By the end of this section, you will be able to:
- Explain how a circular pathway, such as the citric acid cycle, fundamentally differs from a linear pathway, such as glycolysis
- Describe how pyruvate, the product of glycolysis, is prepared for entry into the citric acid cycle
Introduction to pyruvate oxidation and the tca cycle
A note from the instructor
As with the module on glycolysis, there is a lot of material in this module. I do not expect you to memorize specific names of compounds or enzymes. However, I will give you those names for completeness. For exams I will always provide you with the pathways we discuss in class and in the BioStax Biology text modules. What you need to be able to do is understand what is going on in each reaction. We will go over in lecture, problems that will be similar to those I will ask of you on exams. Do not be overwhelmed with specific enzyme names and specific structures. What you should know are the general types of enzymes used and the types of structures found. For example you do
not need to memorize the structures of malate or succinate. You will need to know that both are carboxylic acids if the structure is given to you and should be able to identify the important functional groups. In addition, you will
not need to know which reactions specifically generate GTP or NADH, but if given the reactions you should be able to tell if a red/ox reaction is occurring. Finally, you will
not be expected to memorize enzyme names, but like in glycolysis you will be expected to know the various types of reactions a type of enzyme can catalyze, for example, a
dehydrogenase catalyzes a red/ox reaction. That is the level of understanding I expect. If you have any questions please ask.
Pyruvate oxidation and the tca cycle
The end-product of glycolysis are 2 pyruvate molecules, 2 ATPs and 2 NADH molecules. The question becomes, what does the cell do with them. ATP can be used for a variety of cellular functions including biosynthesis, transport, replication etc. NADH, is a problem, it needs to be recycled to NAD
+ . This occurs either through fermentation, in the absence of an electron transport chain, or can be used to generate a proton motive force (PMF) or "energized membrane", which can then lead to either ATP formation or other forms of work (transport of nutrients, cellular locomotion, etc. and will be discussed in later modules). That leaves the cell to deal with pyruvate.
The fate of cellular pyruvate
- Pyruvate can be used as a terminal electron acceptor in fermentation reactions, as was discussed in Module 7.2.
- Pyruvate could be secreted from the cell as a waste product.
- Pyruvate could be further oxidized to extract even more usable cellular energy, which is what will be discussed below.
The further oxidation of pyruvate
The pruvate formed in glycolysis has a variety of fates depending upon the cell type, physiology and environment the cell is in. In many instances, cells can further oxidize pyruvate, generating additional energy in the form of
GTP and reducing power, the formation of
NADH (and FADH2) along with the production of a variety of additional precursors, which can be used for biosynthesis as required by the cell. In aerobically respiring eukaryotic cells, the pyruvate molecules produced at the end of glycolysis are transported into mitochondria, which are the sites of cellular respiration and house the oxygen consuming electron transport chain. In respiring bacteria and archaea, the pyruvate is further oxidized in the cytoplasm. All three use similar mechanisms to further oxidize the pyruvate to CO
2 . Regardless of the organism, if pyruvate is to be further oxidized, the reactions are basically universal: first pyruvate will be transformed into an acetyl group that will be picked up and activated by a carrier compound called
coenzyme A (CoA) and the resulting
acetyl-CoA feeds directly into the
Tricarboxylic Acid Cycle also referred to as the
TCA cycle or the
Krebs Cycle . This process is detailed below.
