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Preliminary step: oxidative decarboxylation

There are two reasons that make the presence of oxygen significantly increase the efficiency of the energy production process:

  • Pyruvic acid can be fully oxidized to CO2 and H2O and is not used solely to oxidize NADH.
  • In the electron transport chain, electrons are transferred, step by step, up to O2 to form H2O, producing a lot of ATP. 

The pyruvic acid obtained in the glycolysis in the cytosol, binds to specific membrane transporters to be able to cross the double membrane of the mitochondria to reach the mitochondrial matrix.

Within the mitochondria, in the matrixoxidative decarboxylation occurs, by means of an enzymatic complex called pyruvate dehydrogenase, whereby pyruvic acid is decarboxylated, oxidizes its α-keto group and is activated to the form of acetyl-CoA (2 carbon atoms).

The carbohydrates, fatty acids and amino acids, to be harvested energy by the cell, have to turn into groups acetyl, as acetyl-CoA, to enter the Krebs cycle and be oxidized to CO2 and H2O.

This preliminary phase is carried out in several stages:

  • Loss of the carboxyl group of pyruvate, which is released as CO2 (decarboxylation) with the first carbon and two oxygens.
  • The carbonyl group of pyruvate is oxidized (oxidative decarboxylation) and gives up its electrons to a NAD+ that is reduced to NADH, forming an acetyl group (CH3 -CO-).
  • The acetyl group is esterified with the -SH group of the coenzyme. The formed acetyl CoA which enters the citric acid cycle.

The pyruvate dehydrogenase enzyme catalyzes this reaction, reducing a molecule of NAD+ and forming 1 NADH. Since two pyruvate molecules had been obtained from each glucose molecule, 2 NADH are obtained at this stage.

Pyruvate transport to the mitochondria (oxidative decarboxylation):

pyruvate + CoA + NAD+ → Acetyl-CoA + NADH + CO 2

Recalling the processes seen so far, the glucose molecule has been oxidized to two CO2 molecules and two acetyl groups, also forming four NADH molecules (two in glycolysis and two in pyruvic acid oxidation).

Acetyl groups bind to coenzyme A, forming acetyl-CoA. It is in this way that you will enter the next phase of catabolism, the Krebs cycle.

Important to remember: The initial stage of cellular respiration is the oxidation of pyruvic acid to acetyl CoA (oxidative decarboxylation).

Balance oxidative decarboxylation (from 2 pyruvic acid molecules):

  • 2 acetyl-CoA molecules.
  • NADH + 2 H+.
  • 2 CO2.

Questions that have come out in University entrance exams (Selectividad, EBAU, EvAU)

Extremadura, July 2020, question 9

Answer the following questions:

A. Identify the processes that the following enzymes are involved in and what role they play:

Pyruvate dehydrogenase (0.5 points).

Ribulose 1,5-diphosphate carboxylase (0.5 points).

B. Define vaccine and immunological serum and establish two differences between them. (1 point)

Valencian Community, June 2021, question 3.2

3.2. After glycolysis, the pyruvate obtained can have two different destinations:

Pyruvate + NADH + H+ → lactate + NAD+
Pyruvate + CoA + NAD+ → acetyl-CoA + CO2 + NADH

a) Explain under what conditions each of them occurs (2 points).
b) Indicates whether the lactate and acetyl-CoA obtained could continue to oxidize. If so, indicate which metabolic path each of them would enter and whether both processes will ultimately provide the same energy (2 points).

Fundamental insights on oxidative decarboxylation

Oxidative decarboxylation

  • Through glycolysis, in the cytosol, pyruvic acid is obtained, which has to cross the double membrane of the mitochondria to reach the mitochondrial matrix.
  • The oxidative decarboxylation occurs and n the mitochondrial matrix.
  • The pyruvate dehydrogenase enzyme catalyzes this reaction by which pyruvic acid is decarboxylated, and is activated to the form of acetyl-CoA (2 carbon atoms), reducing a molecule of NAD+ and forming 1 NADH. As two pyruvate molecules had been obtained from each glucose molecule, 2 NADH is obtained at this stage.

Pyruvate + CoA + NAD+ → Acetyl-CoA + NADH + CO2


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