The conversion of pyruvate to acetyl-CoA is an important reaction in the metabolic process. In this article, we will discuss how it is done and what happens when a single pyruvate molecule is converted into acetyl-CoA. First, let’s review some metabolism basics:
1) The citric acid cycle (or Krebs cycle) takes place in the mitochondria where there are enzymes that convert ATP molecules into adenosine diphosphates (ADP). Remember that ATP contains three phosphate groups; when it breaks down, one of those phosphate groups transfers to ADP.
2) Another enzyme converts ADP back into ATP by transferring one phosphate group from NADH.
Now, let’s get to the conversion of pyruvate into acetyl-CoA. If there is an excess of pyruvates and NAD+ in the mitochondria, then when one molecule of glucose enters this pathway it will be converted by enzymes either to form oxaloacetate (when oxygen levels are high) or as we shall see later on when oxygen levels are low. The equation is: PYRUVATE + COA –> ACETYL-COA + LACTATE + H+, which you can also remember as “Pyr” plus “Coa” equals “Acetyl-” plus “Lactate”. This reaction takes place with two other products being produced; lactic acid and hydrogen ions.
this is because when pyruvate (or any other three carbons molecule) enters the citric acid cycle, one of those phosphate groups transfers to ADP.
another enzyme converts ADP back into ATP by transferring one phosphate group from NADH.”
This article covers how single pyruvate is converted to acetyl-CoA, including what happens when this reaction occurs in different environments with varying oxygen levels such as high or low. When there are more than enough pyruvates available for use in the mitochondria -as would be seen during periods of intense exercise or fasting-, then oxaloacetate will form instead of using two molecules of COA rather than one.
The most common form of this is when a glycolysis reaction occurs in the absence of oxygen, an erythrocyte will be present to donate its NADPH for use by pyruvate kinase and phosphoenolpyruvic acid dehydrogenase. When there are not enough oxaloacetates available for conversion into Acetyl CoA then alpha-keto acids such as malonyl CoA or succinyl-CoA can take their place -though they are less efficient at functioning with energy production-.
Alternatively, when acetyl-CoA levels exceed demand in high aerobic conditions like those seen during recovery periods after exercise, the excess amounts of COA get converted back into OAA so as not to build up.
If pyruvate joins acetyl CoA, the other product will be an alpha-keto acid such as succinyl-CoA or malonyl Coa which is less efficient when it comes to energy production than acetyl CoA; this happens when there are no oxaloacetates available for conversion into Acetyl CoA.
How To Convert A Single Pyruvate Into Acetyl-CoA: How It Is Done And What Happens When
When there are low oxygen levels, the pyruvate gets converted into acetyl CoA.
This happens when malonyl Coa or succinyl-CoA can take their place -though they are less efficient at functioning with energy production-. The other products of this reaction when single pyruvate is converted to acetyl CoA include alpha-keto acids like malonyl and succinyltacetic acid which are not as good for producing energy as Acetyl CoA. If OAA levels exceed demand in high aerobic conditions like those seen during recovery periods after exercise, then COA gets converted back into an OAA so that it does not build up.
If you want to know more about how this happens and what can happen when acetyl-CoA is produced, click the link below.
How It Is Done: carbon dioxide (CO²) gets released from one end of the Krebs cycle at the end of aerobic respiration. A molecule called pyruvate is created when glucose breaks down, and it can be converted to acetyl-CoA in a process known as reductive carboxylation
This conversion takes place in two stages where one alpha-keto acid has to be removed first before the CO² gets turned into an OAA so that it doesn’t build up:
First step – PEP (phosphoenol pyruvic) goes through decarboxylase enzymes which turn some of its carbon molecules into oxaloacetic acids, turning them from acidic to basic. These then react with phosphorylated malonic semialdehyde or succinyl coenzyme A
to produce acetyl-CoA and CO².
One of the products that come out is alpha-ketoglutarate which goes back to Krebs cycle for a second round as it’s needed in another step
Second Step – PEP gets converted into pyruvate by adding one carbon molecule, this reaction releases hydrogen ions (H+) which then go through Malic enzyme or Isocitrate dehydrogenase enzymes to form NAD+ and FAD+. These reactions also release ATP molecules when they are coupled with other phosphorylation reactions. After all the necessary conversions have been made, oxaloacetate can now react with succinyl coenzyme A again so that the process repeats itself: turn two alpha-keto acids together to form alpha-ketoglutarate, produce NAD+ and FAD+, convert PEP into pyruvate by adding one carbon molecule.