Understanding Aspartate Transcarbamoylase: Cooperativity and Allosteric Regulation

aspartate transcarbamoylase or simply a TCH is an allosteric enzyme and like most allosteric enzymes

it displays cooperativity so in this lecture I'd like to focus on the cooperative nature of a TCS and then

discuss two allosteric effectors to this enzyme so we're going to focus on cytidine triphosphate CTP and adenosine

triphosphate ATP now let's begin by discussing the coronary structure of this enzyme and how its quaternary

structure actually affects the likelihood that the substrate will bind onto the active sites now when the

concentration the substrate molecule is low inside the cell the entire coronary structure of the enzyme exists

predominantly in the T State the 10th state so what do we mean by the 10th state well in the 10th state of this

enzyme the two catalytic trimers and these are the orange structures in this diagram are basically found in close

proximity with respect to one another and they create a very compact and very constrained structure and so what that

means is the active sites in these catalytic trimers are going to have a low affinity for that substrate

molecules and so at low substrate concentrations the substrate molecules are not going to be likely to be

balanced in those active sites and so that will create a low cattle integrate a low catalytic activity now when we

begin to increase the substrate concentration inside the cells of our body the substrate molecules will begin

to bind onto the active side so let's suppose one substrate molecule binds on to one of the active sites of this

enzyme and once the binding takes place what that does is it shifts the equilibrium of this entire equation

slightly to the right side from the T state to that our state and so as we increase the concentration of the

substrate even more more of those substrate molecules begin to bind onto the active sites and eventually that

shifts the entire curve to the right side and at a very high substrate concentration inside ourselves the

entire curve will be shifted this way this error will be very very small and so this enzyme will exist predominantly

into our state now what do we mean by our what do we mean by the our state well the our state is also known as the

relaxed state and that's because these two orange catalytic trimers have now moved far apart they rotated and so they

created this relaxed structure and in this structure the active sides have a high affinity for the substrate molecule

and so what that means is the rate of activity of this enzyme when the enzymes quaternary structure is in the our state

will be much higher than compared to that T state and that's exactly why at high concentration of substrate the

activity of that enzyme will be high because it exists in that our state so what's actually taking place is upon the

binding of the substrate molecules into the active sites the active sites found on the different catalytic trimers

basically begin to interact with one another and they cooperate with one another and that's exactly what shifts

entire curve from one side of the equation to the other side of the equation and this is what we know as the

cooperative behavior of allosteric enzymes such as aspartate transcarbamoylase and this mechanism can

be described by using the concerted model which we actually use to describe the behavior of hemoglobin molecules so

based on this model the activity of this aspartate transcarbamoylase is all-or-nothing it

either exists in the cheese state in which this enzyme is inactive or it exists in the our state in which the

enzyme is fully active and it's concentration of the substrate inside the cell that ultimately determines

where the equilibrium will actually lie and a high substrate concentration the enzymes quaternary structure will exist

in the our state while in while at low substrate concentrations the enzymes quaternary structure will exist in the T

States now if we take a look at the following diagram we have three different curves so the y-axis is the

rate of product formation for this enzyme catalyzed reaction and the x-axis is the concentration of that substrate

molecule now at low concentration of the substrate molecule we're going to exist predominantly in the T state and it's

this green curve that describes the Michaelis Menten curve for this particular enzyme for this particular

quaternary structure and so notice at some particular substrate concentration the rate of activity will be low and

that's because the active sites of the enzyme have a low affinity for that enzyme but as we begin to increase the

concentration of that substrate that begins to shift then tie a curve to the R into the r-state

and once we transform the quaternary structure from the tents to the relaxed state the blue curve is the curve that

basically begins to describe that quaternary behavior and so according to this blue curve at the same substrate

concentration we see that the rate of activity is much higher so the green curve describes the Michaelis Menten

curve for the tight state the blue curve describes for the relaxed state and if we combine these two curves we obtained

the actual black curve that describes the s-shaped the sigmoidal curve for this allosteric enzyme and this is what

we know as the concerted model we have the all-or-nothing behavior in which we either

just in the t-state in which the enzyme is inactivated or the art state in which the enzyme is fully active and dip and

basically carries out that particular catalytic reaction now what about the two types of allosteric effectors that

ourselves actually use to basically control a regulated activity of this enzyme so let's begin by focusing on CTP

so remember this enzyme catalyzes the first step in the biosynthetic process of CTP cytidine triphosphate and it's

the CTP that basically creates a negative feedback loop that goes back and binds onto the allosteric unto the

allosteric sites found on these red regulatory chains and by binding onto these red regulatory change it decreases

and inhibits the activity of that enzyme the question is how does this actually take place well upon the binding of CTP

to the red regulatory chains what it does is it stabilizes the T state of that enzyme so it lowers the energy of

that cheese State it makes it more stable so as these brown CTP molecules bind

onto each one of these regulatory chains as shown here it stabilizes the energy of this T State makes it much more

stable and that shifts the equilibrium from the OO state into the T State and so what that means is when CTP actually

binds onto the regulatory change these CTP molecules make it more difficult for the substrate to actually bond unto the

active sides and transform this quaternary structure from the T state into the our state and so in this manner

our cells use these CTP molecules to actually regulate the behavior of this enzyme so when we have a high

concentration of CTP we don't want to produce any more CTP and so the cell uses

the CTP in a negative feedback loop to basically inhibit the activity of this enzyme now the second type of allosteric

effector of this enzyme is actually ATP adenosine triphosphate in fact adenosine triphosphate just like CTP also binds on

to that same regulatory chain these red chains but unlike CTP when ATP binds onto these regulatory chains it actually

increases the rate of activity of this enzyme and an increase in the rate of activity of the enzyme by displacing the

CTP molecule from that regulatory chain and shifting the entire curve from the T State to that our state so adenosine

triphosphate or ATP is also an allosteric effector of a TCH and bind to the same side on the regulatory chain

but it actually increases the act of activity of that a TCA so at high concentration of ATP the ATP will

displace the CTP from the active from the regulatory side on those regulatory chains and that will shift the

equilibrium from the T state to the our state and make the active size much more likely to actually bind and convert that

substrate molecule into the final product the CTP now the question is what is the physiological significance of

this effect why exactly does a high concentration of ATP activate this enzyme in the first place well for two

reasons first of all if we have a high concentration of ATP that means we have plenty of energy inside ourselves and

that implies if we have plenty of energy we can easily use that energy to actually synthesize these nucleoside

triphosphates such as ATP by using this enzyme right here now the second reason why a high ATP concentration basically

stimulates this enzyme to carry out its process is because of the following reasoning so at

a high concentration of ATP when we have many ATP molecules inside ourselves that means we can use the ATP molecules to

actually synthesize nucleoside triphosphate so RNA and DNA molecules but to actually synthesize these nucleic

acids we not only need that ATP we also need other nucleoside triphosphate specifically per imaging nucleoside

triphosphates such as CTP and if we have a high amount of ATP that usually means we have an unequal distribution of the

per imaging su purine nucleoside triphosphates and so equalize this distribution of

nucleoside triphosphates per imaging securing nuclear nucleoside triphosphates we have to carry out this

reaction to actually increase the amount of CTP the per imaging nucleoside triphosphate that is found inside our

cells so physiologically a high ATP concentration means that there is an unequal distribution of the nucleoside

triphosphates the purine to pyrimidine nucleoside triphosphates and therefore the cell tends to equalize this

distribution by producing more per imaging nucleoside triphosphates such as CTP so we basically conclude that the

reason this ends on displays cooperativity is because the individual subunits of this enzyme actually

interact with one another and by binding of the substrate molecule we essentially shift the equilibrium from the T state

to the our state now in order to regulate the activity of this enzyme inside our cells in order to actually

turn on or turn off the activity of the sound of this enzyme our cells use these two regulatory effectors CTP and ATP by

- the same regulatory chains those allosteric sites found on these red regulatory chains bud they have

opposite effects CTP creates a negative feedback loop that basically decreases the activity inhibits the activity of

this enzyme bud ATP binds and displaces the CTP out of that regulatory chain and it actually increases the activity of

the enzyme by shifting the equilibrium from the tent state to that more active or state