The Citric Acid Cycle - Biochemistry - Lecture Notes, Study notes for Biochemistry. Biyani Girls College

Biochemistry

Description: The Citric Acid Cycle, Citrate Synthase, Isocitrate Dehydrogenase, Pyruvate Dehydrogenase, Dihydrolipoyl Transacetylase, Dihydrolipoyl Dehydrogenase, Succinate Dehydrogenase, Fumarase Catalyzes, Malate Dehydrogenase, Net Reaction of the Citric Acid Cycle are few points from this lecture notes. If you are looking for complete set of lectures on biochemistry, you can find it in my documents section.
Showing pages  1  -  2  of  6
The Citric Acid Cycle
I. Citrate Synthase
The first reaction of the citric acid cycle is the condensation of acetyl-CoA
and oxaloacetate to form citrate and CoA-SH.
The enzyme that catalyzes this reaction is called citrate synthase.
Go’ = -32.2 kJ/mol
The change in free energy based on the steady state concentrations of
oxaloacetate, acetyl-CoA and citrate in the mitochondria of isolated from pig
hearts is:
G = -53.9 kJ/mol
A very exergonic reaction and irreversible.
The mechanism of citrate synthase is
shown to the left. In the active site of the
enzyme we have two histidines and an
aspartate which function as general acids
and bases during catalysis.
The first step is to generate an enol of
acetyl-CoA. Asp-375 functions as a
general base abstracting a proton for the
methyl group of Acetyl-CoA. His-274
concertedly functions as a general acid
donating a proton to the carbonyl to form
the enol.
The enol generated in the first step is
converted into a nucleophile by the
abstraction of the enol hydrogen by His-
274 now functioning as a general base.
The electrons of the double bond attack the
electrophilic center of the ketone of
oxaloactete. His-320 functions in concert
as a general acid donating its proton to the
carbonyl oxygen of oxaloacetate to form
citryl-CoA. Citryl-CoA spontaneously
hydrolyses while it is still bound to the
active site to generate coenzyme A and
citrate.
H3CC
O
SCoA
CO2-
CO
CH2
CO2-
CH2
CCO
2-
CH2
CO2-
CO2-
HO
+
CoASH + H+
CO2
-
O
H2C
-O2C
His274
CH2
N
NH
H
+C
C
O
CoAS
H
H
H
O
-O
Asp375
CO2
-
O
H2C
-O2C
His274
CH2
N
NH
HC
C
O
CoAS
H
H
H
O
O
Asp375
His320
CH2
N
NH
H
+
CO2
-
OH
H2C
-O2C
CH2
C
O
CoAS
His320
CH2
N
NH
H
O
O
Asp375
His274
CH2
N
NH
H
H2O
CH2
CCO
2
-
CH2
CO2
-
CO2
-
HO
CoASH + H+
Docsity.com
Citrate synthase is a homodimer with symmetry as you can see to the left.
It has a sequential order kinetic mechanism.
First the enzyme binds oxaloacetate which induces the large conformational
change shown (b). This is yet another example of induced fit. The
conformation change induced by oxaloacetate binding creates the acetyl-
CoA binding site and seals oxaloacetate form the aqueous solvent.
Now that you are no longer novices regarding metabolic strategies, you
probably already have a sense that citrate synthase is going be allosterically
regulated.
It is the first step of a metabolic pathway.
It catalyzes an irreversible step in the pathway.
It is a homodimer with symmetry.
And your intuitions are correct. This enzyme is a site for allosteric regulation.
One of the products of the citric acid cycle is NADH which allosterically inhibits the enzyme. Succinyl-
CoA, the product of the fifth step of the cycle also inhibits.
II. Aconitase
Citrate is a tertiary alcohol. It is difficult to oxidize tertiary alcohols because forming the
ketone would involve breaking a carbon-carbon bond. To get around this problem, citrate
is isomerized into isocitrate. Isocitrate is a secondary alcohol which can be easily
oxidized to the ketone by NAD+. The enzyme that catalyzes this migration of a hydroxyl
group is aconitase. This enzyme catalyzes the dehydration of citrate of form cis-aconitate
and then rehydrating the double bond to form isocitrate.
Go’ = +6.7 kJ/mol
The change in free energy based on the steady state concentrations of citrate and isocitrate
in the mitochondria of isolated from pig hearts is:
G = +0.8 kJ/mol ; near equilibrium.
Aconitase is an iron sulfur protein.
It contains four iron atoms complexed to four inorganic
sulfides and three cysteine sulfur atoms called a 4Fe-4S
iron- sulfur cluster. One of the iron atoms has an open
coordination site that complexes with the carboxylate
group of C3 and the hydroxyl group of citrate. This
iron residue facilitates the dehydration and rehydration
reaction and accounts for the stereospecifity of the
reaction.
CH2
CCO
2-
CH2
CO2-
CO2-
HO
C
CH
CH2
CO2-
CO2-
OHH
-O2C
C
C
H2C
CO2-
CO2-
H
CO2-
Docsity.com
The preview of this document ends here! Please or to read the full document or to download it.
Document information
Embed this document:
Docsity is not optimized for the browser you're using. In order to have a better experience please switch to Google Chrome, Firefox, Internet Explorer 9+ or Safari! Download Google Chrome