Download ENZYMES

Basics of ENZYMES
Jony Mallik
B. Pharmacy; M. Pharmacy
E-mail: [email protected]
DEFINITION
“Enzymes are the protein made biochemical that’s are synthesized,
stored & released by/in/from exocrine gland of human body.”
“Enzymes are proteins that catalyze (i.e., increase the rates of reaction
by decreasing activation energy) chemical reactions without
consumed in it.”
“Enzyme are biological catalysts, that can catalyze more biological
reaction(digestion, metabolism etc.) leads to a rapid ending of that
reaction with suitable product.”
DEFINITION RELATED TO ENZYME
 In enzymatic reactions, the molecules at the beginning of the
process are called substrates, and the enzyme converts them into
different molecules, called the products.
 The site on the enzyme to which substrate is bind & converted into
product is called as active site of enzyme.
ENZYME-SUBSTRATE COMPLEX
Enzymes are organic catalyst produced by an organisms. The reactant
in an enzyme-catalyzed reaction is called “substrate”.
HOLOENZYME
 A completely active enzyme with all its quality to fit with a
substrate & to accelerate a reaction rate is known as
“Holoenzyme”
 They contain a protein and non-protein part. Both parts must be
present before the enzyme can function.
 The protein part is called the “apoenzyme” and the non-protein
(organic part) is called “coenzyme”.
HOLOENZYME = APOENZYME + COENZYME
HOLOENZYME
COENZYME
Coenzymes are not proteins and so are not inactivated by heat.
Examples of coenzymes are the vitamins or compounds
derived from vitamins. The reaction involving a coenzyme
can be written as follows:

coenzyme + apoenzyme = enzyme
Coenzyme A is essential in the metabolism of carbohydrates,
lipids, and proteins in the body.
COFACTORS
Some enzymes cannot work on their own, they need a molecule called a
cofactor in order to work properly. Cofactors modify the enzyme
complex so that it has the chemical properties necessary to catalyze a
reaction.
There are three kinds of cofactors:
a) Prosthetic group: Organic molecule that is permanently attached to an
enzyme.
b) Coenzymes: Relatively small organic molecules are not permanently
attached to the enzyme molecule.
c) Metal cofactors: Inorganic metal ions that are also known as enzyme
activators.
NOMENCLATURE
 Formerly enzyme were given names ending in “-in”. With no
relation being an indicator between the enzyme and the substance
it affects the substrate.
 The current system for naming enzymes uses the name of the
substrate or the type of reaction involved, with the ending “-ase”.
CLASSIFICATION
Oxidoreductases – are enzymes that catalyze oxidation-reduction
reactions between two substrates. The enzymes of the
oxidation-reduction reactions in the body are important
because these reactions are responsible for the production of
heat and energy.
Transferases – are enzymes that catalyze the transfer of a
functional group between two substrates.
CLASSIFICATION
Hydrolases – hydrolytic enzymes catalyze the hydrolysis of
carbohydrates, esters and proteins.
Lyases – are enzymes that catalyzes the removal of groups from
substrates by means other than hydrolysis, usually with the
formation of double bonds.
Isomerases – are enzymes that catalyze the interconversion of cistrans isomers.
Ligases – or synthetases, are enzymes that catalyze the coupling of
two compounds with breaking of pyrophosphate bonds.
The Chemical nature of enzymes
• Enzymes are globular proteins. They have a complex tertiary and quaternary structure in
which polypeptides are folded around each other to form a roughly spherical or globular
shape. The overall 3D shape of an enzyme molecule is very important: if it is altered, the
enzyme cannot bind to its substrate and so cannot function. Enzyme shape is maintained by
hydrogen bonds and ionic forces.
Enzymes have several important properties:
 Enzymes are specific: each enzyme usually catalyses only one reaction.
 Enzymes combine with their substrates to form temporary enzyme-substrate complex.
 Enzymes are not altered or used up by the reactions they catalyze, so can be used again and
again.
 Enzymes are sensitive to temperature and pH.
 Many enzymes need cofactors in order to function.
 Enzyme function may be slowed down or stopped by inhibitors.
The specificity of enzymes
Two models that may explain how enzymes work are:
1) The lock and key hypothesis
2) The induced fit hypothesis
1) The lock and key hypothesis
 Enzyme has a particular shape into which the substrate or
substrates fit exactly. This is often referred to as the ‘lock and
key’ hypothesis where the substrate is imagined being like a
key whose shape is complementary to the enzyme or lock. The
site where the substrate bonds in the enzyme is known as the
active site and it has a specific shape.
2) The induced fit hypothesis
– The active site in many enzymes is not exactly the same shape
as the substrate, but moulds itself around the substrate as the
enzyme substrate complex is formed.
– Only when the substrate binds to the enzyme is the active site,
the correct shape to catalyze the reaction. As the products of
the reaction from they fit the active site less well and fall away
from it. Without the substrate, the enzyme reverts to its
‘relaxed’ state, until the next substrate comes along.
Factors affecting enzyme activity
The factors that affect enzyme activity also affect the functions of the
cell and ultimately the organism. Enzymes are proteins and their
function is therefore affected by:
 Temperature
 pH
 Substrate concentration
 Enzyme concentration
 Cofactors
 Inhibitors
Temperature
For a non-enzymatic chemical reaction, the general rule is: the higher the
temperature, the faster the reaction. This same rule holds true for a reaction
catalyzed by an enzyme, but only up to about 40-450C. Above this
temperature, enzyme molecules begin to vibrate so violently that the delicate
bonds that maintain tertiary and quaternary structure are broken, irreversibly
changing the shape of the molecule. When this happens, the enzyme can no
longer function and it is said to be denatured.
pH
Like other proteins, enzymes are stable over a limited range of pH. Outside this
range, at the extremes of pH, enzymes are denatured. Free hydrogen ions (H+) or
hydroxyl ions (OH-) affect the changes on amino acid residues, distorting the 3D
shape and causing an irreversible change in the proteins tertiary structure.
Enzymes are particularly sensitive to changes in pH because of the great
sensitivity of their active site. Even if a slight change in pH is not enough to
denature the molecule, it may upset the delicate chemical arrangement at the
active site and so stop the enzyme working.
Substrate concentration
The rate of an enzyme-controlled reaction increases as the substrate
concentration increases, until the enzyme is working at full
capacity. At this point, the enzyme molecules reach their turnover
number and assuming that all other conditions such as temperature
are ideal, the only way to increase the speed of the reaction even
more is to add more enzyme.
Enzyme concentration
In any reaction catalyzed by an enzyme, the number of enzyme
molecules present is very much smaller than the number of
substrate molecules. When an abundant supply of substrate is
available, the rate of reaction is limited by the number of enzyme
molecules present. In this situation, increasing the enzyme
concentration increases the rate of reaction.
Inhibitor
 Inhibitors slow down or stop enzyme reaction. Usually, enzyme inhibition
is a natural process, a means of switching enzymes on or off when
necessary.
 Inhibition can be reversible and the enzyme returns to full activity once
the inhibitor is removed. Drugs and poisons can inhibit particular enzymes,
this type of inhibition is often non-reversible.
 Reversible inhibitors are either competitive or non-competitive.
 Competitive inhibitors
Compete with normal substrate molecules to occupy the active site. A
competitive inhibitor fits into the active site of the enzyme preventing the
real substrate from gaining access. The inhibitor cannot be converted to the
products of the reaction and so the overall rate of reaction is slowed down.
Fig: Competitive inhibitors bind reversibly to the enzyme, preventing the binding of substrate. On
the other hand, binding of substrate prevents binding of the inhibitor. Substrate and inhibitor
compete for the enzyme.
 Non-competitive inhibitors
Non-competitive inhibitors bind to the enzyme away from the active site but change the overall shape of
the molecule, modifying the active site so that it can no longer turn substrate molecules into product.
Non-competitive inhibition has this name because there is no competition for the active site.
Examples
•
Cyanide combines with the Iron in the enzymes cytochrome oxidase.
•
Heavy metals, Ag or Hg, combine with –SH groups.
These can be removed by using a chelating agent such as EDTA.
Irreversible inhibitors
Irreversible inhibitors bind permanently to the enzyme, rendering it
useless. For example, cyanide is an irreversible inhibitor.
Examples: nerve gases and pesticides, containing organophosphorus,
combine with serine residues in the enzyme acetylcholine esterase.
Chemical reactions
• Chemical reactions need an initial input of energy = THE
ACTIVATION ENERGY
• During this part of the reaction the molecules are said to be in a
transition state.
Reaction Pathway
Enzyme controlled pathway
Enzyme controlled reactions proceed 108 to 1011 times faster than
corresponding non-enzymatic reactions.