Which of the following is an irreversible reaction?

The transformation of pyruvic acid into acetyl CoA represents an irreversible reaction due to its role in cellular metabolism and the accompanying regulatory mechanisms. This conversion is catalyzed by the pyruvate dehydrogenase complex and involves a decarboxylation process that releases carbon dioxide and requires coenzymes, making it energetically unfavorable to proceed in the reverse direction under physiological conditions.

In contrast, other reactions listed can be reversed under certain conditions. For example, pyruvic acid can be converted into lactic acid through reduction and is reversible, while glucose-6-phosphate can be transformed into either glycogen or pyruvate, both of which are also reversible processes in metabolic pathways. The nature of these reactions implies that they can adapt to the needs of the cell, allowing for greater versatility in energy production and storage. Thus, the conversion of pyruvic acid into acetyl CoA is a critical, one-way step in central metabolism, contributing significantly to the flow of carbon through the tricarboxylic acid (TCA) cycle.