Biochemical Engineering Fundamentals, Part 1 - Bailey J., Ollis D. 1989

Stoichiometry and Energetics of Metabolic Conversions
Respiration
Tricarboxylic Acid Cycle

First, supplementing Section 5.3 on Carbohydrate METABOLISM, we should note that all the metabolic pathways described therein leading to Pyruvate can also operate during Respiration, with respiration-specific reactions commencing at the pyruvate formation stage. However, unlike Fermentation, pyruvate in respiratory metabolism is not reduced to any end product by the hydrogen atoms generated during glucose breakdown; instead, these hydrogens are utilized for other purposes, which we will briefly review here. Furthermore, The conversion of pyruvate into an acetic acid derivative (acetyl-CoA) generates an additional amount of the universal reducing agent NADH:

Class="center">

Acetyl-CoA (see Fig. 2.9) is also a key intermediate in the Catabolism of Amino Acids and Fatty acids; consequently, all three classes of Organic compounds can be oxidized to acetyl-CoA.

The first phase of this oxidation proceeds via a closed sequence of reactions known as the tricarboxylic acid (TCA) cycle, the Krebs cycle, or The Citric Acid Cycle (Fig. 5.7). Note that the two remaining carbon atoms of pyruvate (one having been previously lost as CO₂ during acetyl-CoA formation) enter the cycle by condensing with a C₄-acid to form a 6-carbon carboxylic acid. In subsequent Stages of the TCA cycle, however, these two carbon atoms are released as CO₂, meaning that all carbon atoms of the original pyruvate are fully accounted for in this initial phase of respiration.

The overall Stoichiometry of the TCA cycle is as follows:

It might appear that the TCA cycle (as depicted in Fig. 5.7) serves a purely catalytic function, since no carbon source other than the substrate is indicated. In reality, however, as shown in Fig. 5.1, the TCA cycle is equally vital as a source of precursors for downstream biosynthetic pathways. Consequently, certain intermediates of this cycle are continually withdrawn for Biosynthesis, and their pools must be replenished. This is accomplished through the synthesis of oxaloacetate from pyruvate or another 3-carbon acid. In some microorganisms, TCA cycle intermediates are replenished via The Glyoxylate cycle, in which a molecule of succinate is formed by the Condensation of two acetate molecules.

FIG. 5.7. The Tricarboxylic Acid Cycle.