Transcript Slide 1

Syllabus to be covered till 31st
November
Naindeep Kaur
Resource person
Biotech(E)- first year
Enzymes
• Enzymes are biological molecules that catalyze(i.e., increase the
rates of) chemical reactions. In enzymatic reactions,
the molecules at the beginning of the process, called substrates,
are converted into different molecules, called products. Almost
all chemical reactions in a biological cell need enzymes in order
to occur at rates sufficient for life .
• Enzyme activity can be affected by other molecules.
•
Many drugs and poisons are enzyme inhibitors. Activity is also
affected by temperature, pressure, chemical environment
(e.g., pH), and the concentration of substrate.
• The catalytic activity of many enzymes depends on the presence of
small molecules termed cofactors.
• an enzyme without its cofactor is referred to as an apoenzyme;
• the complete, catalytically active enzyme is called a holoenzyme.
• Cofactors can be subdivided into two groups: metals and small
organic molecules. The enzyme carbonicanhydrase, for example,
requires Zn2+ for its activity Glycogen phosphorylase , which
mobilizes glycogen for energy, requires the small organic molecule
pyridoxal phosphate (PLP).
• Cofactors that are small organic molecules are called coenzymes.
Often derived from vitamins, coenzymes can be either tightly or
loosely bound to the enzyme. If tightly bound, they are called
prosthetic groups.
• Loosely associated coenzymes are more like cosubstrates
because they bind to and are released from the enzyme just
as substrates andproducts are.
• The use of the same coenzyme by a variety of enzymes and
their source in vitamins sets coenzymes apart from normal
substrates, however.
The Active Sites of Enzymes Have Some
Common Features
1. The active site is a three-dimensional cleft formed by groups
that come from different parts of the amino acid sequence.
2. The active site takes up a relatively small part of the total
volume of an enzyme.
3. Active sites are clefts or crevices.
4. Substrates are bound to enzymes by multiple weak
attractions.
5. The specificity of binding depends on the precisely defined
arrangement of atoms in an active site.
Enzymes Decrease the Activation Energy.
The Michaelis-Menten Model
Consider an enzyme that catalyzes the S to P by the following pathway:
Our starting point is that the catalytic rate is equal to the product of the concentration of
the ES complexand k 2.
The rates of formation and breakdown of ES are given by:
To simplify matters, we will work under the steady-state assumption. In a steady state,
the concentrations of intermediates, in this case [ES], stay the same even if the
concentrations of starting materials and products are changing.
This occurs when the rates of formation and breakdown of the ES complex are equal.
Setting the right-hand sides of
equations 11 and 12 equal gives
By rearranging equation 13, we obtain
Equation 14 can be simplified by defining a new constant, K M, called the
Michaelis constant:
Note that K M has the units of concentration. K M is an important characteristic of
enzyme-substrate interactions and is
independent of enzyme and substrate concentrations.
Inserting equation 15 into equation 14 and solving for [ES] yields
Now let us examine the numerator of equation 16. The concentration of uncombined
substrate [S] is very nearly equal to the total substrate concentration, provided that the
concentration of enzyme is much lower than that of substrate. The
concentration of uncombined enzyme [E] is equal to the total enzyme concentration
[E]T minus the concentration of the ES complex.
Substituting this expression for [E] in equation 16 gives
By substituting this expression for [ES] into equation 10, we
obtain
The maximal rate, V max, is attained when the catalytic sites on the enzyme are saturated
with substrate that is, when
[ES] = [E]T. Thus,
Substituting equation 22 into equation 21 yields the Michaelis-Menten equation:
Competitive inhibitionIn
• In competitive inhibition, the inhibitor and substrate
compete for the enzyme (i.e., they can not bind at the
same time).Often competitive inhibitors strongly
resemble the real substrate of the enzyme
• For example, methotrexate is a competitive inhibitor of
the enzyme dihydrofolate reductase, which catalyzes the
reduction of dihydrofolate to tetrahydrofolate.
Uncompetitive inhibition
• In uncompetitive inhibition, the inhibitor cannot
bind to the free enzyme, only to the ES-complex.
• The EIS-complex thus formed is enzymatically
inactive.
• This type of inhibition is rare, but may occur in
multimeric enzymes.
Non-competitive inhibition
• Non-competitive inhibitors can bind to the enzyme
at the binding site at the same time as the
substrate,but not to the active site.
• Both the EI and EIS complexes are enzymatically
inactive.
• Because the inhibitor can not be driven from the
enzyme by higher substrate concentration (in
contrast to competitive inhibition), the apparent
Vmax changes. But because the substrate can still
bind to the enzyme, the Kmstays the same.
Biological function
• Enzymes serve a wide variety of functions inside living organisms.
They are indispensable for signal transduction and cell regulation,
often via kinases and phosphatases.
• They also generate movement, with myosin hydrolyzing ATP to
generate muscle contraction and also moving cargo around the cell
as part of the cytoskeleton.
• Other ATPases in the cell membrane are ion pumps involved
inactive transport.
• Enzymes are also involved in more exotic functions, such
as luciferase generating light in fireflies.
• Viruses can also contain enzymes for infecting cells, such as the HIV
integrase and reverse transcriptase, or for viral release from cells,
like the influenza virus neuraminidase.
• An important function of enzymes is in the digestive systems of
animals. Enzymes such as amylases and proteases break down large
molecules (starch or proteins, respectively) into smaller ones, so they
can be absorbed by the intestines
• Different enzymes digest different food substances. In ruminants,
which have herbivoros diets, microorganisms in the gut produce
another enzyme, cellulase, to break down the cellulose cell walls of
plant fiber.
• Amylases from fungi and plants:Production of sugars from starch,
such as in making high-fructose corn syrup.In baking, catalyze
breakdown of starch in the flour to sugar. Yeast fermentation of sugar
produces the carbon dioxide that raises the dough.
• Rennin, derived from the stomachs of young ruminant
animals (like calves and lambs):Manufacture of cheese,
used to hydrolyze protein
• Glucose isomerase: Converts glucose into fructose in
production of high-fructose syrups from starchy
materials. These syrups have enhanced sweetening
properties and lower calorific values than sucrose for the
same level of sweetness.
• Restriction enzymes, DNA ligase andpolymerases:Used to
manipulate DNA in genetic engineering, important
in pharmacology ,agriculture and medicine. Essential
forrestriction digestion and the polymerase chain
reaction. Molecular biology is also important in forensic
science.