2,3-BPG is produced by tissues from ___________ in glycolysis.

2,3-BPG, or 2,3-bisphosphoglycerate, is formed from 1,3-BPG (1,3-bisphosphoglycerate) during the glycolytic pathway. Specifically, this transformation occurs in red blood cells, where the enzyme bisphosphoglycerate mutase facilitates the conversion of 1,3-BPG to 2,3-BPG. This is an essential step because 2,3-BPG plays a critical role in modulating hemoglobin's affinity for oxygen, allowing for efficient oxygen release in tissues.

The context of glycolysis further highlights the importance of this process, as it balances the production of ATP and enables the organism to maintain proper metabolic function under various oxygen conditions. The presence of 2,3-BPG ensures that hemoglobin can release oxygen more effectively in areas where it is needed most, such as actively respiring tissues.

The other potential answers do not fit as sources of 2,3-BPG production: ATP and ADP are nucleotides involved in energy transfer, and 3-PG (3-phosphoglycerate) is an intermediate later in the glycolytic pathway, not directly involved in the production of 2,