Oxidative decarboxylation of pyruvate

Structure of the pyruvate dehydrogenase complex

PDC is a complexly organized multienzyme system comprising three types of catalytic protomers ($E_1$, $E_2$, and $E_3$), alongside regulatory subunits (kinase and phosphatase).

The secret to the complex's efficiency lies in its unique spatial organization. All enzymes are positioned in close proximity, enabling simultaneous reactions across multiple sites. Consequently, intermediate metabolic products are not released into the surrounding environment but are directly transferred from one active center to another. This maximizes substrate conversion efficiency.

Coenzymes involved in PDC function are divided into two groups:

• Tightly bound: permanently integrated into the enzyme structure (coenzymes of $E_1$, $E_2$, and $E_3$ protomers).
• Free: NAD⁺ and HS-CoA join the complex exclusively at the moment of the reaction and subsequently leave as finished products — acetyl-CoA and reduced NADH + H⁺.

Catalytic protomers and their coenzymes

The rate of oxidative decarboxylation reactions directly depends on vitamin availability in the body. A deficiency in even one of them causes process activity to drop sharply.

Note: Pyridoxine (vitamin B₆) is not involved in this process.

Stages of oxidative decarboxylation

The net reaction equation is as follows: Pyruvate + NAD⁺ + Coenzyme A $\rightarrow$ Acetyl-CoA + NADH + H⁺ + CO₂

This process is divided into five sequential stages:

1. Stage I. Enzyme $E_1$ performs pyruvate decarboxylation (cleavage of CO₂). The resulting two-carbon fragment is transferred to the active coenzyme TDP to form hydroxyethyl.
2. Stage II. Enzyme $E_2$ oxidizes the hydroxyethyl group and transfers the two-carbon residue to lipoic acid amide. Lipoic acid plays a special role here: bound to lysine residues, it functions as a "swinging arm", shuttling hydrogen atoms and acetyl groups between enzyme active centers.
3. Stage III. The acetylated form of transacetylase interacts with HS-CoA. This synthesizes the high-energy compound acetyl-CoA, which then enters the Krebs cycle. Transacetylase converts into the dihydrolipoamide form.
4. Stage IV. Enzyme $E_3$, containing FAD, dehydrogenates the reduced transacetylase. This allows lipoic acid to regenerate and return to its initial state for a new cycle.
5. Stage V. At the final stage, the reduced FADH₂ is dehydrogenated with the participation of NAD⁺. The resulting NADH + H⁺ is directed to the electron transport chain (ETC), where it drives the synthesis of three ATP molecules.

Regulation mechanisms of PDC activity

The activity of the pyruvate dehydrogenase complex is controlled by altering its state: the dephosphorylated form is active, and the phosphorylated form is inactive. Two regulatory enzymes are responsible for this switching:

• PDC kinase converts the complex into an inactive state. It is activated by cellular energy excess signals — high levels of ATP, NADH, and acetyl-CoA. Thus, excess products shut down the reaction. Kinase inhibitors (keeping PDC active) include pyruvate (the most potent inhibitor), ADP, HS-CoA, and calcium ions.
• PDC phosphatase dephosphorylates and turns on the complex. Its primary activator is $Ca^{2+}$ ions.

Features of the absorptive period: Following a meal, glucose enters cells and is broken down into pyruvate. High pyruvate concentrations trigger PDC via two pathways. First, pyruvate strongly inhibits PDC kinase, preventing it from "turning off" the complex. Second, it allosterically activates the dephosphorylated form of the enzyme along with NAD⁺ and HS-CoA. Consequently, glucose is actively converted into acetyl-CoA, which in the liver is used for fatty acid synthesis.

Class and exam questions

What else to study on this topic

• Calculation of energy yield upon oxidation of 1 mol of pyruvate
• Determination of the oxidative phosphorylation coefficient (P/O) for the reaction
• Stoichiometry of complete pyruvate oxidation in the Krebs cycle
• Pathway of hydrogen and electrons along the electron transport chain (ETC)

Other topics in the "Energy Metabolism" section

Prepared by WhiteCoat Pro faculty in accordance with the medical school curriculum. For educational purposes only.