G.R.
- Thread starter Todd
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You are a very resourseful fellow, there Todd. I feel I can not challenge you, because you have this informational weapon called the net, at your disposal. Oh the horror.....
what to do? OH I HAVE YOU NOW!!!!
Here it goes...Name and discribe the steps, where applicable, of the "Citric Acid Cycle" ...If I can do it, so can you!!!
what to do? OH I HAVE YOU NOW!!!!
Here it goes...Name and discribe the steps, where applicable, of the "Citric Acid Cycle" ...If I can do it, so can you!!!
ManOSafety
Literotica Guru
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- Jan 3, 2000
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ack....
Nightmares from my Bio class just came flashing back, thank you so much Todd...
Nightmares from my Bio class just came flashing back, thank you so much Todd...
Oh the horror...You didn't write that out, ya cheater...
I am sure that you didn't know that, by heart...
I am sorry but you don't get a prize...until you hand write it out like you learnt it.
P.S. I want it in essay from...no pussy till the teach is satisfied..
I am sure that you didn't know that, by heart...
I am sorry but you don't get a prize...until you hand write it out like you learnt it.
P.S. I want it in essay from...no pussy till the teach is satisfied..
The citric acid cycle (also called the Krebs cycle, after its primary discoverer, Sir Hans Krebs) is a central part of the chemistry which converts food components like carbohydrates, fats, andd proteins into ATP and thus provides energy to carry out biological functions. This is shown schematically as follows:
The primary input to the citric acid cycle is Acetyl Coenzyme A (abbreviated acetylSCoA). This molecule is produced from carbohydates and fats by processes we will look at later, and it is also one of the products of amino acid (protein) metabolism. Its structure is:
The function of the citric acid cycle is to convert the two carbons of acetylSCoA to CO2, ATP, NADH and FADH2. As we'll see later, the latter two molecules are converted to ATP by subsequent processes.
Now let's look at the individual reactions in the cycle.
The first step combines oxaloacetate, which is recycled through the whole cycle, with acetylSCoA to form citrate. In this reaction, a proton is removed from the alpha carbon of acetylSCoA (remember aldol and Claisen condensations?) to make a Lewis base which adds to the ketone carbonyl of oxaloacetate. HSCoA (Coenzyme A) is also released in this reaction.
The second step is an isomerization in which an H and an OH swap places. This looks mysterious, but it is the result of two "substeps"; first a dehydration (loss of water) and then a hydration (addition of water). The second "substep" puts the H where the OH was and vice versa.
The third step oxidizes the now secondary alcohol to a ketone. This gives us an NADH molecule as a product, and we also get a CO2. We can also understand why Reaction 2 was necessary, since the tertiary alcohol we had before step two couldn't be oxidized (no H on the C). The ketone produced in 2 is also needed to allow the CO2 to be released, but that's a detail we won't explain.
In the fourth step, alpha-ketoglutarate reacts with coenzyme A (HSCoA) to release another molecule of CO2 replace it with -SCoA. Coupled to this process is the reduction of a molecule of NAD+ to NADH. Two molecules of CO2 have been lost, so the two carbons of the acetyl group of acetylSCoA have effectively been oxidized to CO2.
Step five converts an ADP to ATP, using the energy available from the hydrolysis of the -SCoA ester to a carboxylate ion.
The remaining steps (six through eight) oxidize the one four carbons which are left from the citrate from -CH2- to C=O. Step six uses FAD to oxidize the alkane carbons in the middle of succinate to an alkene, making FADH2 as well. Addition of water to the new double bond occurs in step seven. The secondary alcohol produced in step seven is oxidized to a ketone in step eight, a reaction which is coupled to the reduction of NAD+ to NADH.
At the end of step eight, oxaloacetate is available to react with another molecule of acetylSCoA to begin the cycle again.
Since this cycle is a major part of the energy production system, its control is important so as to balance the cycle's activity and the body's needs. For example, NADH is an inhibitor of the enzyme for step three. If there is lots of NADH, the cycle is slowed down to reduce formation of more. If ATP is being used up by the body's energy needs, the concentration of ADP increases and it activates that same enzyme. If there's lots of ATP, it inhibits the enzyme for step 1. These control mechanisms adjust the cycle's activity so that it keeps in step with the energy needs of the body.
Let's sum up the changes that occur as a result of the citric acid cycle:
We put in:
One acetylSCoA
Four oxidized coenzymes (3 NAD+, 1 FAD)
One ADP and a phosphate.
We get out:
Four reduced coenzymes (3 NADH, 1 FADH2)
One ATP (energy rich)
Two CO2 molecules from an acetyl group
One coenzyme A (to be recycled into acetylSCoA in its synthesis - Chapter 11).
The two carbons of the acetyl group have been oxidized to CO2 and energy from that has been deposited in ATP, NADH and FADH2. The rest of the story is about the oxidation of NADH and FADH and the transfer of the energy stored there to ATP.
Before we look at the events which oxidize NADH and FADH2, let's connect two different descriptions of a reduction reaction. In this course, we've regarded reduction as the addition of H atoms (usually two). The reduction of a C=O group to a HC-OH group is our most familiar example. In inorganic chemistry, reduction is described as the addition of electron(s); for example, Fe+++ + e- --> Fe++. How are these descriptions related?
First, let's think about a hydrogen atom. It is a proton (H+ and an electron e-. If we separate the proton from the electron then the electron can be considered as a reducing agent. The proton can either go along (as in most organic reductions) or go else where (as in most inorganic reductions).
The function of the respiratory chain is to do just this. The H atoms are removed from the NADH and FADH2 molecules, making NAD+ and FAD again available for the citric acid cycle. The H atoms are effectively split into protons and electrons. This process takes place in a membrane, so that the chemistry on one side is kept separate from the chemistry on the other side. The protons are deposited on one side, making that side more acidic. The electrons are used to reduce oxygen to water on the other side of the membrane. This latter reaction also consumes protons, so that the reduction of oxygen to water makes its side of the membrane less acidic.
This difference in acid concentration (about 1.7 pH units) represents stored energy, since protons will tend to flow from a high concentration situation to a low concentration situation. If we prevent that, we store energy in a way which is analogous to storing enery by holding water back from flowing down hill.
The process which recovers this energy in usable form is called oxidative phosphorylation. It is not understood in detail, but its general features are that the passage of protons through an channel which contains the enzyme complex called ATP synthetase provides the energy needed to make ATP from ADP and phosphate.
When the results of the respiratory chain and oxidative phosphorylation are added up, the oxidation of a molecule of NADH to NAD+ yields three ATP molecules, and the oxidation of a molecule of FADH2 to FAD yields two ATP molecules.
We can use this to draw up a balance sheet for the oxidation of glucose
The primary input to the citric acid cycle is Acetyl Coenzyme A (abbreviated acetylSCoA). This molecule is produced from carbohydates and fats by processes we will look at later, and it is also one of the products of amino acid (protein) metabolism. Its structure is:
The function of the citric acid cycle is to convert the two carbons of acetylSCoA to CO2, ATP, NADH and FADH2. As we'll see later, the latter two molecules are converted to ATP by subsequent processes.
Now let's look at the individual reactions in the cycle.
The first step combines oxaloacetate, which is recycled through the whole cycle, with acetylSCoA to form citrate. In this reaction, a proton is removed from the alpha carbon of acetylSCoA (remember aldol and Claisen condensations?) to make a Lewis base which adds to the ketone carbonyl of oxaloacetate. HSCoA (Coenzyme A) is also released in this reaction.
The second step is an isomerization in which an H and an OH swap places. This looks mysterious, but it is the result of two "substeps"; first a dehydration (loss of water) and then a hydration (addition of water). The second "substep" puts the H where the OH was and vice versa.
The third step oxidizes the now secondary alcohol to a ketone. This gives us an NADH molecule as a product, and we also get a CO2. We can also understand why Reaction 2 was necessary, since the tertiary alcohol we had before step two couldn't be oxidized (no H on the C). The ketone produced in 2 is also needed to allow the CO2 to be released, but that's a detail we won't explain.
In the fourth step, alpha-ketoglutarate reacts with coenzyme A (HSCoA) to release another molecule of CO2 replace it with -SCoA. Coupled to this process is the reduction of a molecule of NAD+ to NADH. Two molecules of CO2 have been lost, so the two carbons of the acetyl group of acetylSCoA have effectively been oxidized to CO2.
Step five converts an ADP to ATP, using the energy available from the hydrolysis of the -SCoA ester to a carboxylate ion.
The remaining steps (six through eight) oxidize the one four carbons which are left from the citrate from -CH2- to C=O. Step six uses FAD to oxidize the alkane carbons in the middle of succinate to an alkene, making FADH2 as well. Addition of water to the new double bond occurs in step seven. The secondary alcohol produced in step seven is oxidized to a ketone in step eight, a reaction which is coupled to the reduction of NAD+ to NADH.
At the end of step eight, oxaloacetate is available to react with another molecule of acetylSCoA to begin the cycle again.
Since this cycle is a major part of the energy production system, its control is important so as to balance the cycle's activity and the body's needs. For example, NADH is an inhibitor of the enzyme for step three. If there is lots of NADH, the cycle is slowed down to reduce formation of more. If ATP is being used up by the body's energy needs, the concentration of ADP increases and it activates that same enzyme. If there's lots of ATP, it inhibits the enzyme for step 1. These control mechanisms adjust the cycle's activity so that it keeps in step with the energy needs of the body.
Let's sum up the changes that occur as a result of the citric acid cycle:
We put in:
One acetylSCoA
Four oxidized coenzymes (3 NAD+, 1 FAD)
One ADP and a phosphate.
We get out:
Four reduced coenzymes (3 NADH, 1 FADH2)
One ATP (energy rich)
Two CO2 molecules from an acetyl group
One coenzyme A (to be recycled into acetylSCoA in its synthesis - Chapter 11).
The two carbons of the acetyl group have been oxidized to CO2 and energy from that has been deposited in ATP, NADH and FADH2. The rest of the story is about the oxidation of NADH and FADH and the transfer of the energy stored there to ATP.
Before we look at the events which oxidize NADH and FADH2, let's connect two different descriptions of a reduction reaction. In this course, we've regarded reduction as the addition of H atoms (usually two). The reduction of a C=O group to a HC-OH group is our most familiar example. In inorganic chemistry, reduction is described as the addition of electron(s); for example, Fe+++ + e- --> Fe++. How are these descriptions related?
First, let's think about a hydrogen atom. It is a proton (H+ and an electron e-. If we separate the proton from the electron then the electron can be considered as a reducing agent. The proton can either go along (as in most organic reductions) or go else where (as in most inorganic reductions).
The function of the respiratory chain is to do just this. The H atoms are removed from the NADH and FADH2 molecules, making NAD+ and FAD again available for the citric acid cycle. The H atoms are effectively split into protons and electrons. This process takes place in a membrane, so that the chemistry on one side is kept separate from the chemistry on the other side. The protons are deposited on one side, making that side more acidic. The electrons are used to reduce oxygen to water on the other side of the membrane. This latter reaction also consumes protons, so that the reduction of oxygen to water makes its side of the membrane less acidic.
This difference in acid concentration (about 1.7 pH units) represents stored energy, since protons will tend to flow from a high concentration situation to a low concentration situation. If we prevent that, we store energy in a way which is analogous to storing enery by holding water back from flowing down hill.
The process which recovers this energy in usable form is called oxidative phosphorylation. It is not understood in detail, but its general features are that the passage of protons through an channel which contains the enzyme complex called ATP synthetase provides the energy needed to make ATP from ADP and phosphate.
When the results of the respiratory chain and oxidative phosphorylation are added up, the oxidation of a molecule of NADH to NAD+ yields three ATP molecules, and the oxidation of a molecule of FADH2 to FAD yields two ATP molecules.
We can use this to draw up a balance sheet for the oxidation of glucose
Very good Todd....
I especially liked the inclusion of the reduction info...
I would say that you can have a prize now!
However, the Kreb's cycle follows glycolysis, not the reverse. Where the Kreb's cycle ends, the Electron Transport Chain begins. The ETC is occurs in the mitochondria, where the Reduced NADH and FADH2, are passed along cytochromes and manipulated in a series of exergonic redox reactions, thus resulting in oxidative phosphorylation.
Glycolosis, or the oxidation of pyruvate is a much more archaic form of respiration.
Todd, care to find a copy of the chemiosmotic model?
I especially liked the inclusion of the reduction info...
I would say that you can have a prize now!
However, the Kreb's cycle follows glycolysis, not the reverse. Where the Kreb's cycle ends, the Electron Transport Chain begins. The ETC is occurs in the mitochondria, where the Reduced NADH and FADH2, are passed along cytochromes and manipulated in a series of exergonic redox reactions, thus resulting in oxidative phosphorylation.
Glycolosis, or the oxidation of pyruvate is a much more archaic form of respiration.
Todd, care to find a copy of the chemiosmotic model?
