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How is pyruvate changed before it enters the tricarboxylic acid cycle?
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mitochondrion
Mitochondria (red) are found throughout the cytoplasm of almost all eukaryotic cells (cell nucleus is shown in blue; cytoskeleton is shown in yellow).
Before entering the tricarboxylic acid (TCA) cycle, pyruvate undergoes a process called oxidative decarboxylation, which is catalyzed by the pyruvate dehydrogenase complex. This multienzyme complex facilitates a series of consecutive reactions that convert pyruvate into acetyl coenzyme A.
The transformation involves the following key steps:
- Decarboxylation: Pyruvate reacts with thiamine pyrophosphate (TPP), releasing carbon dioxide and forming a hydroxyethyl-TPP-enzyme complex.1
- Transfer and Oxidation: The hydroxyethyl group is transferred to the coenzyme lipS₂ (6,8-dithio-n-octanoate), where it is oxidized to an acetyl group.1
- Formation of Acetyl CoA: The acetyl group is then transferred to the sulfhydryl group of coenzyme A, resulting in the production of acetyl coenzyme A.1
Throughout this process, two reducing equivalents are liberated in the form of NADH + H⁺. The coenzyme lipS₂ is subsequently reoxidized to allow the cycle to continue, transferring electrons to FAD and then to NAD⁺.1