Citric Acid Lecture
Content
Today we’re going to finish up the citric acid cycle, last time we
finished the basics of the chemistry of the cycle. We bring two carbons
and it’s acetates, condense some deloastic to make the carbon
citrate , I summarized isocitrate from the …….. isocitrate
dehydrogenase so we get nadh and co2 then another ……… with a
ketoglutarate dehydrogenase to make another nadh with co2 and
succilyn co a to use the energy of….. to drive synthesis of gtp by
succilyn co a synthetase make a four carbon Molecule succinate and
then two more oxidations succinate dehydrogenase We get FAD on the
other dehydrogenase use NAD…. Membrane….. donates it an electron
directly into …. Transports …. The product that is fumerate the …
carbon number 1, rehidrate that using fumerase to make secondary
alcohol l mailates which is oxidize on the last step …. The products
three ADHs one FADA2 one GTP and two CO2s .
We look at the citric acid cycle as a catabolic pathway, which is the
oxidation of those two carbons from acitate and acitate to carbon
dioxide , but the citric acid cycle also functions anablolicly and that
means that we use the intermediates in the pathway in that cycle, it’s
the building blocks to make other molecules, as a breakerish to other
molecules and because it has both anabolic and catabolic function we
call the citric acid cycle anphibolic pathway, in other words sometimes
it functions catabolicly and sometimes it’s functioning anabolicly. But
this two things actually conflict with each other some because
remember when we talked about the intermediates in the cycle in the
catalog phase we talked about them being catalytic, in other words
there is no net synthesis or degradation of oxaloacitate or alpha
ketogluterate they are just catalytic amounts and the sultrate you’re
feeding into the cycle and the products are spinning off the cycle so we
begin to use the oxaloacitates to make something else, you can make
aminoacids out of oxaloacitates, eventually you’re going to run out of
intermediates and you slow down or actually even stop the catabolic
phase so that creates a problem, so again this shows, this is figure 17,
it shows both the anabolic and catabolic functions of the pathway, the
catabolic functions of the acitate coming in, spinning in the CO2
spading off. The anabolic functions are being use on things like
oxaloacitates or alpha ketogluterate or fumerate I’m sorry alpha
ketogluterate is … to aminoacids or deplete them use them to make
aminoacids you can use succilyn CoA to make porphyrins you can
use citrate as the building block for fatty acids and cholesterol, and as
we’re going to see in a couple weeks we use oxaloacitates really as the
starting point to make glucose in a process call gluconegenesis. So the
carbons and the oxaloacitates, they’re eventually go thru malate to
make glucose. So all of those are going to deplete the intermediates.
So reactions that deplete the citric acid cycle intermediates are known
as cataplerotic reactions, so all of this red arrows that are go out of the
cycle as a group are called cataplerotic, and cataplerotic literally
means to empty, your emptying the cycle, the intermediates in the
cycle. So in order to replenish the cycle we need to have
complementary reactions that will fill up, these are called the
anaplerotic reactions, so they’ll fill the cycle and that’s where I remind
you that … dehydregenase bringing acetate in, this is not an
anaplerotic reaction, because all the carbons are feeding in to the
cycle, two carbons feed out even is not net synthesis of any of these
intermediates, so we’re not making intermediates by bringing acitates
into the cycle so that’s not an anaplerotic reaction. We have to have
other reactions that will bring the carbons in to refill those
intermediates, to those who think it will directly synthesize
intermediates from other carbon sources, so we want to look at a
couple of examples of both cataplerotic and anapletotic reactions,
somebody don’t understand the concept at all, why would we need to
be able to refill the intermediates in the cycle and why you can’t use
acitate to do that? Ok. Couple of examples of cataplerotic reactions, so
again reaction that are just going to take the intermediates out and
make something else, a lot of aminoacids synthesis starts with
intermediates in the cycle so for instance glutamate dehydrogenase is
going to make the aminoacid glutamate starting with alpha
ketoglutarate, you don’t need to know the details of this reactions, ….,
but you can see this is a 5 carbon backbone, there is a 5 carbon
backbone so all we do is replacing this carbon… with an amino…. So
that is what the reaction will do, so we use the carbons in the alpha
ketoglutarate (CAC intermediate) to synthesize to make glutamate, if
you’re asking we’re using protein synthesis. Ok . Another example,
aspartyl aminotransferase with a whole bunch of aminotransferase ,
that all they do is they’ll take a suxtrate Like oxaloacetate, again a CAC
intemediate, it’s transfer the aminoglut from the alanine to the …
making a sparric acid and releasing pyruvate so your moving
aminoglut back and forward between two molecules ,so again to make
aspartate acid we’re consuming oxaloacetate so a cataplerotin
reaction will take something out of the cycle to make aminoacid.
So the anaplerotic reactions is where I want to spend more time for
what we have been talking about, we were making sure the cycle will
function in a catabolic action so we don’t knock the intermediates to
make sure we can oxidize the acitates , we need to figure out how
make sure we can replenish the intermediates, so a few things that
are getting move, aminoacid brake down, so just like we can consume
oxaloacitates to make aspartate acid , we’re going to metabolize
aspartate acid , we can make oxaloacitate from it, so one of the major
ways of bringing intermediates is to brake down aminoacid and then
again you’ll hear about that in chem. 162 .
Another example that we will look at later on this semester when we
talk about betaoxidation of fatty acids when odd chain fatty acids
(fatty acids that have 15 13 11 17 carbons) when they are broken
down they actually feed in succilyn CoA, so you can actually make
succilyn CoA from the oxidation of odd chain fatty acids. We’ll look at
that reaction late on this semester. The reaction I want to spend some
time on right now is an enzyme called pyruvate carboxylase so this is
not what we’re talking about pyruvate ….and ethanol fermentation
which was a way to make… and CO2 from…. This is pyruvate
carboxylase so we’re going to make a net synthesis of oxaloacetate
from pyruvate, we’re going to us CO2 and ATP, so we need the energy
in ATP to do a carboxylation reaction to put this carboxylase group
under the CO2 on to the pyruvate as a … acid and using the energy, so
a coupled reaction use the energy in the ATP to drive forward the
carboxylation, so again if there is insufficient oxaloacetate in the cycle
because of the cataplerotic reaction depleted things, what’s going to
happen is the cycle will slow down, acetyl CoA builds up, it will
activate this enzyme so this enzyme pyruvate carboxylase is activated
by high levels of acetyl CoA, it will take some of the pyruvate turn it
into oxaloacetatate and that will allow the cycle to continue. That’s sort
of how the enzyme function, what it does biologically. Just a reminder,
the structure of the oxaloacetate, it’s a beta keto acid, beta keto acids
are …. unstable, so again here’s the carboxylic and the beta keto … so
instantaneously carboxylation …. Which should get you energetically
favorable decarboxylation so two step oxaloacitate still in the bench in
water and neutral pH there so you start to see decarboxylation of the
molecule which will spontaneously degrade. So again is a unstable
molecule, what we’re are going to be doing is essentially putting
energy in to drive this backwards, we’re gonna carboxylate pyruvate
cut this CO2 backbone , so you can see here the pyruvate, even
enough pyruvate that is going to put the CO2 in croboxic acid. So here
is the enzyme pyruvate carboxylase, it’s gonna take the pyruvate, it’s
gonna take the CO2 in the form of a byproduct so start the… to disolve
form of CO2 using the energy in the ATP to drive the carboxylation of
this carbon and … on the pyruvate to make it to oxaloacetate. In a
couple reaction using the energy of the ATP to drive … and the Acytil
CoA is the required allosteric activator. It there’s not acytil CoA around
the enzyme doesn’t function, it’s complete inactive in the absence of
acytil CoA. And acytil CoA is not a substray is a allosteric activator. I
gonna be drawing, we’ll see a lot of allosteric activators and inhibiters
as we go on I’ll draw the allosteric activators in green or something
approaching green, with a plus and a circle around, that signifies that it
is an allosterig …. On the …. This enzyme requires the cofactor known
as biotin, this double ring along valerate side chain and the … acid
move here. This is going to allow to be covalently associated with
enzymes thru the….amino glucoclyzimes so there is…. Linkage here
this make a long molecule chain with the business end, so this should
be remanecent of … acid with the 14 angstrons chain with the business
end out. So this is going to again allow this biotin to carry in just two
forms this pyruvate carboxylase we carboxilated the biotin, it’s going
to be carried that provaxylated group from one part of the enzyme to
the other acting sort of like a crane, most like a … function in the
pyruvate dehydrognase …….. again it’s funtions in carboxylation
reactions and it is an activated … because that is what is it does. Biotin
actually it is a vitamin but is not something you will find in your cereal
boxes because the bacteria in our gut make enough biotin to get us by,
so we don’t actually need to have it in our diet. Biotin deficiencies are
extremely rare, it only happen in certain athletes that like to consume
raw egg , there is a protein in egg white called avidin, avidin ,has
extremely high affinity for biotin, binds it extremely tightly and prevent
its absorption so we can’t take much of avidin, if you cook the egg
thenit … the avidin protein and no longer binds the biotin and it’s no
longer a problem , but if you consume raw egg then the avidin can
essentially bind the biotin in your system so only Rocky type athletes
have biotin deficiency, so it’s a extremely rare disorder, and usually
can be easily diagnose and remedy, just stop eating raw eggs. And you
have to consume actually a lot of raw eggs. So the mechanism of the
enzyme is on figures 16-1 again the reason it’s in chapter 16 is that
this enzyme is going to participate not only in the anaplerotic reaction
that we were talking about here, but also in gluconeogenesis in order
to make glucose from pyruvate , this is the fisrt step, so will come back
to that system later. On the enzyme we have to active sites, so there is
sort of two phases of the reaction, each of them happens onone active
site. the fist one it’s going to be the activation of that bycarb and the
formation of carboxybiotinyl, so that’s going to happen in active site
one. And on active site two we’re going to use carboxybiotinyl to
donate the carbisilic acid move to the pyruvate to make the
oxaloacetate. So we’ll zoom in … so looking at phase one or active site
one, this will remind you as I said before CO2 disolves in water forms
carbonic acid se we’re going to be looking at this from a CO2 or bypart
which is shown here, and on active site one it will bind the bypart, it
will bind the ATP in such a way in catalisis by proximity this is gonna...
gamma fostate, you release the ADP and you form a transient of
molecular carboxyphophate , its not a very stable molecule but is form
transiently and very efficiently, it’s a very energetically favorable
reaction because again we’re using the energy in the ATP to drive this
reaction, this happens very rapidly and efficiently, it will also
spontaneously brake down to a molecule of ….phosohate and the CO2,
so again we essentially made a small amount of CO2 right in the active
site of the enzyme and again just by proximity the biotin is sitting right
there forming a … in the CO2 and it carboxylates itself, so… happens in
the active site driven by hydrolysis of that ATP . so first reaction site
one carboxylation of biotin driven by the hydrolisys of ATP.
Now in site 2 we have the carboxybiotinyl, in site 2 is where the
pyruvate binds , now again what is going to happen is that the
carboxylic acid is going to disassociate again … forming that CO2
here , we’re going to make a nuclear file out of the pyruvate , we have
seen this before we can make a … once you make an ….. form which is
going to be encouraged by acid base chemistry, you can add and
substract protons, you made a nuclear file here you can form a
nuclear file attack on the CO2 that would actually form the carboncarbon bond. So we’ve seen this strategy over and over again where
we form the enolate . enolate allow resin structures and … forms
transicly disposition allowing the liquid filling pad. So that’s how the
carboxylation of pyruvates occur, it occurs by the pyruvate enolate
which allow us to make a nuclear file as we brought the CO2 into the
active site originally attach to the carboxybiotinyl. The first site we’re
going to carboxylate the biotin, the second site the carboxybiotinyl
donates the CO2to the pyruvate to make oxaloacetate.
So I just drew kind of a cartoon of this because the site one and site
two are actually physically distinct sites of the pyruvate carboxylase
enzyme. Here is the biotine which again with a 14 angstroms chain
which acts like a crane, so this is the unmodified biotin it will swing into
active site one where the ATP and the bycarbon
gonna bind, and you’re going to carboxylate this biotin braking down
the ADP making carboxybiotinyl. So that now swings out of the active
site, it will swing down to the active site two where the pyruvate will
donate that carboxylic acid group to the pyruvate to make
oxaloacetate.
So in the beginning state we have the unmodified
bicarb because that is like a crane, it can swing down, still attach it can
swing down into active site one where the ATP and bypart bind, we are
going to perform the first reaction, the site one reaction which is the
carboxylation of the biotin, we see that product here, here is the
caboxybiotinyl. That then swings out of there , out of that site, down
into active site two , it’s where the pyruvate is binded. Once the
pyruvate binds make the enolate form of the pyruvate, we dissociate
the carboxylic acid to CO2 … oxaloacetate.
So two sites enzyme, again the biotin functioning as a crane very much
like we saw the … fuctioning in the pyruvate dehydrogenase .
We’ll se pyruvate functioning in a couple of other enzymes this
semester yet as always we’re gonna work this way, it’s always gonna
be an activated suit and carrier.you gonna have site one it’s gonna
always be activation of the biotin, with the carboxylic acid on the biotin
and site two is just gonna bind various substrates that we’ll watch
later.
Just to remind you again, what this anapletoric reaction allow us to do
is to bring carbon in to the CAC, into intermediates from the CAC
without going thru acetates . because again bringing acetates there is
no net synthesis of oxaloacetate or alpha ketoglutarate or any
intermediate from… from those carbons you have to bring’em in a
different way 22:48
finished the basics of the chemistry of the cycle. We bring two carbons
and it’s acetates, condense some deloastic to make the carbon
citrate , I summarized isocitrate from the …….. isocitrate
dehydrogenase so we get nadh and co2 then another ……… with a
ketoglutarate dehydrogenase to make another nadh with co2 and
succilyn co a to use the energy of….. to drive synthesis of gtp by
succilyn co a synthetase make a four carbon Molecule succinate and
then two more oxidations succinate dehydrogenase We get FAD on the
other dehydrogenase use NAD…. Membrane….. donates it an electron
directly into …. Transports …. The product that is fumerate the …
carbon number 1, rehidrate that using fumerase to make secondary
alcohol l mailates which is oxidize on the last step …. The products
three ADHs one FADA2 one GTP and two CO2s .
We look at the citric acid cycle as a catabolic pathway, which is the
oxidation of those two carbons from acitate and acitate to carbon
dioxide , but the citric acid cycle also functions anablolicly and that
means that we use the intermediates in the pathway in that cycle, it’s
the building blocks to make other molecules, as a breakerish to other
molecules and because it has both anabolic and catabolic function we
call the citric acid cycle anphibolic pathway, in other words sometimes
it functions catabolicly and sometimes it’s functioning anabolicly. But
this two things actually conflict with each other some because
remember when we talked about the intermediates in the cycle in the
catalog phase we talked about them being catalytic, in other words
there is no net synthesis or degradation of oxaloacitate or alpha
ketogluterate they are just catalytic amounts and the sultrate you’re
feeding into the cycle and the products are spinning off the cycle so we
begin to use the oxaloacitates to make something else, you can make
aminoacids out of oxaloacitates, eventually you’re going to run out of
intermediates and you slow down or actually even stop the catabolic
phase so that creates a problem, so again this shows, this is figure 17,
it shows both the anabolic and catabolic functions of the pathway, the
catabolic functions of the acitate coming in, spinning in the CO2
spading off. The anabolic functions are being use on things like
oxaloacitates or alpha ketogluterate or fumerate I’m sorry alpha
ketogluterate is … to aminoacids or deplete them use them to make
aminoacids you can use succilyn CoA to make porphyrins you can
use citrate as the building block for fatty acids and cholesterol, and as
we’re going to see in a couple weeks we use oxaloacitates really as the
starting point to make glucose in a process call gluconegenesis. So the
carbons and the oxaloacitates, they’re eventually go thru malate to
make glucose. So all of those are going to deplete the intermediates.
So reactions that deplete the citric acid cycle intermediates are known
as cataplerotic reactions, so all of this red arrows that are go out of the
cycle as a group are called cataplerotic, and cataplerotic literally
means to empty, your emptying the cycle, the intermediates in the
cycle. So in order to replenish the cycle we need to have
complementary reactions that will fill up, these are called the
anaplerotic reactions, so they’ll fill the cycle and that’s where I remind
you that … dehydregenase bringing acetate in, this is not an
anaplerotic reaction, because all the carbons are feeding in to the
cycle, two carbons feed out even is not net synthesis of any of these
intermediates, so we’re not making intermediates by bringing acitates
into the cycle so that’s not an anaplerotic reaction. We have to have
other reactions that will bring the carbons in to refill those
intermediates, to those who think it will directly synthesize
intermediates from other carbon sources, so we want to look at a
couple of examples of both cataplerotic and anapletotic reactions,
somebody don’t understand the concept at all, why would we need to
be able to refill the intermediates in the cycle and why you can’t use
acitate to do that? Ok. Couple of examples of cataplerotic reactions, so
again reaction that are just going to take the intermediates out and
make something else, a lot of aminoacids synthesis starts with
intermediates in the cycle so for instance glutamate dehydrogenase is
going to make the aminoacid glutamate starting with alpha
ketoglutarate, you don’t need to know the details of this reactions, ….,
but you can see this is a 5 carbon backbone, there is a 5 carbon
backbone so all we do is replacing this carbon… with an amino…. So
that is what the reaction will do, so we use the carbons in the alpha
ketoglutarate (CAC intermediate) to synthesize to make glutamate, if
you’re asking we’re using protein synthesis. Ok . Another example,
aspartyl aminotransferase with a whole bunch of aminotransferase ,
that all they do is they’ll take a suxtrate Like oxaloacetate, again a CAC
intemediate, it’s transfer the aminoglut from the alanine to the …
making a sparric acid and releasing pyruvate so your moving
aminoglut back and forward between two molecules ,so again to make
aspartate acid we’re consuming oxaloacetate so a cataplerotin
reaction will take something out of the cycle to make aminoacid.
So the anaplerotic reactions is where I want to spend more time for
what we have been talking about, we were making sure the cycle will
function in a catabolic action so we don’t knock the intermediates to
make sure we can oxidize the acitates , we need to figure out how
make sure we can replenish the intermediates, so a few things that
are getting move, aminoacid brake down, so just like we can consume
oxaloacitates to make aspartate acid , we’re going to metabolize
aspartate acid , we can make oxaloacitate from it, so one of the major
ways of bringing intermediates is to brake down aminoacid and then
again you’ll hear about that in chem. 162 .
Another example that we will look at later on this semester when we
talk about betaoxidation of fatty acids when odd chain fatty acids
(fatty acids that have 15 13 11 17 carbons) when they are broken
down they actually feed in succilyn CoA, so you can actually make
succilyn CoA from the oxidation of odd chain fatty acids. We’ll look at
that reaction late on this semester. The reaction I want to spend some
time on right now is an enzyme called pyruvate carboxylase so this is
not what we’re talking about pyruvate ….and ethanol fermentation
which was a way to make… and CO2 from…. This is pyruvate
carboxylase so we’re going to make a net synthesis of oxaloacetate
from pyruvate, we’re going to us CO2 and ATP, so we need the energy
in ATP to do a carboxylation reaction to put this carboxylase group
under the CO2 on to the pyruvate as a … acid and using the energy, so
a coupled reaction use the energy in the ATP to drive forward the
carboxylation, so again if there is insufficient oxaloacetate in the cycle
because of the cataplerotic reaction depleted things, what’s going to
happen is the cycle will slow down, acetyl CoA builds up, it will
activate this enzyme so this enzyme pyruvate carboxylase is activated
by high levels of acetyl CoA, it will take some of the pyruvate turn it
into oxaloacetatate and that will allow the cycle to continue. That’s sort
of how the enzyme function, what it does biologically. Just a reminder,
the structure of the oxaloacetate, it’s a beta keto acid, beta keto acids
are …. unstable, so again here’s the carboxylic and the beta keto … so
instantaneously carboxylation …. Which should get you energetically
favorable decarboxylation so two step oxaloacitate still in the bench in
water and neutral pH there so you start to see decarboxylation of the
molecule which will spontaneously degrade. So again is a unstable
molecule, what we’re are going to be doing is essentially putting
energy in to drive this backwards, we’re gonna carboxylate pyruvate
cut this CO2 backbone , so you can see here the pyruvate, even
enough pyruvate that is going to put the CO2 in croboxic acid. So here
is the enzyme pyruvate carboxylase, it’s gonna take the pyruvate, it’s
gonna take the CO2 in the form of a byproduct so start the… to disolve
form of CO2 using the energy in the ATP to drive the carboxylation of
this carbon and … on the pyruvate to make it to oxaloacetate. In a
couple reaction using the energy of the ATP to drive … and the Acytil
CoA is the required allosteric activator. It there’s not acytil CoA around
the enzyme doesn’t function, it’s complete inactive in the absence of
acytil CoA. And acytil CoA is not a substray is a allosteric activator. I
gonna be drawing, we’ll see a lot of allosteric activators and inhibiters
as we go on I’ll draw the allosteric activators in green or something
approaching green, with a plus and a circle around, that signifies that it
is an allosterig …. On the …. This enzyme requires the cofactor known
as biotin, this double ring along valerate side chain and the … acid
move here. This is going to allow to be covalently associated with
enzymes thru the….amino glucoclyzimes so there is…. Linkage here
this make a long molecule chain with the business end, so this should
be remanecent of … acid with the 14 angstrons chain with the business
end out. So this is going to again allow this biotin to carry in just two
forms this pyruvate carboxylase we carboxilated the biotin, it’s going
to be carried that provaxylated group from one part of the enzyme to
the other acting sort of like a crane, most like a … function in the
pyruvate dehydrognase …….. again it’s funtions in carboxylation
reactions and it is an activated … because that is what is it does. Biotin
actually it is a vitamin but is not something you will find in your cereal
boxes because the bacteria in our gut make enough biotin to get us by,
so we don’t actually need to have it in our diet. Biotin deficiencies are
extremely rare, it only happen in certain athletes that like to consume
raw egg , there is a protein in egg white called avidin, avidin ,has
extremely high affinity for biotin, binds it extremely tightly and prevent
its absorption so we can’t take much of avidin, if you cook the egg
thenit … the avidin protein and no longer binds the biotin and it’s no
longer a problem , but if you consume raw egg then the avidin can
essentially bind the biotin in your system so only Rocky type athletes
have biotin deficiency, so it’s a extremely rare disorder, and usually
can be easily diagnose and remedy, just stop eating raw eggs. And you
have to consume actually a lot of raw eggs. So the mechanism of the
enzyme is on figures 16-1 again the reason it’s in chapter 16 is that
this enzyme is going to participate not only in the anaplerotic reaction
that we were talking about here, but also in gluconeogenesis in order
to make glucose from pyruvate , this is the fisrt step, so will come back
to that system later. On the enzyme we have to active sites, so there is
sort of two phases of the reaction, each of them happens onone active
site. the fist one it’s going to be the activation of that bycarb and the
formation of carboxybiotinyl, so that’s going to happen in active site
one. And on active site two we’re going to use carboxybiotinyl to
donate the carbisilic acid move to the pyruvate to make the
oxaloacetate. So we’ll zoom in … so looking at phase one or active site
one, this will remind you as I said before CO2 disolves in water forms
carbonic acid se we’re going to be looking at this from a CO2 or bypart
which is shown here, and on active site one it will bind the bypart, it
will bind the ATP in such a way in catalisis by proximity this is gonna...
gamma fostate, you release the ADP and you form a transient of
molecular carboxyphophate , its not a very stable molecule but is form
transiently and very efficiently, it’s a very energetically favorable
reaction because again we’re using the energy in the ATP to drive this
reaction, this happens very rapidly and efficiently, it will also
spontaneously brake down to a molecule of ….phosohate and the CO2,
so again we essentially made a small amount of CO2 right in the active
site of the enzyme and again just by proximity the biotin is sitting right
there forming a … in the CO2 and it carboxylates itself, so… happens in
the active site driven by hydrolysis of that ATP . so first reaction site
one carboxylation of biotin driven by the hydrolisys of ATP.
Now in site 2 we have the carboxybiotinyl, in site 2 is where the
pyruvate binds , now again what is going to happen is that the
carboxylic acid is going to disassociate again … forming that CO2
here , we’re going to make a nuclear file out of the pyruvate , we have
seen this before we can make a … once you make an ….. form which is
going to be encouraged by acid base chemistry, you can add and
substract protons, you made a nuclear file here you can form a
nuclear file attack on the CO2 that would actually form the carboncarbon bond. So we’ve seen this strategy over and over again where
we form the enolate . enolate allow resin structures and … forms
transicly disposition allowing the liquid filling pad. So that’s how the
carboxylation of pyruvates occur, it occurs by the pyruvate enolate
which allow us to make a nuclear file as we brought the CO2 into the
active site originally attach to the carboxybiotinyl. The first site we’re
going to carboxylate the biotin, the second site the carboxybiotinyl
donates the CO2to the pyruvate to make oxaloacetate.
So I just drew kind of a cartoon of this because the site one and site
two are actually physically distinct sites of the pyruvate carboxylase
enzyme. Here is the biotine which again with a 14 angstroms chain
which acts like a crane, so this is the unmodified biotin it will swing into
active site one where the ATP and the bycarbon
gonna bind, and you’re going to carboxylate this biotin braking down
the ADP making carboxybiotinyl. So that now swings out of the active
site, it will swing down to the active site two where the pyruvate will
donate that carboxylic acid group to the pyruvate to make
oxaloacetate.
So in the beginning state we have the unmodified
bicarb because that is like a crane, it can swing down, still attach it can
swing down into active site one where the ATP and bypart bind, we are
going to perform the first reaction, the site one reaction which is the
carboxylation of the biotin, we see that product here, here is the
caboxybiotinyl. That then swings out of there , out of that site, down
into active site two , it’s where the pyruvate is binded. Once the
pyruvate binds make the enolate form of the pyruvate, we dissociate
the carboxylic acid to CO2 … oxaloacetate.
So two sites enzyme, again the biotin functioning as a crane very much
like we saw the … fuctioning in the pyruvate dehydrogenase .
We’ll se pyruvate functioning in a couple of other enzymes this
semester yet as always we’re gonna work this way, it’s always gonna
be an activated suit and carrier.you gonna have site one it’s gonna
always be activation of the biotin, with the carboxylic acid on the biotin
and site two is just gonna bind various substrates that we’ll watch
later.
Just to remind you again, what this anapletoric reaction allow us to do
is to bring carbon in to the CAC, into intermediates from the CAC
without going thru acetates . because again bringing acetates there is
no net synthesis of oxaloacetate or alpha ketoglutarate or any
intermediate from… from those carbons you have to bring’em in a
different way 22:48
