During the catalytic cycle of pyruvate dehydrogenase, pyruvate becomes covalently attached to a prosthetic group, thiamine pyrophosphate (TPP). This enzyme-bound intermediate loses carbon dioxide to form a second intermediate, which then donates a two-carbon unit to the lipoamide prosthetic group to form acetyl lipoamide. The structure of thiamine pyrophosphate is given. Modify the structure by drawing the covalent intermediate that donates a two-carbon unit to lipoamide.