Pyruvate dehydrogenase Rx  and  the KREBS CYCLE.

The Kreb's cycle (citric acid cycle, tri-carboxylic acid cycle) is common pathway for oxidation of all fuel molecules including amino acids, sugars, and fatty acids. The main point of entry for carbon molecules is as acetyl CoA (2C) which condenses with oxaloacetate (4C) to form citrate (6C). The citric acid cycle generates the reduced coenzymes, NADH and FADH 2, as well as GTP, and provides intermediates for biosynthetic reactions. The citric acid cycle occurs in the mitochondrial matrix of the cell. The reduced coenzymes enter oxidative phosphorylation (electron transfer chain) where the majority of the cell's ATP is synthesized, by the ATP synthase.     [see overall figure]

The first reaction to be considered is the oxidative decarboxylation of pyruvate to form acetyl CoA. This reaction, catalyzed by the enzyme pyruvate dehydrogenase complex, is not part of the citric acid cycle directly, but generates the acetyl CoA that enters the KC. This reaction provides the link between glycolysis and the KC. The reaction is as follows.
Pyruvate + CoA + NAD+ acetyl CoA + CO2 + NADH

The Pyruvate Dehydrogenase Complex responsible for carrying out this reaction contains 3 kinds of enzymes and 5 different many coenzymes. In a four step reaction sequence pyruvate is converted into acetyl CoA.   [figure]

One of the important cofactors necessary for the activity of the pyruvate dehydrogenase complex is thiamine pyrophosphate (TPP). TPP is derived from the vitamin thiamine, also called vitamin B 1. The function of TPP in the pyruvate dehydrogenase complex is to destabilize the bond between the carbonyl and carboxyl groups of pyruvate. TPP is also a cofactor for the transketolase enzymes. The structure of TPP is shown below. A deficiency in thiamine results in the disease called beriberi. The oral manifestations of thiamine deficiency include "old rose" colored tongue, some depapillation at the periphery of the tongue, and a deeper than expected red color of the oral mucosa.

Lipoamide or lipoic acid contains 2 thiol groups which are essential for its function as a cofactor. Lipoic acid is linked to the dihydrolipoyl transacetylase component of the pyruvate dehydrogenase complex. The thiol groups can exist in the oxidized form (as shown in the figure below) or in the reduced form (2 free HS- groups). Because of its ability to undergo oxidative-reduction reactions, lipoate can serve as an electron carrier as well as an acyl carrier. The pyruvate dehydrogenase reaction depends on both of these functions.

The mechanism of action of the PDH Complex is for the enzyme pyruvate decarboxylase to oxidatively decarboxylate the pyruvate by removing a CO 2.  Next, the enzyme lipoamide reductase transacetylase transfers the pyruvate carbon skeleton onto coenzyme-A.  The third enzyme, dihydrolipoyl dehydrogenase removes a hydrogen onto NAD making NADH.  The net result is the release of CO 2 and the making of NADH and acetyl-coenzyme-A for each pyruvate.   [figure]

Note the additional molecule of CO 2 and additional molecule of NADH formed by the pyruvate dehydrogenase reaction add to the totals of aerobic metabolism. However, these reaction products is not considered directly a part of the Krebs Cycle.

 

After the production of acetyl CoA, the initial step of the KC is the condensation of oxaloacetate and acetyl CoA to form citrate. This reaction is catalyzed by the enzyme citrate synthase. The reaction involves a condensation of reactants to form the intermediate citryl CoA followed by a hydrolysis reaction yielding citrate and CoASH. 

The net reaction for the citric acid cycle is:

Acetyl CoA + 3 NAD+ + FAD + GDP + Pi + 2 H 2O

                            2 CO 2 + 3 NADH + FADH 2 + GTP + H+ + CoA

Many of the reactions of the Krebs cycle result in the formation of important cellular molecules.

 

Note:    a new value for ATP per NADH is 2.5 and ATP per FADH2 is 1.5.
You may see the numbers ATP per NADH = 3   and   ATP per FADH 2 = 2 in some texts. For more information on the re-evaluation of the number of ATP's/nucleotide coenzyme see Hinkle, et al.  Mechanistic stoichiometry of mitochondrial oxidative phosphorylation. Biochemistry 30:3576-82, 1991.

The different values of 30 or 32 ATP/glucose depend on the method used to transport cytoplasmic NADH, formed by glycolysis, into the mitochondria, i.e. the shuttles.

Summary

  1. Pyruvate is converted to acetyl CoA by the pyruvate dehydrogenase complex
  2. The pyruvate dehydrogenase complex contains several enzyme activities and cofactors and is the key step regulating the flow of 2 carbon fragments into the citric acid cycle.
  3. Acetyl CoA condenses with oxaloacetate to form citrate as the first step of the Krebs cycle
  4. One round (1 Acetyl-coA) of citric acid cycle results in the formation of 
    1 GTP,   3 NADH,   1 FADH 2,  and   2 CO 2