Understanding Enzymes: The Catalyst of Biological Reactions
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Introduction
Enzymes are biological molecules that play a crucial role in catalyzing chemical reactions within our bodies. This discussion focuses on the pivotal properties of enzymes, emphasizing their ability to significantly increase reaction rates without altering the overall equilibrium of the processes involved.
The Role of Enzymes in Biological Reactions
Enzymes are often referred to as catalysts in biochemistry. Their primary function is to accelerate the rate at which chemical reactions occur, effectively reducing the time require to reach equilibrium. Understanding how they accomplish this is fundamental to grasping the principles of biochemistry and cellular function.
How Enzymes Speed Up Reactions
- Catalysis: Enzymes lower the activation energy needed for reactions. This energy barrier is what ordinarily slows down reaction rates.
- Equilibrium Considerations: Although enzymes speed up reactions, they do not affect the equilibrium state of the reaction. The concentrations of reactants and products at equilibrium remain unchanged regardless of whether the reaction is catalyzed or uncatalyzed.
Understanding Gibbs Free Energy
The Gibbs free energy ( ΔG) of a reaction indicates its spontaneity. A negative ΔG implies an exergonic reaction, meaning energy is released, and the products are more stable than the reactants.
- Reaction Example: Consider a reaction where we have reactants A and B converting into products B and C. The ΔG is derived from the difference in energy levels between the products and reactants:
- If the energy of the products is lower than that of the reactants, the reaction is favorable.
- Enzymes do not alter the energy levels of products or reactants, thus ΔG remains unchanged.
The Transition State and Activation Energy
What is Transition State?
- The transition state represents the highest energy state during a reaction. It is a fleeting moment where old bonds are broken, and new bonds form.
- The activation energy is the energy required to reach this transition state from the reactants.
Role of Enzymes in Stabilizing Transition States
- Enzymes facilitate the formation of the transition state by stabilizing it. They provide an optimal environment for the existing bonds to break and new bonds to form.
- This stabilization reduces the overall activation energy, accelerating the reaction.
Enzyme Kinetics: Understanding Maximum Velocity
Maximum Velocity Definition
- The maximum velocity of an enzyme refers to the peak rate at which an enzyme can catalyze a reaction. It is dependent on the concentration of substrates and active sites available on the enzyme.
Factors Affecting Maximum Velocity
- Substrate Concentration: When substrate concentration increases, enzyme activity also rises until saturation is achieved—at which point all active sites are occupied.
- Active Site Availability: Once all active sites are filled, the reaction reaches its maximum velocity and cannot increase further with additional substrate.
Graphical Representation
Enzyme activity is often depicted on a graph where:
- The Y-axis represents enzyme activity/velocity.
- The X-axis represents substrate concentration.
- The curve rises steeply at first then levels off upon reaching maximum velocity, illustrating saturation.
Conclusion
Enzymes are integral to life, ensuring that chemical reactions proceed efficiently and swiftly. They lower activation energies without altering the equilibrium state of reactions, allowing for the maintenance of cellular functions. By understanding enzyme kinetics and their properties, we unlock insights into numerous biological processes, paving the way for advancements in biotechnology and medicine. The next discussion will delve deeper into the mechanisms by which enzymes stabilize transition states and function at the molecular level.
in our discussion on the properties of enzymes we mentioned one very important property of enzymes namely the ability
of the enzyme to catalyze to the biological reactions that take place inside our body and inside our cells now
what that ultimately means is enzymes speed up the rate at which chemical reactions take place now by speeding up
a chemical reaction enzymes essentially decrease the time that is needed for that particular chemical reaction to
actually reach equilibrium now this is a very important thing to remember about enzymes enzymes decrease the time that
is needed to reach equilibrium but enzymes do not actually change the equilibrium itself they do not change
the energy of the products and reactants nor they actually change the amount of products or reactants that is formed at
equilibrium so to see what we let's take a look at the following energy diagram so let's suppose we have a hypothetical
Elementary single step reaction in which the reactants are these and the products are these so on the reactant side we
have a bond between A and B and C Exist by itself on the product side we now have a bond between B and C and A Exist
by itself now based on the following energy diagram so the Y AIS is the gives free energy and the xaxis is the
reaction progress so we go from reactants to products notice that if we compare the y-coordinate value of the
products to the y-coordinate value of the reactants this is lower in energy than this and what that means is if we
take the free energy of the products and we subtracted from the free energy of the reactants we get a negative value
and what that means is free energy will be produced will be released when this reaction takes place
the Delta G is negative and so that implies this reaction is exergonic it is spontaneous and as long as we have
enough energy to overcome the activation barrier this quantity here the reactants will spontaneously and naturally form
these products because they are lower in energy and therefore more stable so once again enzymes do not affect the
free energy value of the products nor will they affect the free energy value of the reactants and since the energy
value of the products and reactants will not be affected that means the Delta G the difference between this and this
will not be affected as well now because it's the energy of the products and reactants and specifically it's the
difference between the energy of the products and reactants that determines the concentrations of products and
reactants that will exist at equilibrium because the enzymes do not affect the energy values of the products and
reactants they will not affect the concentration of the products and reactants that will exist at equilibrium
so once again we know that enzymes do not affect the free energy of the reactants and products this implies that
they will not change the equilibrium of the reaction that is the same concentration of products and reactants
will be formed in the presence as in the absence of the enzyme so this is the case where we have our uncatalyzed
reaction but if we add an enzyme the energy value of the reactants and products will not actually
what is actually changed well recall that the kinetics of the chemical reaction is determined by the energy of
the transition state and the transition state is this transient molecule transient stage that exists between the
reactants and our products now what exactly will the transition state look like when we go from the reactants to
the products well we have a single Elementary re a single step Elementary reaction in which on the reactant side
we have a bond between a and b and on the product side we have a bond between B and C and what that implies is to
actually go from the reactants to the products we have to break the bond between a and b and we have to form the
bond between B and C so in that transition stage what we're going to see is a bond breaking between a and B so B
will begin to move away from a and because the electron density will basically move away that bond between A
and B will begin to break and that can be described by using a dashed line so this dashed Line Between A and B
basically moves B is moving away and the bar and and that bond is being broken on the other hand because B is approaching
C the electron densities of these two atoms or molecules is overlapping and so we begin to form we begin to form that
Bond so we have a partially formed Bond here and a par and a partially broken bond here and because the electron
densities aren't overlapping very well that will increase the energy of the transition state in fact according to
the diagram the energy of the transition state represents the highest possible I free energy value on the following curve
and so if we are to actually mark down on the curve where that transition state actually is this highest most Peak on
the curve the Apex represents that transition state so we can basically write that this is in fact that
transition state it represents the highest the maximum energy value in that particular chemical reaction and this
daggered symbol describes the energy states so to calculate the free energy value of the transition stage of that
molecule of that chemical reaction we simply take this y-coordinate value and we subtract the energy of the reactants
so the energy of the transition State minus the energy of the reactants give gives us this quantity known as the free
energy uh thee energy of activation or simply the activation energy the activation barrier of this particular
so we have the enzyme in the enzyme we have this special location in that enzyme that we're going to discuss in
much more detail in the next lecture but this special location in the enzyme is known as the active side and it's the
active side that that creates a micro environment and binds to these molecules here so these reactants will move into
the active side of that enzyme creating the enzyme substrate complex and what the enzyme actually does inside that
active side is it stabilizes this partially broken Bond and this partially form Bond and by
stabilizing these partially broken and form bonds it lowers the energy of activation it stabilizes the transition
state lowering that free energy of activation and if we lower that free energy of activation we increase the
rate at which that reaction takes place so once again enzymes bind specific substrates on regions called active
sides to form the enzyme substrate complex by binding substrat to the active sides enzymes stabilize the
energy of the transition state which in turn stimulates the breakage of the old bonds and the formation of the new bonds
to form that product molecule as shown in this particular diagram so if we look at the following diagram once more when
we go from the uncatalyzed to the catalyzed case we see that the energy of the products or reactants is not changed
this energy is the same as this energ energy value and this energy is the same as this energy so the change in Gibs
free energy between our products and reactants is not changed what is changed is the energy between the transition
state and the reactants so here we see that there's a stabil a stabilization of the transition state and a lowering an
energy and that means the difference between the energy of this transition state in the reactants will be smaller
in this case than in this case and this is precisely what makes that reaction actually go quicker so by adding an
enzyme we increase the or we decrease the time it takes for equilibrium to actually establish but once equilibrium
is actually established the same concentrations of products and reactants are formed in the catalyzed case as that
uncatalyzed case now let's discuss something called the maximum velocity of enzymes so the
maximum velocity of enzymes basically describes the maximum activity at which the enzyme will actually operate so
let's suppose in our mixture we have a total of 100 enzymes and each one of these enzymes will have its own active
site now if we add 50 substrate molecules then only 50 enzymes will contain active sides that are filled and
that means our entire mixture because only half of the enzymes are filled the entire activity of all the enzymes will
be half of its maximum value but if we continue adding substrate molecules so that we add let's say 100 substrate
molecules then all the active sides and all the enzymes will be filled and that means the entire activity of our mixture
of enzy enes will be at a maximum velocity and that's exactly what this graph actually represents so the Y AIS
is the enzyme activity also called the enzyme velocity and the xaxis is the substrate concentration so basically
this dashed line represents the maximum velocity at which that enzyme or the mixture of enzymes will actually operate
and notice we have this line and this Curve will never actually cross this line it will never actually go higher
than the line because if we have for example 100 enzymes we only have 100 possible active sides so even if we add
let's say 200 substrate molecules because we have an excess of substrate molecules and only 100 active sides we
have a maximum activity at which only 100 active sides at a time can actually be filled so as we we increase our
substrate concentration we see that that the enzyme activity increases up to a certain value known as the maximum
enzyme activity or maximum enzyme velocity so we see that at a constant concentration of enzyme the enzyme
activity will continue to rise until a certain maximum value is reached known as the maximum velocity and the maximum
activate so let's change that so all the active sites act active so all the active sides are filled with
the appropriate substrate so that basically represents the condition in which if let's say we have a thousand
enzymes each enzyme has one active side if we have 1,000 substrate molecules then all the active sides will be filled
and that is the point at which that enzyme is operating at a maximum enzyme velocity at a maximum enzyme activity so
in the next lecture we're going to focus on how that active side actually stabilizes that transition state and
we're going to focus much more on what that active side actually does and how it binds to its appropriate substrate