Understanding Allosteric Regulation: The Role of Aspartate Transcarbamoylase

the first enzyme regulation mechanism that we're going to focus on will be alisic regulation and to demonstrate how

this is actually used and how it works inside ourselves let's take a look at a specific type of alisic enzyme known as

asate transc carbom moate so in this lecture and the next several lectures we're going to focus on how this enzyme

actually works and how it is regulated so let's begin by discussing the reaction that this enzyme actually

catalyzes so the reaction is shown on the board we have carbom moil phosphate that reacts with aspartate so these are

the two substrate molecules to the aspartate transc carbom moas and this catalyzes the conversion of these

molecules into these two products we have or we have orthop phosphate and we also have the narbon moil

aspartate now the first question you might be thinking is what's the big deal with this this reaction what is the

physiological significance of this reaction inside our body well as it turns out this reaction is actually the

first step in the very long biological synthesis of nitrogenous basis perimidine bases and the peridin are

actually used to produce the perimidine based nucleo Tri phosphates for instance the citadine triphosphates or simply CTP

so the ultimate result of this reaction is the production of this CTP molecule and so this can be seen in the following

reaction pathway so we have the carbom moil phosphate reacts with the asate in the presence of this enzyme to produce

the NC carbom moil aspirate as well as that p as well as that orthop phosphate and then these products react many many

many times to ultimately form that citadine triphosphate and these nucleotide triphosphates are basically

used to produce to build DNA molecules inside our body so this reaction is a pretty big

deal now the question is how do we know that this enzyme is in fact an alisic enzyme how do we know that there exists

a biological molecule inside our body that is used to basically control the activity of this enzyme well the first

evidence that this is an alisic enzyme came from early studies that basically showed that the rate of formation of

this and carbom mooil aspartate depends on the concentration of this final product the CTP and this is described in

the following graph so we have the y axis is the rate of formation of the narbon moil aspartate and the xais is

the concentration of this final product in this reaction here the CTP the citadine triphosphate so what this curve

basically shows us what the what the blue curve tells us is when inside ourselves we have a low concentration of

citadine triphosphate the rate of formation of this intermediate will be relatively high and so what that implies

is the activity of the ATC that aspartate transc carbom moas will also be high but as we increase the

concentration of a CTP as we produce more and more CTP and in fact once our cell's concentration of CTP is planful

we're going to see that somewhere here if we look at the rate of formation of this molecule it will be much lower than

in this particular case and so what that implies is as the concentration of CTP increases it

somehow goes back to this reaction here and affects the activity of this atcas because ultimately it's the atcas that

controls the rate of this reaction of the production of this intermediate and carbon moil

aspartate now the question is where exactly does the CTP molecule actually bind to on the

atcas well can the CTP bind onto the active side of this enzyme the only way to bind onto the active side is if the

CTP actually resembles has the same structure as either aspartate or carbom moil phosphate and we know that the

structure of this looks nothing like the structure of either of these two substrates and so what that what that

means is the CTP to actually inhibit the activity of atca it must bind onto some other side other than the active side

and those sides as we said previously are known as alisic signs regulatory signs so we see that in this biological

synthesis of CTP it's the end product it's the CTP itself that goes back to the beginning

to the first step in the reaction and inhibits the activity of this asate trans carbon moas and we know from basic

biology that this type of pathway is known as the negative feedback loop or negative feedback inhibition and this

CTP molecule is known as an alisic inhibitor of this enzyme and that's how we know that this enzyme is controlled

alisic inside our cell so once again these results suggest that the end product of the at tca's initiated

reaction must bind onto and inhibit the activity of that aspartate trans carbon moas and this is known as negative

feedback inhibition now since the structure of the CTP the cadine triphosphate is

nothing like the structure of these substrate molecules that means this molecule does not bind to the active

side but it binds to some other regulatory side known as the alisic side and because it inhibits the activity of

that enzyme we call the CTP an alisic inhibitor too this enzyme here so we see that at low concentrations of the

citogene triphosphate in ourselves there's not enough CTP to actually bind onto the atcas and so the activity of

atcas will be high and the rate of production of the narbon moil asp will also be high and this is shown in the

following curve at low concentrations of ctps somewhere here the rate of production will be high somewhere here

but as we increase the concentration of CTP the rate will drop and that's because now we have ample amount of CTP

and some of them will go back and bind onto this enzyme basically inhibit that enzyme and that decreases the rate of

production of this intermediate molecule and that makes sense because if we have abundant amounts of CTP we do not want

to waste energy and produce this intermediate molecule and so that's exactly why we limit the production of

an carbon moil aspartate by controlling the activity of this enzyme alisic now before we actually go into

our discussion of the structure of this enzyme the final thing that I'd like to focus on in this lecture is the fact

that aspartate transc carbom moas observes Cooperative Behavior so like most alisic enzymes

atcas actually exhibits cooperativity and so what that means is The Binding to one side affects the binding Affinity of

the other sides on that same enzyme and if we graph the relationship between the rate at which we produce this product

molecule the narbon moil aspirate with respect to the concentration of the substrate the aspirate as shown in this

diagram we're going to see not the typical Micha Menan curve but we're going to see the sigmo s shape curve and

that's because this enzyme and alisic enzymes in general observe Cooperative Behavior now what do we mean by

Cooperative Behavior so let's think back to to hemoglobin so when we discussed hemoglobin we discussed what it means

for an enzyme or a protein to behave in a Cooperative fashion so in our discussion on hemoglobin we said that

hemoglobin behaves cooperatively because it consists of different subunits and so it has different active sides it has

more than one active side it has more than one side to which the oxygen actually binds to and so because of that

every time an oxygen binds onto one of the sides it basically creates a confirmational change in the structure

of that enzyme and that induces it increases the Affinity of the other sides for that same substrate for that

oxygen molecule and so what that implies is the reason atcas exhibits Cooperative behavior is because it consists of

multiple subunits and those subunits must have additional active sides and so when one of those active sides is filled

with a substrate molecule such as the asate the other active sides become much more likely to actually bind to that

substrate molecule that's why we have this sigmo curve so this Cooperative Behavior implies that atcas must consist

of multiple subunits and hence multiple active sides it has multiple active sides as a substrate binds onto one of

those active sides it changes the Affinity of the other active sides for that substrate molecule and this is due

to the interaction between the different subunits and we'll discuss much more why this actually takes place and how they

interact with one another when we discuss the structure of this enzyme so in the next lecture we're going to focus

on the structure the three-dimensional shape and the subunits that are found inside the aspartate transc carbom moas

or ATC alisic enzyme