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This tutorial will give you an overview of the Kreb Cycle process and focus on the Pyruvate Dehydrogenase reaction that generates the Acetyl-CoA required for ...
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Welcome to part one of our lecture series on the Kreb Cycle. This tutorial will give you an overview of the Kreb Cycle process and focus on the Pyruvate Dehydrogenase reaction that generates the Acetyl-CoA required for entry into the Kreb Cycle.
The Amazing Mitochondria Image from Villareal, M.R. Following the completion of glycolysis, pyruvate will move into the matrix of the mitochondria during aerobic respiration. It will then be converted to Acetyl-CoA which can then enter into the reactions of the Kreb Cycle (which is also called the Citric Acid Cycle or Tricarboxylic Acid Cycle)
Amino Acid Metabolism Image from : Keministi Amino acid synthesis is tied with the processes of carbohydrate metabolism It is interesting to note that while the glycolytic pathway and the Kreb cycle are known for their abilities to produce electron carriers that will generate large amounts of ATP (producing energy for the cell), the intermediates within the pathway can also be converted to amino acids that will be utilized during protein synthesis. For example, you have already seen in the process of gluconeogenesis, that oxaloacetate can be converted into aspartate, and alpha-ketoglutarate into glutamate. These can be further modified to form other amino acids such as Asparagine and Methionine, or Proline and Arginine. Many branched chain amino acids can also be produced from pyruvate, including alanine.
Image from : Keenleyside W. Microbiology: Canadian Edition Nucleotide Biosynthesis Further branching off from these metabolite pools are the production of the nucleotide monomers used in DNA and RNA biosynthesis. Other important cofactors such as the heme in hemoglobin are also produced from intermediates in the Kreb Cycle. Thus, this metabolic pathway sets the stage for providing both the resources and energy for building the major macromolecules within the body. They are all connected here, which is why we spend so much time studying this system.
Conversion of Pyruvate to Acetyl-CoA By the Pyruvate Dehydrogenase (PDH) Complex Image from Narayanese, WikiUserPedia, YassineMrabet, TotoBaggins -CoA But before we can get into the reactions that are within the Kreb Cycle, pyruvate must first be converted into Acetyl-CoA. This is mediated by a major enzyme complex (perhaps one of the largest complexes known!) that is called the Pyruvate Dehydrogenase Complex. It is common for this complex to have more than 60 subunits and it is visible by electron microscopy. Within this reaction Pyruvate and Coenzyme-A(SH) are substrates that end in producing a molecule of CO 2 and Acetyl-CoA. A molecule of NADH is also produced during this process.
Pyruvate Dehydrogenase Complex
Pyruvate Dehydrogenase (PDH) Complex Image from : Goodsell, D. (2012) Molecule of the Month, Protein Database
Electron Microscopy of PDH Complex Image from : Vijayakrishnan, S., et al (2010) J Mol Bio 399(1):71- This is a partial recombinant structure of the Pyruvate Dehydrogenase (PDH) Protein Complex analyzed by negative-stain electron microscopy. I love the technologies that allow us to visualize the realtime structure of biological molecules. It makes me stop and reflect on the wonders of life and how complex we all really are.
Thiamine Pyrophosphate (TPP) Derivative of Vitamin B Image from : CSIRO Thiamine Pyrophosphate is derived from vitamin B1. Good sources of Thiamine (vitamin B1) from the diet, include legumes, nuts, oats, eggs, milk, beef, and liver. Other food sources are often fortified with vitamin B1 such as rice, pasta, breads, cereal and flour.
Decarboxylation Occurs at E Image modified from : Yikrazuul H 3 H^3 H 3 3 H E1 Subunit E1 Subunit = Pyruvate Decarboxylase Activity During the reaction mechanism of PDH, pyruvate and thiamine pyrophosphate (TPP) are bound by the E1 subunint of PDH. The thiazolium ring of TPP is in a zwitterionic form, and the anionic C2 carbon performs a nucleophilic attack on the C2 (ketone) carbonyl of pyruvate. The resulting intermediate undergoes decarboxylation leaving a 2 carbon intermediate attached to TPP.
Pyruvate Dehydrogenase Complex Image from : Goodsell, D. (2012) Molecule of the Month, Protein Database
Decarboxylation Occurs at E Image modified from : Yikrazuul H 3 H^3 H 3 3 H E1 Subunit E1 Subunit = Pyruvate Decarboxylase Activity Once the acetyl group has been attached to the TPP in the E1 subunit, the oxidized form of the lipoamide cofactor enters the subunit. The ene-ol carbon in the TPP-intermediate structure has anionic character, enabling it to attack the disulfide bond of the oxidized lipoamide cofactor. This releases and restores the TPP cofactor in the E1 subunit and opens the ring structure of lipoamide to create one reduced thiol functional group and the other sulfur loaded with the acetyl group from pyruvate. The E1-catalyzed process is the rate- limiting step of the whole pyruvate dehydrogenase complex.
Pyruvate Dehydrogenase Complex Image from : Goodsell, D. (2012) Molecule of the Month, Protein Database
Reduction and Transacetylation at E Image modified from : Yikrazuul H 3 H^3 H 3 3 H E2 Subunit E2 Subunit = Lipoamide Reductase Activity At this point, the hydrolipoate-thioester functionality is positioned into the dihydrolipoyl transacetylase (E2) active site, where a transacylation reaction transfers the acetyl from the "swinging arm" of lipoamide to the thiol of coenzyme A. This produces acetyl-CoA, which is released from the enzyme complex and subsequently enters the citric acid cycle. It also fully reduced the lipoamide cofactor to dihydrolipoamide. Thus, the E2 subunit can also be called lipoamide reductase-transacetylase.
