BIOCHEMISTRY - Textbook - Ostapchenko L. I. - 2012

Chapter 16. ENERGY METABOLISM

16.3. The Final Stage of Catabolism as the Main Source of Hydrogen Donors for the ETC

16.3.1. Oxidative Decarboxylation of Pyruvate

The oxidative decarboxylation of Pyruvate (see subsection 12.1.3) takes place in the mitochondrial matrix. Pyruvate is transported into the mitochondrial matrix across The inner mitochondrial membrane via a specific carrier protein through an H⁺ symport mechanism (Fig. 16.14). The conversion of pyruvate into acetyl-CoA is described by the following overall equation:

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Fig. 16.14. Transport of pyruvate across the mitochondrial membrane

During this reaction, Oxidative Decarboxylation of pyruvate occurs, As a result of which the carboxyl group is released as _CO2, while the acetyl group is incorporated into acetyl-CoA. One hydrogen atom is incorporated into NADH, while another is released into the medium as H⁺. The reaction is irreversible, since ∆G⁰ = -33.5 kJ/mol.

Structure OF THE pyruvate dehydrogenase complex. The process of oxidative decarboxylation is catalyzed by the highly organized pyruvate dehydrogenase complex (PDC). The PDC includes three Enzymes: pyruvate decarboxylase (E₁), dihydrolipoyl transacetylase (E₂), and dihydrolipoyl dehydrogenase (E₃), as well as five Coenzymes: Thiamine diphosphate (TDP), Lipoic Acid (LA), FAD, NAD⁺, and CoA. In addition, the complex contains Regulatory Subunits: protein kinase and phosphoprotein phosphatase (Table 16.4).

All these Enzymes and Coenzymes are assembled into a multienzyme system that contains varying amounts of each enzyme and has a molecular weight exceeding 6 ·10⁶ Da.

At the center of the complex lies dihydrolipoyl transacetylase (E₂), which forms its core. The molecules of pyruvate decarboxylase (E₁) and dihydrolipoyl dehydrogenase (E₃) are attached to dihydrolipoyl transacetylase.

Table 16.4

Mammalian Pyruvate Dehydrogenase Complex

Pyruvate decarboxylase contains TDP tightly bound to its protein moiety, whereas dihydrolipoyl dehydrogenase contains FAD.

The lipoylated groups of the core enzyme (E₂) function as swinging "arms" that transfer hydrogen atoms and acetyl groups from one enzyme molecule of the complex to another.

Reactions Catalyzed by the PDC. The conversion of pyruvate into acetyl-CoA involves five stages (Fig. 16.15).

Fig. 16.15. Sequence of reactions catalyzed by the pyruvate dehydrogenase complex:

1 — E₁ catalyzes the decarboxylation of pyruvate and The transfer of the C₂-fragment to TDP; 2 — E₂ catalyzes The oxidation of the hydroxyethyl group and the transfer of the C₂-fragment to LA; 3 — the acetylated dihydrolipoyl transacetylase interacts with CoA to yield the reduced form of lipoic acid and acetyl-CoA;

4 — the oxidized form of transacetylase is regenerated with the participation of E₃;

5 — the oxidized form of E₃ is regenerated with the participation of NAD⁺

In The First stage, pyruvate binds to TDP within E₁ and undergoes decarboxylation:

This reaction yields a TDP derivative with a hydroxyethyl group attached to the thiazole ring (Fig. 16.16).

Fig. 16.16. Thiamine diphosphate and hydroxyethyl-ThDP:

The functional moiety of ThDP is the thiazole ring, which attaches the pyruvate decarboxylation product, the hydroxyethyl group

Second Stage: dihydrolipoyl transacetylase (E₂) catalyzes the transfer of a hydrogen atom and an acetyl group from ThDP to the oxidized form of lipoylysine residues, yielding an acetyl thioester of lipoic acid (Fig. 16.15).

In the Third Stage, CoA reacts with the acetyl derivative of E₂, resulting in the Formation of Acetyl-CoA and a fully reduced lipoyl residue, which serves as the prosthetic group of E₂.

In the Fourth Stage, dihydrolipoyl dehydrogenase (E₃) catalyzes the transfer of hydrogen atoms from the reduced lipoyl groups to FAD, the prosthetic group of the E₃ enzyme.

In the fifth stage, the reduced FADH₂ transfers hydrogen to NAD⁺ to form NADH.

The pyruvate dehydrogenase complex features a large negative redox potential, which, alongside coenzyme reduction (NADH), drives The formation of the high-energy thioester bond in acetyl-CoA.

The structural assembly of these Three types of enzymes enables the coordination of individual steps in this complex enzymatic reaction. All intermediates of pyruvate oxidative decarboxylation remain tightly bound to the complex, thereby enhancing the overall reaction rate and minimizing Side Reactions.

Like all Proteins involved in TCA cycle reactions, the pyruvate dehydrogenase complex is encoded by nuclear DNA. The transport of PDC subunits into Cell/35.html">Mitochondria is a complex process driven by ATP energy or the transmembrane Electrochemical Potential, mediated by heat Shock proteins (chaperones) that prevent their premature folding prior to entering the mitochondrial matrix or inner mitochondrial membrane.

Link between pyruvate oxidative decarboxylation and the ETC. Oxidative decarboxylation of pyruvate is accompanied by the generation of NADH, which supplies electrons to the Respiratory Chain and drives the synthesis of 3 moles of ATP per 1 mole of pyruvate via Oxidative Phosphorylation.

Since the ADP/ATP and NADH/NAD⁺ ratios within The Cell remain relatively constant, accelerated ATP utilization leads to increased ADP concentrations and faster NADH oxidation in the respiratory chain. A rise in NAD⁺ concentration, in turn, stimulates pyruvate oxidative decarboxylation. Conversely, elevated levels of ATP and NADH suppress this process. Thus, shifts in the ADP/ATP and NADH/NAD⁺ ratios serve as vital signals reflecting cellular energy demands and regulating The rate of pyruvate oxidative decarboxylation. The catalytic activity of the pyruvate dehydrogenase complex decreases when the cell has an adequate supply of "fuel" in the form of Fatty acids and acetyl-CoA.