Download The Kinetics of Enzyme Catalyzed Reactions

The Kinetics of Enzyme
Catalyzed Reactions
Dr. Saleha Shamsudin
Introduction
Classification of enzymes according to the
reaction catalyzed and how enzymes work.
Enzymes
• There are many chemical compounds in the
living cell.
• How they are manufactured and combined at
sufficient reaction rates under relatively mild
temperature and pressure?
• How does the cell select exactly which
reactants will be combined and which
molecule will be decomposed?
• The answer is catalysis by enzyme.
Enzymes
•Enzymes are biological catalysts that are protein molecules in
nature- react in mild condition
•They are produced by living cells (animal, plant, and
microorganism) and are absolutely essential as catalysts in
biochemical reactions.
•Almost every reaction in a cell requires the presence of a specific
enzyme– related to its particular protein structure.
•A major function of enzymes in a living system is to catalyze
the making and breaking of chemical bonds.
•Therefore, like any other catalysts, they increase the rate of
reaction without themselves undergoing permanent chemical
changes.
•The catalytic ability of enzymes is due to its particular
protein structure.
•A specific chemical reaction is catalyzed at a small
portion of the surface of an enzyme, which is known as
the active site.
•Some physical and chemical interactions occur at this
site to catalyze a certain chemical reaction for a certain
enzyme.
Catalysts
• A catalyst is unaltered during the course of a
reaction and functions in both the forward and
reverse directions.
• In a chemical reaction, a catalyst increases the
rate at which the reaction reaches equilibrium.
• For a reaction to proceed from starting material to
product, the chemical transformations of bondmaking and bond-breaking require a minimal
threshold amount of energy, termed activation
energy.
• Generally, a catalyst serves to lower the
activation energy of a particular reaction.
Enzyme
Enzyme is protein or nucleic acid.
• Catalyze biochemical reactions
– breaking, forming and rearranging bonds.
• Specificity
– Dictated by the enzyme active site.
– Some active sites allow for multiple substrates.
• Cofactors/Coenzyme (Non protein group)
– Vitamin derivatives, metals (minerals) can bind as cosubstrates or remain attached through multiple catalytic
cycles
– Cofactors: metal ions, Mg, Zn, Mn, Fe.
– Coenzyme: complex organic molecule, NAD, FAD, CoA or
some vitamins
Enzymes
• Proteins that assist in chemical reactions may be
Enzymes
– Specific because of conformational shape
• Enzymes are catalysts
– Catalyst: chemical that changes the rate of a reaction
without being consumed
– Recycled (used multiple times)
• Enzymes reduce the activation energy of a reaction
– Amount of energy that must be added to get a reaction to
proceed
The activation energy
for the decomposition
of hydrogen peroxide
varies depending on the
type of catalysis.
Type of
catalysis
Activation
energy
Uncatalyzed
reaction at
20°C
18
kcal/mol
Enzymatically
catalyzed
(catalase)
7 kcal/mol
Chemically
catalysed (by
collodial
platinum)
13
kcal/mol
Enzyme lower the activation energy of the reaction by binding
the substrate and forming an enzymes-substrate complex.
Important terms (Enzyme)
• active site - a region of an enzyme comprised of
different amino acids where catalysis occurs or a
small portion of the surface of an enzyme which a
specific chemical reaction is catalyzed
• substrate - the molecule being utilized and/or
modified by a particular enzyme at its active site
• co-factor - organic or inorganic molecules that are
required by some enzymes for activity. These
include Mg2+, Fe2+, Zn2+ and larger molecules
termed co-enzymes like nicotinamide adenine
dinucleotide (NAD+), coenzyme A, and many
vitamins.
Important terms (Enzyme)
• holoenzyme - a complete, catalytically active
enzyme including all co-factors OR an enzyme
containing a nonprotein group
• apoenzyme - the protein portion of a holoenzyme
minus the co-factors OR the protein part of
holoenzyme (holoenzyme=apoenzyme+cofactor)
• isozyme - (or iso-enzyme) an enzyme that
performs the same or similar function of another
enzyme that occur in several different molecular
forms.
Nomenclature of enzyme
Originally enzymes were given nondescriptive names such as:
rennin : curding of milk to start cheese-making processor
pepsin : hydrolyzes proteins at acidic pH
trypsin : hydrolyzes proteins at mild alkaline pH
The nomenclature was later improved by adding the suffix -ase to
the name of the substrate with which the enzyme functions, or to
the reaction that is catalyzed, for example:
Nomenclature of enzyme
Enzyme reactions are different from chemical
reactions, as follows:
1. An enzyme catalyst is highly specific, and catalyzes only one
or a small number of chemical reactions. A great variety of
enzymes exist, which can catalyze a very wide range of
reactions.
2. The rate of an enzyme-catalyzed reaction is usually much
faster than that of the same reaction when directed by
nonbiological catalysts at mild reaction condition.
3. A small amount of enzyme is required to produce a desired
effect.
4. Enzymes are comparatively sensitive or unstable molecules
and require care in their use.
Enzymatic Reaction Principles
• Biochemically, enzymes are highly specific for their
substrates and generally catalyze only one type of
reaction at rates thousands and millions times
higher than non-enzymatic reactions.
• Two main principles to remember about enzymes
are
a) they act as CATALYSTS (they are not consumed in a
reaction and are regenerated to their starting state)
and
b) they INCREASE THE RATE of a reaction towards
equilibrium (ratio of substrate to product), but they do
not determine the overall equilibrium of a reaction.
Reaction Rates
• The rate of the reaction is determined by several factors
including the concentration of substrate, temperature and
pH.
• For most standard physiological enzymatic reactions, pH
and temperature are in a defined environment (pH 6.97.4, 37oC).
• This enzymatic rate relationship has been described
mathematically by combining the equilibrium constant,
the free energy change and first or second-order rate
theory.
Keq = e−∆Go/RT
• The net result for enzymatic reactions is that the lower the
activation energy, the faster the reaction rate, and vice
versa.
Specificity
• Most synthetic catalyst are not specific i.e.,
they will catalyze similar reactions involving
many different kinds of reactants. While
enzymes are specific. They will catalyze only
one reaction involving only certain substances.
Binding Energy
• The interaction between enzyme and its substrate is
usually by weak forces.
• In most cases, van der Waals forces and hydrgen
bonding are responsible for the formation of ES
complexes.
• The substrate binds to a specific site on the enzyme
known as the active site.