reactii ciclul krebs

8/10/2019 Reactii Ciclul Krebs

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24.10.2013 Citric Acid Cycle Reactions

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Citric Acid Cycle

Carbohydrate

Metabolism

Overview

Overview

Metabolism &

Energy Summary

Elmhurst College

Form Acetyl CoACitric Acid Cycle

SummaryCori Cycle Chemistry Department

Citric Acid Cycle

ReactionsHormone Control Virtual ChemBook

Click for larger image

Citric Acid Cycle Reactions

Introduction:

Under aerobic conditions the end product of glycolysis is

pyruvic acid. The next step is the formation of acetyl

coenzyme A(acetyl CoA) - this step is technically not a part of

the citric acid cycle, but is shown on the diagram on the top left.

Acetyl CoA, whether from glycolysis or the fatty acid spiral, is

the initiator of the citric acid cycle. In carbohydrate metabolism,

acetyl CoA is the link between glycolysis and the citric acid

cycle.

The initiating step of the citric acid cycle occurs when a four

carbon compound (oxaloacetic acid) condenses with acetyl

CoA (2 carbons) to form citric acid (6 carbons).

The whole purpose of a "turn" of the citric acid cycle is to

produce two carbon dioxide molecules. This general oxidation

reaction is accompanied by the loss of hydrogen and electrons

at four specific places. These oxidations are connected to the

electron transport chain where many ATP are produced.

The reactions for the citric acid cycle are shown in the graphic

on the left. These reactions are more familiar than those from

glycolysis. Unless the instructor states otherwise, you should

study these reactions so that you can: tabulate the ATP and

CO2generated; name the type of reaction at each step; and

writethe structure of any compound which has been blanked

out. You should not memorize these structures but derive them

from a knowledge of reaction principles.

Citric Acid Cycle- with white background for printing

Overview of Metabolism

Reaction 1: Synthesis of Citric Acid

Acetyl CoA and oxaloacetic acid condense to form citric acid.

The acetyl group CH3COO is transferred from CoA to

oxaloacetic acid at the ketone carbon, which is then changed to

an alcohol. The net effect is to join a 2 carbon piece with a 4

carbon piece to make citric acid which is 6 carbons. This is just

called the synthesis of citric acid.

This reaction is catalyzed by citric acid synthetase.

Reaction 1 - Chime in new window
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Link to Chime: Citric Acid Synthetase- Department of

Biochemistry,The University of Arizona

Reaction 2: Dehydration of an alcohol

Two steps (Rx. 2 and 3) are required to isomerize the position

of the -OH group on citric acid. This first step is a dehydration

of an alcohol to make an alkene. The cis-aconitic acid remains

bound to the enzyme aconitase in readiness for the next step.

This reaction is catalyzed by aconitase.


Reaction 3: Hydration to make alcohol

This reaction is a hydration reaction of an alkene to make an

alcohol. This hydration does not follow Markovnikov's Rule.

The net effect of reactions 2 and 3 has been to move the -OH

group from C-3 to C-2, which is isocitric acid.

This reaction is catalyzed by aconitase.


Link to Chime: Aconitase- Department of Biochemistry,The

University of Arizona

Reaction 4: Oxidation

This is the first oxidation reaction in which an alcohol is

converted to a ketone. Two hydrogens and 2 electrons are

transferred to NAD+to NADH + H+. This is the entry point

into the electron transport chain.

The product of this reaction, oxalosuccinic acid, remains

attached to the isocitrate dehydrogenase for the next step.

This reaction is catalyzed by isocitrate dehydrogenase.

Reaction 4 Chime in new window

Reaction 5: Decarboxylation

This is the first step where a carbon group is lost as carbon

dioxide in a decarboxylation reaction. The remaining compound

now has 5 carbons and is called alpha-ketoglutaric acid.

This reaction is also catalyzed by isocitrate dehydrogenase.

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Reaction 6: Oxidation, Decarboxylation, Thiol Ester

Synthesis

This complex oxidative decarboxylation is guided by three

enzymes in much the same fashion as the formation of acetyl

CoA from pyruvic acid. This is actually the only non-reversible

step in the entire cycle and prevents the cycle from operating in

the reverse direction.

This is the second oxidation reaction in which an alcohol is

converted to a ketone. Two hydrogens and 2 electrons are

transferred to NAD+to NADH + H+. This another the entry

point into the electron transport chain.

This is the second step where a carbon group is lost as carbon

dioxide in a decarboxylation reaction. Essentially, although not

the exact same carbons, the two carbons from the acetyl CoA

have been converted to carbon dioxide at the end this step/.

The remaining 4 carbon group is attached to the CoA through a

thiol ester high energy bond. Notice that the final product,

succinyl CoA, has 4 carbons in the succinate group at one end

of the CoA molecule.

This reaction is catalyzed by alpha-ketoglutarate dehydrogenase

complex.


Reaction 7: Hydrolysis of Succinyl CoA;

Synthesis of ATP

The hydrolysis of the thioester bond (exothermic) is coupledwith the formation of ATP (Actually guanosine triphosphate is

formed first but is further coupled with the ADP to make ATP).

This is the only "visible" ATP formed in the entire cycle.

Succinic acid,a 4 carbon acid, is the product of this reaction.

This is the start of the return to the beginning of the cycle.

This reaction is catalyzed by succinyl CoA.



This slightly unusual oxidation reaction results in the removal of

the hydrogens from saturated alkyl carbons to form an alkene,

fumaric acid. The hydrogen acceptor is the coenzyme FAD

instead of the more usual NAD+. This will be significant when

the ATP is tabulated from the electron transport chain, since this

coenzyme is in the enzyme complex 2. Only 2 ATP result from

this reaction in the electron transport chain.

This reaction is catalyzed by succinate dehydrogenase.


Link to Chime: Succinate dehydrogenase- Department of

Biochemistry,The University of Arizona
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Reaction 9: Hydration to form an alcohol

This is a simple hydration reaction of an alkene to form an

alcohol. Take your pick where you place the -OH group since it

must be adjacent to a carboxylic acid group in either case and

forms malic acid.

This reaction is catalyzed by fumarase.

Reaction 9 - phosphoenol pyruvic acid Chime in new window


This is the final reaction in the citric acid cycle. The reaction is

the oxidation of an alcohol to a ketone to make oxaloacetic

acid. The coenzyme NAD+ causes the transfer of two

hydrogens and 2 electrons to NADH + H+. This is a final entry

point into the electron transport chain.

This reaction is catalyzed by malate dehydrogenase.


Conclusion:

Starting with acetyl Co A with 2 carbons, the citric acid cycle

spins these 2 carbons off as two carbon dioxide molecules.
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