso ramres soyf nLthDeHs is(eLda cbtya tdei fdfeerhenytd rgoegneens aasned) tissue and organ specific.

INnidtruogceibnlaes ee,n zymes - When formation of enzyme is induced by substrate availability. eg. Lactase, -galactosidase.

tEexmtrpeemraotuzyrem) ees.g .- TEanqz pyomlyems,e rwahseic.h may also function at extremely adverse conditions (very high

Abzymes - When the monoclonal antibodies (Mab) are used as enzymes.

pBriootdeeotlyetricg eenntzsy -m Eensz.ymes used in washing powders are known as bio-detergents eg.-amylase, lipase,

eHsosuesneti akl eteop cinegll./ cEoxn.s Etintuztyimvee se nozf ycmeell sr e-s Wpirhaitciohn a. re always present in constant amount & are also

NOMENCLATURE AND CLASSIFICATION

Enzyme commision of IUB-1961 divides all enzyes into 6 major classes on basis of types of reactions catalysed by them. It also proposed an international code of 4 digits for each enzyme. Suffix ‘-ase’ is used
for a enzyme.

Class-1 : Oxido-reductases - These enzymes involve in oxidation-reduction reactions.
it involves 3 sub classes (i) Oxides (ii) dehydrogenase (iii) reductase
e.g. Cytochrome oxides.

Class-2 : Trasferases - These Enzymes transfer specific group from one substrate to another.
e.g. Transaminase, Hexokinase.

Class-3 : Hydrolases - These Enzymes involve in hydrolysis reactions with help of H2O
e.g. Protease, Lipase, Carbohydrase. AB + HOH  AH + BOH

Class-4 : Lyases - Spilt the sustrate molecule without water. These Enzymes splits the specfifc
covalent bonds without hydrolysis or H2O addition. AB  A + B
e.g. Aldolase

Class-5 : Isomerases - Rearrangement of molecular structure to form isomers.
e.g. Isomerase, Epimerase

Class-6 : Ligases (Synthases) - Covalent bonding of two substrates to from a large molecule. A + B  AB
e.g. Citrate synthetase, Ligase, ATPsynthase

MODE OF ACTION OF ENZYME

Lock and Key theory or template theory :-

Given by Emil Fischer
According to this theory active sites of enzymes serve as a lock, at which the reactant/substrate
fits like a key. Supported by competitive inhibition.

Enzyme-substrate complex theory :-

Given by Michaelis & Menten
E+S  ESC  ESC  E+P

Induced fit theory : -

Given by D.E. Koshland (1973-74)
G According to this theory active site is not static, but it undergoes a conformational change which is induced by specific substrate. The active site has two groups, (1) Buttressing (supporting) group & (2) Catalytic group.

Butterssing group is meant for supporting the substrate, while the catalytic group break the substrate into product.

ALLOSTERIC MODULATION

Jacob & Monad. - First discovered L-threonine dehydratase, inhibited by its product isoelucine.
In the allosteric modulation, chemical or products fits in allostric sites & bring change in shape of active site of enzyme.
Chemicals which bind at allostric site of allostric enzymes are known as allostric modulators.
These modulators may be activators (positive modulators) or inhibitors (nagetive modulators) of allosteric enzymes.
If allostric modulator positively change the configuration of active site, then called positive allosteric modulation and if negatively change then called negative allosteric modulation respctively by+ve modulator (activators) and-ve modulator (inhibitors).

Ex. Phosphofructokinase inhibited by ATP, activated by ADP.
some times product also binds at allostetric site and inhibits the enzyme.

Ex. Hexokinase inhibited by glucose-6P and exhibits feed back inhibition.
This is a type of reversible and non-competitive inhibition found in allosteric enzymes.
All allosteric modulation are not feed-back inhibition.

FACTORS

pH - Enzymes very sensitive to pH.
Temperature - High temp inactivates enzyme causing their denaturation. They also get inactive at lower temp. Generally all enzymes better perform at body temp. of organism.
Enzyme concentration - Increase in conc. of enzymes will increases the rate of enzymetic reaction till enough the substrate.
Substrate conn- Increase in substrate conn increases the activity of enzymes until all the active sites of enzyme ae saturated.

Inhibitors/Enzyme inhibition :-

Competitive inhibitors or competitive inhibition and reversible type -
These are substrate analogues, which bind to the active site and enzymes get inhibited, such inhibition is called as competitve inhibition.
Eg. Succinic dehydrogenase is inhibited by its competitor malonate.
This is reversible inhibition. Malonate is known as substrate analogue of succinate.
Similarly sulpha drugs are substrate analogue of para-amino benzoic acid (PABA) used in folic acid synthesis in bacterical cells. Hence these drugs are used to kill bacterial cells.

Non-competitve inhibitors or non competitive inhibition and irreversible type :-

In this type of inhibition, inhibitor substance can bind simultaneously to an enzyme, at any site
and destroy the sulfhydryl (S-H) group of enzyme.
Example :- Toxic metals, CO, CN poisoning of cytochrome oxidase.
Such inhibition are irreversible inhibition.

Non competitive & reversible type :-
When inhibitor binds at allosteric site reversibly.
So, product inhibition is always reversible inhibition.
If competitive inhibitor (Reversible) is present, then, km -  and Vmax - No change.
If Non competitive (reversible) inhibitor is present, then, Km-No change, Vmax -Decrease
Competitive inhibition is overcome by increase in concentration of substrate.

SPECIAL POINTS OF ENZYMES
NAD = Nicotinamide Adenine Dinucleotide
DPN = Diphospho Pyridine Nucleotide
NADP = Nicotinamide Adenine Dinuclotide Phosphate
TPN = Triphospho Pyridine Nucleotide
FAD = Flavin Adenine Dinucleotide
FMN = Flavin Mono Nucleotide
Co A = Adenosine Triphospho Panta thenyl thio Ethanol Acetyl Amine
I ATP = 7.6 K cal
Important Coenzymes are
Co-1 (NAD)/DPN Niacin
Co-II (NADP)/TPN Niacin
FAD Riboflavin
FMN Riboflavin
TPP Thiamine
CO-Q Ubiquinone
CO-R Biotine
Co-A Pantothenic Acid
Important Co-factor :-
Fe++ = Cytochrome Oxidase, Catalase, Peroxides, Aconitase
Cu++ = Cytochrome oxidase, Tyrosinase
Zn++ = Carbonic anhydrase, Alcohol dehydrogenase, Caboxy peptidase
Mg++ = Hexokinase, Glucokinase, Pyruvate kinase, Pepcase, Rubisco
K+ = Pyruvate kinase
Mn++ = Arginase, Ribonucleotide reductase, Decarboxylase.
Mo = Nitrogenase complex, Nitrate reductase
Se = Glutathione peroxidase