Citric Acid Cycle

Carbohydrate Metabolism Overview
Overview Metabolism &
Energy Summary
 Elmhurst College
Form Acetyl CoA Citric Acid Cycle Summary Cori Cycle  Chemistry Department
Citric Acid Cycle Reactions Hormone Control    Virtual ChemBook


Click for larger image 

Citric Acid Cycle Summary

Introduction to Citric Acid Cycle:

The diagram at the left puts the citric acid cycle into perspective as the most important part of metabolism. All of the other types of metabolism feed into it. This is the most important link to the electron transport chain where most of the ATP needed for energy reactions is produced.

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 source of acetyl CoA is glycolysis or the fatty acid spiral.

As the cycle proceeds, 2 carbons are converted into two CO2 molecules at steps 5 and 6.

Another important aspect is the series of oxidation reactions which are 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.


Click for larger image

Energy Production:

"Visible" ATP:

In the citric acid cycle, there is only one reaction which indirectly produces an ATP and this is at step 7.

Connections to Electron Transport and ATP:

Reactions 4, 6, and 10 involve oxidations of an alcohol group to a ketone group with the coenzyme NAD+, which result in the removal of 2 hydrogens and 2 electrons. This is the entry point into electron transport chain. Recall that starting with NAD+, results in the production of 3 ATP for each use of the electron transport chain.

Electron Transport Diagram

How many ATP are produced from one turn of the citric acid cycle starting with NAD+ in the electron transport chain?

Steps 4 = NAD+ = 3 ATP
Steps 6 = NAD+ = 3 ATP
Steps 10 = NAD+ = 3 ATP
NET ATP from = NAD+ = 9 ATP


Step 8 is another oxidation involving the coenzyme FAD. Since FAD is part of the second enzyme complex, only 2 ATP are made with the electron transport chain.

Step 8 = FAD = 2 ATP

Quiz: Starting with pyruvic acid to make acetyl CoA, how many ATP are made?  
 Starting with pyruvic acid to make acetyl CoA and continuing into the citric acid cycle, how many ATP are made?  

ATP Summary for Citric Acid Cycle:

Starting with acetyl CoA, how many ATP are made using the citric acid cycle? E.T.C = electron transport chain

 Step

 ATP produced

7

 1

Step 4 (NAD+ to E.T.C.)

3

Step 4 (NAD+ to E.T.C.) 

 3

Step 4 (NAD+ to E.T.C.)

 3

Step 8 (FAD to E.T.C.)

2

 NET

12 ATP

 
Quiz:
Starting with glycogen producing one glucose-6-phosphate, what is the end product of aerobic glycolysis?
 
Starting with glycogen to make glucose-6-phosphate, how many ATP are made using aerobic glycolysis?  
Starting with pyruvic acid, how many carbon dioxide molecules are produced upon complete metabolism in the citric acid cycle?  
 Starting with glucose, how many carbon dioxide molecules are produced upon complete metabolism in the citric acid cycle?  
 Starting with glycogen to make glucose-6-phosphate, how many ATP are made using aerobic glycolysis and the citric acid cycle?  

 

 

 ATP Summary for Glucose - Complete Metabolism:

The phrase "complete metabolism" means do reactions until you end up with carbon dioxide and water. This also means to use glycolysis, citric acid cycle, and electron transport as needed.

Starting with glucose (six carbons) how many ATP are made using glycolysis? This is a review of glycolysis. E.T.C = electron transport chain

 Step

 ATP (used -) (produced +)

 1

 -1

 3

 -1

 5 - NADH to E.T.C to FAD = 2
step 5 used twice

 
2 x 2 = +4

 6 used twice

 1 x 2 = + 2

 9 used twice

 1 x 2 = + 2

 NET

 6 ATP

The glycolysis end product is pyruvic acid, but remember that one glucose (6 carbons) produces two pyruvic acid (3 carbons) molecules.

Starting with one pyruvic acid, how many ATP are made using the citric acid cycle? E.T.C = electron transport chain

 Step

 ATP produced

Pyruvic acid to acetyl CoA
(NAD+ to E.T.C.)

3

Step 7

  1

Step 4 (NAD+ to E.T.C.)

3

Step 4 (NAD+ to E.T.C.) 

 3

Step 4 (NAD+ to E.T.C.)

 3

Step 8 (FAD to E.T.C.)

2

 NET (one pyruvic)

15 ATP

 NET (2 pyruvic)

2 x 15 =  30 ATP

 Step

 ATP

Glycolysis

6

Citric Acid Cycle
w/ 2 pyruvic acid

2 x 15 =  30 ATP

 Net complete glucose

 36 ATP