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

Stoichiometry and Energetics of Metabolic Transformations
Carbon Catabolism
Alternative Pathways of Carbohydrate Catabolism

First of all, let us examine two other reaction sequences through which Glucose Catabolism takes place. The pentose phosphate cycle or Pentose Phosphate Pathway (also referred to as the Hexose monophosphate pathway or shunt) begins with The oxidation of glucose-6-phosphate:

Class="center">Глюкозо-6-фосфат+NАDР⁺→ 6-фосфоглюконат + NADPH + H⁺

The primary function of the pentose phosphate pathway is to supply The Cell with NADPH, which serves as an electron carrier in biosynthetic reactions. Although the scheme of all reactions occurring in this pathway is rather complex (see p. 456 in ref. [1]), the overall Stoichiometry of the pentose phosphate pathway can be expressed by the following relatively simple equation:

Глюкоза + 12NADP⁺ + 7H₂O + ATP →

→ 6СО₂ + Р_i + 12 (NADPH + H⁺) + ADP      (5.15)

Thus, the net result of the pentose phosphate pathway is the Complete oxidation of glucose-6-phosphate to СО₂ with The transfer of all electrons (H) to NADP.

ATP is consumed in this pathway. To provide the cell with ATP, the pentose phosphate cycle can be intercepted at a stage where the overall stoichiometry of the process is given by the equation

3 Глюкоза + 6NADP⁺ + АТР →

→ 2 фруктозо-6-фосфат+глицеральдегид-3-фосфат + 3СO₂+

+ 6(NADPH + H⁺) + ADP + P_i      (5.16)

If the subsequent conversion of fructose-6-phosphate and glyceraldehyde-3-phosphate to Pyruvate proceeds via the EMP pathway, the phosphorylation of 6 ADP molecules (per 3 glucose molecules) will also occur along this route. As a result, 5/3 moles of ATP are generated per mole of glucose, yet the overall energy yield remains lower than that of the EMP metabolic pathway.

At the same time, the pentose phosphate pathway has its own advantages, primarily due to the fact that it yields ribose-5-phosphate and erythrose-4-phosphate as intermediates, which are crucial precursors for the Synthesis of Purines and Pyrimidines in the cell. Such compounds are not formed in the EMP metabolic pathway. For this reason, microorganisms such as lactic acid Bacteria, which rely exclusively on the EMP pathway for Carbohydrate Catabolism, can grow under anaerobic conditions only on complex nutrient media containing pentoses, purines, and pyrimidines. In contrast, E. coli is capable of growing on simpler media even under anaerobic conditions, as it simultaneously employs both the EMP pathway and the pentose phosphate pathway (under certain conditions, for instance, 75% of CARBOHYDRATES are catabolized via the EMP pathway and 25% via the pentose phosphate pathway). This fact illustrates the general principle that Cells can catabolize a given nutrient through multiple pathways concurrently, which presumably ensures their optimal growth by satisfying demands for both energy and precursor metabolites.

The last of the glucose catabolism pathways considered here is the Entner-Doudoroff (ED) pathway. The overall stoichiometry of this reaction sequence is as follows:

Глюкоза + ATP + NADP⁺→

→ глицеральдегид-3-фосфат+пировиноградная кислота +

   + ADP + NADPH + H⁺      (5.17)

Considering that the subsequent conversion of glyceraldehyde-3-phosphate to pyruvate via the same scheme as in the EMP pathway is accompanied by the phosphorylation of two ADP molecules, it becomes evident that from an energetic standpoint, the ED pathway—which yields one mole of ATP per mole of glucose—is not very efficient. Before proceeding to the catabolism of other nutrients, it should be emphasized that the main glucose catabolism pathways described above by no means exhaust the metabolic capabilities of bacteria.

As mentioned earlier, microorganisms can utilize a variety of compounds as carbon sources. A detailed analysis of this issue can be found in ref. [3]; here, we shall limit ourselves to a few Examples illustrating solely the principal catabolic routes for substances other than glucose.

The utilization of other sugars frequently begins with their conversion, via one or more reactions, into glucose or into one of the intermediates of its Catabolic pathways. As an example, Fig. 5.5 illustrates The sequence of reactions by which E. coli initiates the assimilation of the disaccharide lactose. A distinctive feature of this catabolic route is the specific mechanism ensuring galactose utilization. Note the cyclic nature of uridine diphosphate (UDP) utilization, whereby it Functions catalytically. A second example is the utilization of fructose by certain bacteria; in this case, two enzyme-catalyzed reactions convert fructose into the corresponding 1,6-diphosphate, which can then enter the EMP pathway. The bacterial utilization of pentoses typically begins with a series of reactions yielding xylulose-5-phosphate, which may either enter the pentose phosphate pathway or undergo Cleavage via the phosphoketolase pathway (Fig. 5.6).

FIG. 5.5. Catabolism of lactose by E. coli (UDPG, uridine diphosphate glucose; UDPGal, uridine diphosphate galactose).

The processes of microbial utilization of other Organic compounds, particularly Hydrocarbons, are exceptionally interesting and diverse, as are the accompanying chemical transformations. Here we shall only mention the existence in the microbial world of enzyme systems capable of oxidizing hydrocarbons with or without altering their carbon Skeleton, converting hydrocarbon substrates into Amino Acids, Lipids, Vitamins, other microbial cell components, and various Other Compounds. Furthermore, certain microorganisms are able to degrade toxic substances such as phenols and polychlorinated biphenyls. Obviously, such MICROORGANISMS AND THE Enzymes they synthesize are potentially of great importance as catalysts for specific chemical transformations that may prove useful in solving a range of environmental protection problems. An Introduction to the biochemistry of hydrocarbon metabolic pathways and their corresponding enzymes is provided in the excellent book by Dowell [3].

FIG. 5.6. Simplified scheme of ribose catabolism via the phosphoketolase pathway.