TCA- The Citric Acid Cycle



The above diagram depicts the citric acid/Krebs cycle. This occurs in the matrix of the mitochondria after the Link Reaction (the conversion of 2 molecules of pyruvate to 2 molecules of acetyl-CoA). The acetyl-CoA produced from this reaction enters this cycle. Well you can already see from the diagram all the enzymes and products involved. NOTE that TWO molecules of pyruvate is converted to TWO molecules of acetyl-CoA. This means that this citric acid cycle has to occur TWICE.

The main products to note PER TURN of this cycle are:

  • 1 ATP produced
  • 3 NADH produced
  • 1 FADH2

Pyruvate Ventures into TCA Cycle (Link Reaction)


One of the 3 uses of the pyruvate that is made from glycolysis is its entry into The Citric Acid Cycle, also known as The Krebs Cycle. It first has to go through the Link Reaction, which occurs under aerobic conditions (i.e when oxygen is present) and takes place in the matrix of mitochondria. A little summary of this is seen in this picture:

This diagram outlines the main points of this first main use of pyruvate. We see that the 2 molecules of pyruvate, under aerobic conditions (oxygen present), undergoes oxidative decarboxylation and loses 2 molecules of carbon. The products formed from this are 2 molecules of Acetyl CoenzymeA (acetyl-CoA), which now enters the Citric Acid Cycle, and 2 molecules of carbon dioxide.

The next diagram explains in detail this Link Reaction:


This process occurs for each of the 2 pyruvate molecules. The enzyme pyruvate dehydrogenase (along with the five co-factors: CoA-SH, NAD+, TPP, lipoate and FAD) catalyse the conversion of pyruvate (3C) into acetyl-CoA (2C). The other two products of this reaction are carbon dioxide and NADH. As said previously, the acetyl-CoA now enters the Krebs/Citric Acid Cycle.

Purpose of Glycolysis


Now let’s look at the bottom half of that pik I posted 2 posts ago:

Now the main reason why glycolysis happens, is to generate ATP (energy) from glucose. Ok so maybe in the first half of the pathway we USED 2ATP molecules. But hey, gotta use some to gain some, right? 

So, let’s first observe where the “made-back” 2ATP molecules were made:

NOTE: that’s actually supposed to be a double arrow because that reaction is reversible (sorry about that!)
So the regeneration of the 2ATP that was used in the first phase of glycolysis occurs here, in the conversion of 2 molecules of 1,3-bisphosphate to 2 molecules of 3-phosphoglycerate. Can you guess what enzyme number7 is? If you still have your glycolysis pathway pik that I posted, you would see that its phosphoglycerate kinase. This entire conversion is called the “first ATP forming reaction” and it is the substrate-level phosporylation that takes place in the cytosol of cells.

The second 2ATP that are made are the ATP molecules that were gained after glycolysis.


This reaction has a single arrow because it is the ONLY irreversible reaction in the payoff phase of glycolysis. 
It is from the final conversion of 2 molecules of phosphoenolpyruvate to 2 molecules of pyruvate by (enzyme number10) pyruvate kinase that 2 ATP molecules are generated. This conversion is called the “second ATP forming reaction” and it is also a form of substrate-level phosphorylation.

So in summary, in the Payoff Phase of Glycolysis (the second phase of glycolysis), a total of 4 ATP molecules are made. 2 of these are “paid back” because they were used in the first phase of glycolysis (the Prepatory Phase). This leaves 2 molecules of ATP to be gained from this enitre phase.

Glycolysis Pathway- the Video to go with the Previous Pik


This video is where my awesome biochemistry lecturer explains the whole process of glycolysis; showing the enzymes involved, the reactants and products evolved from glycolysis, the number of molecules of ATP and NAD+ used etc. The topic is clearly explained in this video and I advise that you all watch it to the very end because I assure it it will be worth it ^_^. Certain parts of this process will be further discussed in the next post.