Biochemistry and Molecular Biology - Belyasova N.A. 2002

Metabolism. Processes Requiring Energy Input
Metabolic Integration
The Role of Key Intermediates in Metabolic Integration

At any given moment, a metabolizing Cell carries out several hundred Chemical Reactions, all of which are, to some extent, interconnected. This is made possible through the tight integration of cellular METABOLISM and its multi-level regulation. The integration of anabolic (constructive) and catabolic (energy) metabolism was partly discussed in Chapter 8; here, we will focus on key intermediate metabolites—substances that allow The Cell to coordinate metabolic processes involving various classes of Organic compounds.

Figure 19.1 shows a simplified diagram illustrating the integration of Major Metabolic Pathways common to most Cells and organisms. As can be seen, the breakdown and Biosynthesis of complex organic compounds (Proteins, Nucleic Acids, CARBOHYDRATES, Lipids) are interconnected primarily via Pyruvate, acetyl-CoA, and the intermediates of The Tricarboxylic Acid Cycle.

The principal key metabolites include pyruvate, acetyl-CoA, oxaloacetate, and a-ketoglutarate. Let us examine their roles in cellular metabolism.

Pyruvate. This three-carbon a-keto acid links Glycolysis and Gluconeogenesis, as well as Carbohydrate Metabolism with the metabolism of lipids, proteins, Isoprenoids, and Ketone Bodies (Fig. 19.1).

In cells, pyruvate is formed primarily through hexose Catabolism, The oxidation of lactate (which accumulates in Muscles during Lactic acid Fermentation), and the deamination of Alanine. Hexose catabolism was described in Chapter 9. Lactic acid fermentation is a major metabolic process (Chapter 10) that actively occurs in the cells of vigorously working Human and Animal muscles when oxygen supply is insufficient. Here, lactate production enables the Organism to rapidly regenerate NAD⁺, which is essential for ATP generation via glycolysis (the contraction and relaxation of Muscle fibers require ATP Hydrolysis).

Lactate accumulating in the muscles enters the bloodstream and is transported to the Liver, where it is re-oxidized into pyruvate. During gluconeogenesis, Pyruvate can be converted back into glucose. This sequence of reactions is known as the Cori cycle.

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Fig. 19.1. Interconnection of major cellular processes (simplified diagram): 1 — glycolysis; 2 — Pentose Phosphate Pathways; 3 — gluconeogenesis; 4 — Calvin cycle; 5 — ß-Oxidation of Fatty acids; 6 — deamination; 7 — amination of keto acids; 8 — Urea Cycle; TCA — tricarboxylic acid cycle

Alanine is formed from pyruvate through Transamination reactions (Fig. 16.8) involving Amino Acids produced during protein proteolysis. Carried by the bloodstream to the liver, alanine is converted back into pyruvate, serving as a carrier of ammonium nitrogen, which is subsequently incorporated into The urea cycle in the liver. Additionally, alanine derived from pyruvate can be incorporated into Peptides.

The transformations of pyruvic acid (Fig. 19.2) are also linked to Carboxylation and Decarboxylation reactions. The first process, which takes place in the Mitochondria, yields oxaloacetate; this can either enter the TCA cycle (serving as a crucial anaplerotic reaction) or be converted into phosphoenolpyruvate and subsequently into glucose (gluconeogenesis).

In the second case, pyruvate undergoes oxidative decarboxylation to form acetyl-CoA, which is likewise a key metabolite.

Another important aspect of pyruvate metabolism involves various Types of fermentation in which pyruvic acid serves as a substrate.

Acetyl-CoA. The primary sources of this key intermediate are pyruvate and Fatty acids (via oxidative decarboxylation and ß-Oxidation, respectively). Furthermore, acetyl-CoA is produced from ketogenic amino acids during The breakdown of their carbon skeletons.

The Fate of acetyl-CoA, like that of other key metabolites, depends on the metabolic demands of the cell (organism): it can be fully oxidized in the tricarboxylic acid cycle as a substrate; it can undergo sequential Condensation to form 3-hydroxy-3-methylglutaryl-CoA, a precursor of Cholesterol, Terpenes (isoprenoids), and ketone bodies (acetoacetic acid, 3-hydroxybutyric acid, and acetone); or it can be converted into fatty acids (Fig. 19.2). As previously noted, mammalian organisms cannot convert acetyl-CoA into carbohydrates. However, in plant and microbial cells, the synthesis of gluconeogenesis precursors—and consequently carbohydrates—from acetyl-CoA is indeed possible (Fig. 11.7).

Fig. 19.2. Main transformations of pyruvate and acetyl-CoA in cellular metabolism

In addition to the reactions listed above, acetyl-CoA participates in the synthesis of amino acids—specifically Arginine, leucine, and Lysine (in fungal cells) and Cysteine (in certain microorganisms)—and serves as a substrate for Butyric Acid and acetone-butanol fermentation.

The involvement of the main TCA cycle intermediates (oxaloacetate, a-ketoglutarate) in Metabolic Integration can be easily traced in the diagrams presented in Figures 19.1 and 19.2.

To sum up, it should be emphasized once again that all metabolic processes occurring within a cell (organism) are interconnected via intermediate metabolites, which act as products of certain metabolic pathways and substrates for others. This interconnectedness and interdependence allow the cell (organism) to coordinate its capabilities with its metabolic demands and rapidly "fine-tune" its metabolic rate in response to changing environmental conditions. Naturally, this delicate mechanism would be impossible without the Regulation of Metabolic pathways.