What molecule is indicated by the letter D$?$A scheme of cellular respiration. At box A$,$ which is located in the cytosol, glucose turns into pyruvate with release of ATP and electrons carried by NADH. Pyruvate from this stage is used in a cycle in box B$,$ which is located in the mitochondrion. As a result, ATP, CO$2,$ and electrons carried by NADH and FADH$2$ are released. The electrons carried by NADH and FADH$2$ from boxes A and B are used in box C $($inside the mitochondrion$)$ for transformation of a substance marked D $($that comes from the cytosol$)$ into a substance marked E $($released to the cytosol$)$ with release of ATP.